Breeding for Resistance to 13 Fire Blight

Yves Lespinasse1 and Herb S. Aldwinckle2

1INRA–CR d’Angers–Unité d’Amélioration des Espèces Fruitières et Ornementales, Beaucouzé, France; 2Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva, New York, USA

Introduction

Nearly all cultivars of economically important fruits and ornamental plants that belong to the Maloideae are propagated by cloning. Furthermore, given favourable environmental conditions, cultural practices may increase the sever- ity of fire blight to destructive levels. A commercial planting of fruit trees usu- ally consists of one cultivar and its pollinizer, which can also result in an increased level of damage if the cultivar is susceptible to fire blight. A second important factor explaining the destructive effect of fire blight is the long life of an orchard, which allows the build-up of high populations of the bacterium. No commercial crop receives more chemical pesticides than fruits. This is due, in addition to the cultural factors mentioned above, to the complete range of plant parts which may be attacked and to the long period during the grow- ing season when the pathogens may be active. High plant densities, while improving some aspects of production efficiency, can increase the incidence of fire blight. No chemical can completely control the bacterium (Psallidas and Tsiantos, Chapter 11), and the use of antibiotics, where permitted, has resulted in the development of resistant strains (Jones and Schnabel, Chapter 12). The need for highly disease-resistant cultivars is more pressing than ever. Genetic disease immunity or resistance is recognized as an important feature of integrated pest management (IPM). Breeding disease-resistant fruit and orna- mental tree cultivars usually involves combining disease resistance with the best characters of susceptible cultivars. Horizontal resistance, which implies the pos- sible development of at least some infection, might be useful as long as crop loss is held to an acceptably low level – as in the case of fire blight. Genetic resis- tances are more often identified in primitive species or forms, or in obsolete

© CAB International 2000. Fire Blight: the Disease and its Causative Agent, Erwinia amylovora (ed. J.L. Vanneste) 253 254 Yves Lespinasse and Herb S. Aldwinckle cultivars with mediocre appearance and quality. The several generations required to combine the best characteristics of one cultivar with disease resis- tance explain the necessity for long-term and carefully planned breeding pro- grammes. The long generation time in fruit and ornamental trees may require continued breeding efforts through several decades. The pomologist, if working alone as a disease-resistance breeder, must therefore be highly knowledgeable about the disease in question. Preferably, he/she should work in close coopera- tion with a plant pathologist. Even better is an integrated team approach, involving efforts of scientists in both disciplines working in two or more areas representing somewhat different sets of environmental conditions. Team efforts have the greatest potential for rapid progress toward planned objectives.

Evaluation of species and cultivar susceptibility

Fruit scion and rootstock cultivars Natural infection in orchard or nursery Traditionally, resistance to fire blight has been evaluated by observing seedlings or trees growing under nursery or orchard conditions. Several factors other than plant genotype can affect these ratings, since natural sources of inoculum, cultural practices and environmental conditions vary from orchard to orchard, and even within orchards (Aldwinckle and Beer, 1978). Rootstocks (Keil and van der Zwet, 1975), tree age (Shaw, 1934), orchard topography and soil type (Fisher et al., 1959) and inoculum level (Beer, 1978) affect the disease and thus may obscure inherent genotypic differences in susceptibility. Nevertheless, reports based on field observations provide an important check on the validity of more artificial but better controlled experiments and, for some cultivars, are the only information available. The most comprehensive reports about apple susceptibility are those of Thompson (1972) and Aldwinckle et al. (1976), and about pear susceptibility the report of Oitto et al. (1970). Scores used by van der Zwet et al. (1970) ranked from 0 to 10, with the higher scores (10–8) indicating the least damage. This system is an overall appraisal which does not take account of the origin of the infection, usually flowers or shoots. In fact, it was demonstrated (van der Zwet, 1975) that sus- ceptibility could vary widely depending on the organs affected. Studying a large number of pear selections and cultivars in different locations (Maryland and Ohio), van der Zwet et al. (1984) showed inconsistencies in the comparative results, especially in the moderately resistant and resistant classes and occa- sionally even among the fairly susceptible classes. Van der Zwet and Keil (1979) published ratings of cultivars for fire blight resistance based on the literature; these ratings covered European and Asian pear, and apple cultivars. They also reported an overall degree of fire blight resis- tance for different Pyrus species. The cultivars were grouped into four classes: highly resistant, resistant, moderately resistant and susceptible. Breeding for Resistance to Fire Blight 255

PEAR CULTIVARS (Pyrus communis). Of the 287 cultivars named prior to 1920, only 11% are resistant to highly resistant; of the 113 cultivars released between 1920 and 1978, about one-third were reported to be predominantly resistant. As sources of resistance, van der Zwet and Keil (1979) ranked, in descending order based on their degree of fire blight resistance, Pyrus ussuriensis, Pyrus calleryana, Pyrus betulaefolia, Pyrus pyrifolia and Pyrus communis. Of the 49 clones of sand pear (P. pyrifolia synonymous with Pyrus serotina), only 28% were in the resistant classes.

PEAR ROOTSTOCKS. Quince, which is largely used as a pear rootstock, is usually susceptible to fire blight. A new series of rootstocks are being developed from pear, mainly from the cultivar ‘Old Home’, which is considered resistant to fire blight (Lombard and Westwood, 1987).

APPLE CULTIVARS (Malus domestica). Of the 193 cultivars introduced before 1920, about 28% are resistant; of the 197 cultivars released between 1920 and 1978, 41% are resistant.

APPLE ROOTSTOCKS. Several clonal apple rootstocks are highly susceptible to Erwinia amylovora (Cummins and Aldwinckle, 1973). Of the 28 clonal rootstocks (Malling and Malling–Merton series), only nine were in the light susceptibility class. The most important rootstock, M.9, is in the severe susceptibility class.

INTERACTION BETWEEN SCION AND ROOTSTOCK CULTIVARS. Rootstock influence on the growth habit of the scion may also affect scion susceptibility to fire blight. For example, a later flowering period, when conditions are more favourable for infection by E. amylovora, can lead to an increase in fire blight incidence.

Artificial inoculation To avoid the effect of the large variations in inoculum levels that prevail under natural conditions, artificial inoculation techniques have been developed for selection in breeding programmes. The most useful index for fire blight suscep- tibility is the extent of lesion development on the shoot. Measurements of this type have been shown to be strongly correlated with the field susceptibility of apple cultivars in several independent observations. Quamme et al. (1976) have also provided evidence that determination of fire blight susceptibility of pear cul- tivars by artificial inoculation is a valid procedure. Blossom susceptibility of apple and pear cultivars has received less atten- tion than susceptibility of vegetative tissues. However, susceptibility to blossom infection may be important in determining how readily infections are initiated in the orchard (Aldwinckle and Norelli, 1981). Correlation between suscepti- bility of shoots and of flowers is weak (0.25 < r < 0.44) (Thibault and Le Lezec, 1990); a good illustration is the cultivar ‘Gala’, which is only slightly susceptible on shoots but highly susceptible on flowers. 256 Yves Lespinasse and Herb S. Aldwinckle

In orchard or nursery plantings, artificial inoculation of either shoot tips or blossoms may be more or less successful depending on weather conditions. However, if trees under similar growing conditions are inoculated identically and simultaneously, they can be compared in terms of susceptibility. Several pome-fruit breeding programmes currently evaluate progeny for susceptibility to fire blight in the greenhouse. These methods appear to be the most efficient in eliminating highly susceptible individuals and are a great improvement over observations of natural infections in the field (Aldwinckle and Preczewski, 1976; Quamme, 1977; Aldwinckle and van der Zwet, 1979). Aldwinckle and Preczewski (1976) artificially inoculated 92 apple culti- vars; results indicate that, in contrast to pears, some important apple cultivars do have considerable resistance to fire blight – for example, ‘Delicious’ and its sports, and ‘Winesap’. Some of the scab-resistant cultivars are also resistant to fire blight, such as ‘Priscilla’ and ‘Liberty’; fire blight-resistance genes were prob- ably transmitted fortuitously along with scab-resistance genes from Malus flori- bunda 821, the source of the Vf gene. Moreover, Aldwinckle and Preczewski (1976) found that two spur-type apple cultivars were significantly less suscep- tible to fire blight than their standard-type parent cultivars. In most artificial inoculation experiments, a certain number of twigs do not react after inoculation. At the Institut National de la Recherche Agronomique (INRA) Angers laboratories, it was decided to take into account both the fre- quency (F, 0–1) and the severity (S, 0–100) of infection; this led to the index of varietal susceptibility (IVS) (Thibault et al., 1987): IVS = F S, (0–100) The IVS is divided into five classes (I to V) from 0 to 100 (from resistance to high susceptibility). This index is more reliable with data from several years; the eval- uation performed at INRA Angers associated incidence (classes 1 to 5), sever- ity (classes 1 to 5) and IVS (0–100) to characterize a cultivar. Seventy-six apple cultivars and 85 pear cultivars were assessed in this way (Le Lezec et al., 1985; Thibault and Le Lezec, 1990). A complete report of these studies was recently published (Le Lezec et al., 1997). In this review, we present ratings of the most important apple and pear cul- tivars (scions and rootstocks) currently grown in the world. These results were obtained in France over 3 years after artificial inoculation of the shoots in the field with E. amylovora (strain CFBP 1430) using a syringe; 36 shoots were inoculated per cultivar and per year. Major apple cultivars are shown in Table 13.1. The group originating from the cultivar ‘Delicious’ is ranked highest for resistance (I = 1, S = 2, IVS = 11); more recent commercial cultivars, such as ‘Gala’, ‘Jonagold’, ‘Fuji’ and ‘Elstar’ are in the same class as ‘Golden Delicious’. Major pear cultivars are shown in Table 13.2. Among rootstocks (Table 13.3), the quince selections are moder- ately susceptible (class III). The new pear selections show good results (classes I and II); all share the same parent ‘Old Home’. Most commonly used apple root- stocks in Europe (M.9, M.26, MM.106) were susceptible or very susceptible in Breeding for Resistance to Fire Blight 257

Table 13.1. Ratings of the main apple cultivars grown in the world (from Le Lezec et al., 1997). Incidence Severity Index Classes Cultivar (1–5) (1–5) (IVS)

I 0–20 ‘Delicious Spur’a 1211 (‘Starkrimson’) ‘Belle de Boskoop’ 1 2 12 ‘Delicious Standard’a 1315 (‘Hi-Early’) ‘Golden Spur’a 1315 II 20–40 ‘Jonagold’ 2 2 21 ‘Gala’ 3 3 24 ‘Reinette Grise du Canada’ 4 2 27 ‘Golden Delicious’a 3333 ‘Elstar’ 3 4 34 ‘Fuji’ 5 2 36 III 40–60 ‘Granny Smith’ 5 3 45 ‘Braeburn’ 5 3 52 ‘Melrose’ 5 3 54 IV 60–80 ‘Rome Beauty’ 5 4 60 ‘Cox’s Orange Pippin’ 5 4 68 ‘Idared’ 5 4 68 ‘Reine des Reinettes’ 5 5 71 a The spur types are less susceptible than the standards (‘Delicious’ and ‘Golden’ types).

this trial. In the USA, the natural incidence of fire blight infection has been most severe on M.9 and M.26, with few problems on MM.106. There is a great need for a fire blight-resistant rootstock that confers similar size control to that of M.9. There are other reports of the susceptibility of apple and pear (Zeller, 1978, 1990; Maroofi and Mostafavi, 1996). Twenty-four cultivars of Asian pears were evaluated in the field after inoculation; most of the ‘Nashi’ cultivars (P. pyrifolia) are susceptible; the ‘Li’ cultivars (P. ussuriensis and Pyrus bretschneideri) appeared less susceptible than the ‘Nashi’ cultivars (Lecomte, 1993). Most of the cider apples grown in the south-west of England (Gwynne, 1984) and in the west of France (Paulin et al., 1990, 1993b; Chartier et al., 1992) are suscepti- ble to fire blight; the results obtained in France showed that shoot and blossom susceptibility is more extreme than in dessert apples. Crab-apple cultivars have also been evaluated (Cline, 1985; Bonn and Elfving, 1990); some fire blight- resistant crab-apples may be suitable both for apple orchards as pollinizers and for the ornamental industry. 258 Yves Lespinasse and Herb S. Aldwinckle

Table 13.2. Ratings of the main pear cultivars grown in the world (from Le Lezec, et al., 1997). Incidence Severity Index Classes Cultivar (1–5) (1–5) (IVS)

I 0–20 ‘Harrow Sweet’ 2 1 11 ‘Conference’ 3 1 16 ‘Beurré Bosc’ 2 3 18 ‘Blanquilla’ 4 1 18 ‘Coscia’ 2 2 18 II 20–40 ‘Docteur Jules Guyot’ 4 2 21 ‘Beurré Hardy’ 2 3 25 ‘Beurré d’Anjou’ 2 5 30 ‘Rocha’ 4 3 39 III 40–60 ‘Abbé Fetel’ 3 4 43 ‘Bartlett’ 4 4 57 IV 60–80 ‘Passe Crassane’a 4571 ‘Packham’s Triumph’ 5 4 72 V 80–100 ‘Doyenné du Comice’ 5 5 90 a Very susceptible on secondary blossom.

Ornamentals

Cotoneaster, Pyracantha and Crataegus are the most important genera of orna- mentals susceptible to fire blight. Several studies on fire blight susceptibility of species or cultivars of Cotoneaster have been carried out in northern Europe (Zeller, 1979; Bouma, 1990a; Cadic and Lecomte, 1990). Results of about 55 cultivars of Cotoneaster have been summarized by Lecomte and Cadic (1993); most cultivars were susceptible. Limited experimental data are available on seedlings of Crataegus species (Maas Geesteranus and Heyting, 1978; Bouma, 1990a). Paulin et al. (1993a) assessed 84 Crataegus species after shoot inoculation in the field; results showed that susceptibility varied widely within this genus – cases of extreme suscepti- bility were observed, as well as cases of apparently complete resistance. Paulin and Cadic (1997) listed some interesting resistant clones that are commercially useful ornamentals. Bouma (1990a) and Cadic and Lecomte (1990) reported fire blight sus- ceptibility of several species and cultivars of Pyracantha. This genus appeared less susceptible than Cotoneaster. Some cultivars of Pyracantha atalantioides, including ‘Gibsii’ and ‘Debussy’, are very susceptible and have disappeared over time; some others, including ‘Shawnee’ and ‘Mozart’, are quite resistant. Breeding for Resistance to Fire Blight 259

Table 13.3. Ratings of the main rootstocks for pear and apple grown in the world (from Huet and Michelesi, 1990; Le Lezec, et al., 1997). Incidence Severity Index Classes Cultivar (1–5) (1–5) (IVS)

I Pyrus 0–20 INRA® ‘Pyriam’ (OH11) 1 2 11 Delbard® 333 ‘Brokmal’ 1 2 13 Farold® 87 ‘Daytor’ 3 2 14 Farold® 282 ‘Dayre’ 3 2 15 Farold® 40 ‘Daygon’ 4 2 18 Delbard® 51 ‘Broklyl’ 2 2 20 II Pyrus 20–40 Farold® 69 ‘Daynir’ 3 2 26 III Cydonia 40–60 BA 29 5 3 53 C.EM 5 3 55 A.EM 4 2 58 ‘Sydo’ 5 3 58 ‘Adam’s’ 5 3 58 I Malus 0–20 ‘Novole’ 1 1 10 MM.104 1 1 10 ‘Robusta 5’ 1 1 11 M.2 3 2 15 MM.109 2 3 15 MM.111 1 3 17 II Malus 20–40 M.7 4 2 24 M.25 3 3 28 III Malus 40–60 M.27 5 3 50 ‘Pajam 2’ (M.9) 5 3 51 ‘Pajam 1’ (M.9) 5 4 60 IV Malus 60–80 ‘Supporter 4’ (PI80) 5 4 69 M.26 5 4 73 MM.106 5 4 79

Breeding programmes

Van der Zwet and Keil (1979) reviewed breeding programmes on pear, apple, Pyracantha, hawthorn and Cotoneaster. This review emphasizes the breeding work from the late 1970s to the present.

Development of inoculum

Greenhouse screening requires the identification, isolation, maintenance and testing of the pathogenicity of the inoculum. Tests should include isolates from 260 Yves Lespinasse and Herb S. Aldwinckle diverse geographical areas and from plants from the same or closely related species and genera. The breeder must determine as quickly as possible the level of resistance in potential parents against isolates selected for their pathogenic- ity. Plant pathologists have developed a wide range of techniques for successful preservation of virulent inoculum and for the inoculation of host tissue. Differences in virulence (amount of disease caused by a pathogen) are com- mon among strains of E. amylovora. Unless there is an interaction between one or several cultivars tested and the strain, a strain is expected to have the same level of virulence, whatever the inoculated cultivar. Specific interactions, called differential virulence, have been found between some strains of E. amylovora and some apple cultivars by Norelli et al. (1984, 1986, 1987). Strains with differential virulence to apple are not rare in North America (about 10% of the strains tested by Norelli, 1986), but they have never been found in Europe (J.P. Paulin, France, 1998, personal communication). In contrast, no evidence for such interaction has been found in Pyrus (Quamme and Bonn, 1981). In another study with pear, Bell and van der Zwet (1996), found no consistent isolate host genotype interaction, suggesting that differ- ential virulence is not a major concern in breeding for fire blight resistance in pear using the current sources of resistance. The use of a mixture of different strains, as recommended by Norelli (1986), can offer a solution to this problem. Paulin and Lespinasse (1990) sug- gested the inoculation of several selected strains in successive independent steps, since the use of a mixture of strains did not give a higher overall incidence and severity than the most virulent strain. Bell et al. (1990) studied 11 genotypes of pear (P. communis) and five strains of E. amylovora in various combinations at three locations for 2 years in order to assess the degree of variability in disease expression due to random strain and environmental effects. None of the inter- actions resulted in significant differences. Therefore, the choice of standard resistant and susceptible hosts can be arbitrary. Standard strains should, of course, be chosen on the basis of a high general virulence and frequency of suc- cessful infection.

Pear scion breeding programmes

Plants derived from P. ussuriensis and P. serotina led to several cultivars, includ- ing ‘Magness’, ‘Lee’, ‘Mac’ and ‘Star’, which were a compromise between resis- tance and fruit quality. Certain cultivars (e.g. ‘Magness’) showed an unexpected susceptibility to trunk blight. P. ussuriensis and P. serotina do not appear to be sources of resistance superior to that of the European pear, P. communis (Layne, 1964; Bell et al., 1996). Breeding for Resistance to Fire Blight 261

The pear breeding programme conducted at the US Department of Agriculture (USDA) Appalachian Fruit Research Station, Kearneysville, West Virginia This programme has generated many selections resistant to fire blight. At pre- sent, eight pear clones from a collection of 200 second-test selections have been proposed for further trials. All have shown resistance to infection equal to or better than that of ‘Seckel’, from which their resistance is derived (Bell and van der Zwet, 1993). Studying some factors affecting selection for fire blight resistance in pear, van der Zwet et al. (1981) indicated that the presence of bloom and the transi- tion to the adult phase are not necessarily a major factor in susceptibility to fire blight. They also supported early screening of juvenile seedlings.

The Agriculture and Agri-Food Canada pear breeding programme at Harrow, Ontario Many sources of resistance have been used, including cultivars and selections from P. communis, P. ussuriensis and P. pyrifolia, with the fruit characteristics of P. communis being recovered by back-crossing to selected P. communis cultivars. Actively growing seedlings 30–40 cm tall grown in the greenhouse were inoc- ulated near the shoot tip with a standardized suspension of six strains of E. amylovora. Upon evaluation 2 months later, seedlings were discarded if the inoculated lesion extended beyond 25–30% of the shoot length. The more resis- tant seedlings were planted out in the field, where the incidence and severity of natural fire blight infections were rated annually, using the USDA scale (van der Zwet et al., 1970). Seedlings were screened again for fire blight resistance when they began to fruit, usually 5–7 years after establishing the seedling orchard, using the same technique as in the greenhouse (Hunter, 1993). ‘Harrow Delight’ and ‘Harvest Queen’ were named and released in 1981 (Quamme and Spearman, 1983) and ‘Harrow Sweet’ in 1990 (Hunter et al., 1992). Quamme et al. (1990) tested the efficacy of early selection for fire blight resistance. A set of progenies was tested for fire blight resistance by needle inoc- ulation at 3 months of age in the greenhouse and then 5 years later in the orchard. Correlation of fire blight resistance at the two stages of growth was weak or absent on a single-plant basis. Nevertheless, genetic gain based on the field measurements appeared to be possible if plants were selected in the green- house with less than 20% of shoot length blighted. It was proposed that selec- tion for fire blight be carried out at both stages of growth – in the greenhouse, as simple measurements on young plants, and again in the field, when repli- cated measurements can be made.

The INRA pear breeding programme at Angers, France Thibault (1981) developed an initial programme for fire blight resistance breed- ing of pear from a half-diallel, including seven parents: four resistant American 262 Yves Lespinasse and Herb S. Aldwinckle selections and three old European cultivars. In contrast to a diallel cross, which is the set of all possible matings between several genotypes (varieties or selected clones), a half-diallel in pear does not include the reciprocal crosses and the combinations resulting from self-fertilization (the pear is self-sterile). Twenty- one progenies were obtained and 4000 hybrids were studied (Thibault and Maas Geesteranus, 1984). Later on, the programme was limited to P. communis parents with only low or moderate susceptibility. There were 55 parents and 200 progenies with a total of 55,000 hybrid seedlings (Thibault, 1990). Today 10,000 seedlings from this programme are under evaluation in the field for pomological traits, such as fruit quality (Le Lezec et al., 1991). Thibault et al. (1987) compared two methods of selection of young seedlings: selection in the greenhouse on 30–40-cm-tall plants and selection in the field during the juve- nile phase. Clearly, it appears that the selection in the greenhouse was too severe. The pear seedlings in the greenhouse are now selected for resistance if less than 50% of shoot length is blighted after inoculation with a single strain (CFBP 1430). This strategy permits maintaining enough plants per cross in the field to be able to evaluate the genetic interest of the parents and to increase the efficiency of the selection procedure for releasing new cultivars with desirable pomological traits. The other major character selected against is secondary blos- som, which is only visible in the field at the adult stage. This trait is heritable (Thibault et al., 1983). The strategy developed emphasizes the absence of sec- ondary blossoms and lowers the percentage of shoot length blighted. The most recent pear cultivar released, ‘Angelys’ (1998), is a good example of this strategy; the future development of this new cultivar will be a good test of the paradigm.

The pear breeding programme conducted at the Istituto Sperimentale per la Frutticoltura, Rome and Forli, Italy The programme emphasizes the breeding of dwarf and semi-dwarf cultivars suitable for high density orchards to be managed from the ground (Fideghelli et al., 1984; Quarta, 1990). The population studied consisted of 107 progenies. The seedlings obtained were inoculated in the nursery, where conditions varied from year to year; the same progeny inoculated partly one year and partly another year could give quite different results in percentage of selected seedlings. In these conditions, standard cultivars, such as ‘Stark Delicious’, ‘Bella di Giugno’, ‘Coscia’ and ‘Max Red Bartlett’, susceptible or with low resis- tance, gave offspring with useful resistance. On the contrary, resistant cultivars, such as ‘Morgan’, ‘Dr Molon’, ‘Sirrine’ and ‘US 309’, gave progeny with very low resistance. Furthermore, the fruit quality of the latter was quite poor. It is concluded that selection for resistance within the more commercially adapted breeding population may be more effective (Bagnara et al., 1996). From this programme, five selections are under advanced tests, including the named cul- tivar ‘Tosca’. Breeding for Resistance to Fire Blight 263

Apple scion breeding programmes

Apple scion breeding programmes for fire blight resistance have not been devel- oped to the same extent as the pear breeding programmes, probably because apples in general are less susceptible than pears. Fire blight resistance has been included in programmes in which scab and mildew resistance are more impor- tant. The research stations of Cornell University (Geneva, New York), INRA (Angers, France) and the Institute for Fruit Breeding (Dresden-Pillnitz, Germany), have developed such programmes, with varying degrees of empha- sis on fire blight resistance. At Geneva, New York, each seedling was inoculated with E. amylovora and the material was selected, before fruiting, for resistance to fire blight; in recent years, this screening has been applied later in the breeding process. Gardner et al. (1980b) found dominant genes which confer resistance in certain highly resistant Malus species, M. robusta 5 and a selection of Malus sublobata, called ‘Novole’, which were studied for rootstock breeding. These clones are not com- pletely resistant and a differential genotype strain interaction was found with some clones (Norelli et al., 1984, 1986). Certain scab-resistant cultivars and selections are more promising for scion breeding (Aldwinckle and van der Zwet, 1979; Lespinasse and Paulin, 1984, 1990; Korban et al., 1988). Resistance is quantitatively controlled (Gardner et al., 1980b; Korban et al., 1988; Lespinasse and Paulin, 1990; Fischer and Richter, 1996). Cultivars such as ‘Liberty’ and ‘Priscilla’ carry a high level of resistance under polygenic control with additive gene effects. Korban et al. (1988) studied the segregation of seedlings derived from 16 controlled crosses after fire blight inoculation; the results indicate that there is no evidence of a close linkage between the Vf gene from M. floribunda 821 coding for scab resistance and genes for fire blight resistance. At Dresden-Pillnitz, in Germany, parents including Malus robusta, Malus floribunda, as well as several new Pillnitz Pi- and Re- cultivars®, are used as sources of fire blight resistance. Seedlings are tested in the field; the growing shoots and blossoms are inoculated with a suspension of three highly virulent isolates (Fischer and Richter, 1996). The plant material is scored from 1 (very susceptible) to 9 (no symptoms). Resistance is evaluated after shoot and flower inoculation, since there is no correlation between results obtained with these two assays (Fischer and Schaefer, 1990).

Rootstock breeding

At Dresden-Pillnitz, within the rootstock selection programme, numerous pop- ulations from crosses between species and cultivars were evaluated in the green- house for their susceptibility to fire blight. Seedlings with low susceptibility were selected from progenies of ‘M.4’, ‘M.11’ and Malus micromalus, as well as of Pyrus canescens Pyrus serrulata and P. betulaefolia P. ussuriensis (Fischer, 1996). 264 Yves Lespinasse and Herb S. Aldwinckle

The apple rootstock breeding programme at the New York State Agricultural Experiment Station (Cornell University), Geneva, New York, has emphasized excellent orchard performance combined with disease resistance over a range of size control since its inception in 1968 (Cummins and Aldwinckle, 1982). Over 350,000 hybrid seedlings have been screened and have resulted in a group of about 20 élite selections, from which four rootstocks have already been released and others are being tested. The programme is con- tinuing actively with breeding and evaluation. The Geneva programme has used various sources of resistance, including crab-apples and other Malus species (Gardner et al., 1980a), hybridized with existing rootstocks (e.g. M.9 and M.26). All seedlings were subjected to chal- lenge from zoospore inoculum of mixed virulent isolates of Phytophthora cacto- rum. Surviving seedlings were subsequently subjected to multiple inoculations by shoot tip injection with virulent strains of E. amylovora, including strains showing differential virulence. Only seedlings showing a high degree of resis- tance to E. amylovora were selected. They were screened for resistance to woolly apple aphid, Eriosoma lanigerum, but this was not a determinant factor. Selections were placed in stool beds and further selected. The preselections com- ing from the stool-bed trial were tested as rootstocks grafted to non-precocious cultivars. The best performers were then subjected to further testing as root- stocks grafted to several cultivars in various locations in the eastern USA. Subsequently, several selections and named rootstocks have been and continue to be tested in the multi-state orchard trials of the NC-140 project. The four rootstocks released from the programme to date are all resistant to fire blight, but differ in other characteristics (Robinson et al., 1997). Geneva 65 (1992) produces a smaller tree than M.9 and may be sufficiently productive only with vigorous cultivars, with irrigation. Several nurseries have found it dif- ficult to propagate. Geneva 16 (1997) produces a tree with similar size and pro- ductivity to that of M.9. It requires further testing before being recommended for large-scale use. Geneva 11 (1993) produces trees of M.26 size, or smaller, with similar productivity. Geneva 30 (1994) produces trees of M.7 size, but with greatly superior precocity and productivity. However, it is rather spiny in the stool bed. CG41, an élite selection that has not yet been released, also looks very promising in the M.9 size range, with excellent precocity and productivity.

Ornamental breeding

Cotoneaster A breeding programme was developed in Germany in the 1970s, shortly after the introduction of fire blight; this programme resulted in two field-resistant cul- tivars in the species Cotoneaster dammeri. Breeding efforts have also been made with types of the upright-growing species Cotoneaster franchetii, Cotoneaster sali- cifolius and Cotoneaster wateri (Persiel and Zeller, 1981, 1990); this programme is now discontinued. Breeding for Resistance to Fire Blight 265

Pyracantha A breeding programme was developed in The Netherlands by Bouma (1990b); 135 crosses were made and 4700 seedlings were tested. Crosses between par- ents with a high level of resistance, e.g. ‘Dart’s Red’ ‘Buttercup’ and ‘Dart’s Red’ ‘Shawnee’, resulted in a high percentage of plants without symptoms following inoculation. In France, the Pyracantha breeding programme, funded by a commercial association (GIE SAPHYR®), started in 1982 to create new cul- tivars with resistance to scab and fire blight. In total, 80 progenies and 14,000 seedlings were obtained; about 7000 scab-resistant hybrids were inoculated with a single strain of E. amylovora. After multiple inoculations on the succes- sively selected populations, 330 hybrids were planted in the field for selection of ornamental traits (Cadic, 1983, 1984, 1987; Bertrand et al., 1992; Decourtye and Cadic, 1993). From this programme three new cultivars were released: SAPHYR® orange – ‘Cadange’ and SAPHYR® rouge – ‘Cadrou’ originating from the cross ‘Shawnee’ ‘Mozart’ (Cadic et al., 1990), and SAPHYR® jaune – ‘Cadaune’ originating from the cross ‘Shawnee’ ‘Golden Glow’ (Decourtye and Cadic, 1993). These three cultivars are now widely propagated and have replaced the most susceptible cultivars in landscaping in France. During the past 8 years, approximately 1 million plants of these resistant cultivars have been sold in France and in other countries. This breeding programme is con- tinuing to increase the resistance to fire blight, as well as the cold hardiness needed for the North European market.

Somaclonal variation and mutation breeding

Somaclonal variation and mutation breeding can result in improvement of existing cultivars in a shorter period of time than it usually takes to produce a new cultivar by conventional breeding. These breeding methods make possible the selection of new plant types retaining the most favourable properties of the mother cultivar. Furthermore, in vitro techniques allow the early selection of new genotypes, thus significantly reducing the number of plants to be tested in the field. Viseur (1990) regenerated plants from calluses initiated on roots of the pear cultivar ‘Durondeau’; in vitro screening was performed by artificial inoculation of a virulent strain of E. amylovora. Two out of the four somaclonal variants selected with partial fire blight resistance were identified as tetraploids; this ploidy level and the resulting different growth pattern could explain the lower susceptibility to fire blight. The partial resistance of the other two variants, which were diploid, could, according to Viseur (1990), be due to their branch- ing development. Donovan et al. (1994) reported somaclonal variation for fire blight resis- tance among regenerants from the apple cultivar ‘Greensleeves’. Among 270 somaclones regenerated from leaf discs, four were selected on the basis of in vitro 266 Yves Lespinasse and Herb S. Aldwinckle shoot and greenhouse inoculations. Chevreau et al. (1998) tested the stability for resistance of these four clones in vitro, in the greenhouse and under field con- ditions. Overall results indicated that only one clone was clearly less susceptible than the control. This clone is a ‘spur’ variant with the same level of ploidy (2) and the same zymograms as the control; again, the different growth habit could explain the lower susceptibility to fire blight. Irradiation of in vitro explants and subsequent adventitious regeneration has been tested for four cultivars of pear in order to select mutants with a reduced susceptibility to fire blight (Pinet-Leblay et al., 1992). Such a system could be an alternative to the standard in vivo irradiation of buds, which usu- ally generates a high frequency of chimeric plants. An in vitro screening for resistance to fire blight using a pathogenicity mutant of E. amylovora has been developed on detached leaves to select among a mutagenized population, the regenerants showing a hypersensitive reaction (Pinet-Leblay et al., 1996). This programme, initiated at INRA, Angers, is now discontinued and has been replaced by genetic transformation of the pear.

Prospects and breeding strategies

Plant exploration

Plant exploration, particularly in regions where a pathogen has co-evolved with the host, holds great promise for the discovery of useful germplasm for resis- tance breeding. Introduction of wild material as seeds allows a more comprehensive sam- pling of possible genotypes within a species or in the area explored, with a cor- respondingly greater chance of recovering desirable traits, than does collection of more perishable tissues for asexual propagation. This is well illustrated by the apple exploration programme in North America, Central Asia and China con- ducted by the USDA Plant Genetic Resources Unit and Cornell University at Geneva, New York (Hokanson et al., 1997). Extensive exploration for pear germ- plasm in Central and Eastern Europe has been carried out by the USDA Appalachian Research Station, Kearneysville, West Virginia (van der Zwet and Bell, 1990, 1995).

Progeny size for genetic tests

Optimum progeny size may be determined after a preliminary sample estimate of the variance of the population for that character has been obtained. Without such advance knowledge, a good rule is to produce progenies of at least 100 individuals. Estimation of quantitative genetic parameters, for which the vari- ance between families, as well as the variance within families, is needed, will require that a number of families with a wide range of values for fire blight Breeding for Resistance to Fire Blight 267 resistance be produced. For breeding purposes, in which multiple trait selection is to be carried out, much larger progeny sizes are usually desired, with the exact number to be determined by the percentage of seedlings to be discarded by selection for each trait (Dayton et al., 1983).

Selection of parents

Given the long generation time of some fruit crops or ornamental trees, breed- ing progress may in some cases be enhanced by selecting parents with less than the highest level of resistance available but which retain more nearly acceptable horticultural characteristics; this is the case for apple and pear. Fire blight resistance present in Asian pear species, P. pyrifolia (Burm) Nak., P. calleryana Dene. and P. ussuriensis Max. is often associated with poor flavour, grittiness, small fruit size or other undesirable characters. Although there appears to be no strict genetic association within hybrid progenies as a whole, the proportion of seedlings with acceptable levels of resistance and fruit quality is low, due to the dominance of species characters. Because a specific type of inheritance pattern characteristic of any of the three species studied (P. com- munis, P. ussuriensis, P. pyrifolia) was not detected, it was concluded that the same or similar genes for resistance may be present in each species (Bell et al., 1996). For quantitatively inherited traits, estimates of heritability and of general and specific combining ability may provide an indication of the reliability of the phenotypic scores as indicators of an individual’s breeding value (Brown, 1975; Bell et al., 1977). According to Bell et al. (1977) and Bagnara et al. (1992), the additive sources of variation in pear progeny means exist in sufficient amount (higher than 50%) to enable a reasonable rate of genetic advance in resistance to fire blight. If additive genetic effects predominate over non-additive genetic effects and the environmental variance is low, heritability will be high and par- ents may be selected on the basis of their own phenotypes. Tests of combining ability may be applied to identify prepotent parents. The best approach to breeding is multistage selection, aimed at increasing the frequency of favourable genes to resistance. This requires a large number of crosses, instead of a large number of seedlings per cross. In order to combine fire blight resistance and fruit quality, selection for resistance within the high-quality susceptible cultivars may be the most effec- tive. Selection should thus be applied for many traits at the same time (Bagnara et al., 1996). Again, with pear, Quamme et al. (1990) found a high general combining ability and a low specific combining ability for fire blight resistance. Genetic vari- ance was, therefore, predominantly additive. Genetic advance for fire blight resistance should be obtained by selecting parents on the basis of phenotypic values. Subsequent studies have not supported the hypothesis of a major gene for susceptibility, proposed by Thompson et al. (1975), although progenies in 268 Yves Lespinasse and Herb S. Aldwinckle these studies included both ‘Bartlett’ and ‘Doyenné du Comice’, used as parents by Thompson et al. (1975).

Succession of crosses

Polygenic resistance – that is, resistance conferred by numerous genes of small and more or less equal effect – may be characterized by the method of quanti- tative genetics. Heritability and combining-ability estimates may be of use to the breeder, if proper precautions are taken in experimental design and the results are interpreted with consideration of the assumptions of the analysis (Bell, 1978). When resistance is under polygenic control, test or sib-cross generations must be interposed between each back-cross generation in order to recover recessive genes in homozygous form, to identify heterozygous parents or to reconcentrate polygenes. The most feasible method of working with polygeni- cally controlled resistance appears to require development of a pool of individ- uals, all with as good resistance as can be found by appropriate testing methods and resembling as much as possible the desired cultivar type. Build-up of a poly- gene system requires that screening procedures be refined to the point that small differences in resistance, conditioned by small numbers of minor genes, can be identified. Intercrossing would provide the large base necessary for hor- ticultural selection. The search for molecular markers flanking the quantitative trait loci (QTL) for fire blight resistance would facilitate marker-assisted selection. Up to now, no research programme emphasizes the search for molecular markers linked to fire blight resistance; therefore, breeders must continue the breeding efforts already under way, taking into account that host response is inherited in a quantitative fashion and is due to predominantly additive genetic effects, with dominance or epistasis playing only a minor role (Bell et al., 1996). Simultaneous selection for combined high fruit quality or ornamental attrib- utes and fire blight resistance is clearly possible.

References

Aldwinckle, H.S. and Beer, S.V. (1978) Fire blight and its control. Horticultural Reviews 1, 423–474. Aldwinckle, H.S. and Norelli, J.L. (1981) Varietal differences in incidence of infection of apple blossoms and shoots by Erwinia amylovora (fire blight). Acta Horticulturae 117, 71. Aldwinckle, H.S. and Preczewski, J.L. (1976) Susceptibility of vegetative tissues of apple cultivars to invasion by Erwinia amylovora. Phytopathology 66, 1439–1444. Aldwinckle, H.S. and van der Zwet, T. (1979) Recent progress in breeding for fire blight resistance in apples and pears in North America. EPPO Bulletin 9, 13–25. Breeding for Resistance to Fire Blight 269

Aldwinckle, H.S., Way, R.D., Livermore, K.G., Preczewski, J.L. and Beer, S.V. (1976) Fire blight in the Geneva apple collection. Fruit Varieties Journal 30, 42–55. Bagnara, G.L., Rivalta, L., Laghi, M. and Quarta, R. (1992) Valutazione di diverse com- binazioni d’incrocio di pero per la resistenza al colpo di fuoco batterico. Agricultura Ricerca 140, 73–76. Bagnara, G.L., Rivalta, L., Laghi, M. and Quarta, R. (1996) Evaluation of fire blight resis- tance in pear: combining ability and breeding strategy. Acta Horticulturae 411, 383–392. Beer, S.V. (1978) Technique for field evaluation of spray materials to control fire blight of apple and pear blossoms. In: Zher, E.I. (ed.) Methods for Evaluating Plant Fungicides, Nematicides, and Bactericides. American Phytopathological Society, St Paul, Minnesota, USA, pp. 46–50. Bell, R.L. (1978) Breeding and genetic studies in pear (Pyrus spp.). PhD thesis, Purdue University, West Lafayette, Indiana, USA. Bell, R.L. and van der Zwet, T. (1993) New fire blight resistant advanced selection from the USDA pear breeding program. Acta Horticulturae 338, 415–419. Bell, R.L. and van der Zwet, T. (1996) Stability of host resistance of pear to fire blight. Acta Horticulturae 411, 413. Bell, R.L., Janick, J., Zimmerman, R.H. and van der Zwet, T. (1977) Estimation of heri- tability and combining ability for fire blight resistance in pear. Journal of the American Society for Horticultural Science 102(2), 133–138. Bell, R.L., van der Zwet, T., Bonn, W.G., Thibault, B. and Lecomte, P. (1990) Environmental and strain effects on screening for fire blight resistance. Acta Horticulturae 273, 343–350. Bell, R.L., Quamme, H.A., Layne, R.E.C. and Skirvin, R.M. (1996) Pears. In: Janick, J. and Moore, J.N. (eds) Fruit Breeding, Vol. 1: Tree and Tropical Fruits. J. Wiley and Sons, New York, USA, pp. 441–514. Bertrand, H., Cadic, A. and Belin, J. (1992) Pyracantha. Origine, description et clé de déter- mination des principaux taxons. PHM, Revue Horticole, Collection Repères tech- niques, 101 pp. Bonn, W.G. and Elfving, D.C. (1990) Evaluation of crabapple cultivars and selections for resistance to fire blight. Acta Horticulturae 273, 311–317. Bouma, A.S. (1990a) Testing Cotoneaster, Crataegus and Pyracantha for fire blight sus- ceptibility in the Netherlands. In: Fire Blight of Pomoïdae (Erwinia amylovora, Burrill, Winslow et al.). Applied Research in Europe (1978–1988), EUR 12601, ECSC-EEC-EAEC, Brussels-Luxembourg, ed., pp. 146–165. Bouma, A.S. (1990b) Breeding Pyracantha for fire blight resistance. Acta Horticulturae 273, 327–334. Brown, A.G. (1975) Apples. In: Janick, J. and Moore, J.N. (eds) Advances in Fruit Breeding. Purdue University Press, West Lafayette, Indiana, USA, pp. 3–37. Cadic, A. (1983) La sélection du Pyracantha pour la résistance au feu bactérien (Erwinia amylovora) et à la tavelure (Fusicladium pyracanthae). L’Horticulture Française 155, 15–17. Cadic, A. (1984) Pyracantha breeding program in France: first results. Acta Horticulturae 151, 307–313. Cadic, A. (1987) Breeding Pyracantha for fire blight resistance. Acta Horticulturae 217, 299–303. Cadic, A. and Lecomte, P. (1990) Evaluation de la sensibilité au feu bactérien des Cotoneaster, Pyracantha et Malus en France. In: Fire Blight of Pomoïdae (Erwinia 270 Yves Lespinasse and Herb S. Aldwinckle

amylovora, Burrill, Winslow et al.). Applied Research in Europe (1978–1988), EUR 12601, ECSC-EEC-EAEC, Brussels-Luxembourg, ed., pp. 166–177. Cadic, A., Paulin, J.P. and Belin, J. (1990) New Pyracantha resistant to scab (Spilocaea pyracanthae (Otth.) Rostrup) and to fire blight (Erwinia amylovora (Burr.) Winsl. et al.). Acta Horticulturae 273, 303–306. Chartier, R., Boré, J.M. and Paulin, J.P. (1992) Sensibilité des pommiers à cidre au feu bactérien. Phytoma – La Défense des Végétaux 440, 24–31. Chevreau, E., Brisset, M.N., Paulin, J.P. and James, D.J. (1998) Fire blight resistance and genetic trueness-to-type of four somaclonal variants from the apple cultivar ‘Greensleeves’. Euphytica 104, 199–205. Cline, M.N. (1985) Stopping fire blight requires knowing its symptoms and acting promptly. American Nurseryman 161(11), 83–88, 93–95. Cummins, J.N. and Aldwinckle, H.S. (1973) Fire blight susceptibility of fruiting trees of some apple rootstock clones. HortScience 8(3), 176–178. Cummins, J.N. and Aldwinckle, H.S. (1983) Breeding apple rootstocks. In: Janick, J. (ed.) Plant Breeding Reviews 1, AVI, Westport, pp. 294–394. Dayton, F.D., Bell, R.L. and Williams, E.B. (1983) Disease resistance. In: Moore, J.N. and Janick, J. (eds) Methods in Fruit Breeding. Purdue University Press, West Lafayette, USA, pp. 189–215. Decourtye, L. and Cadic, A. (1993) Sélection de Pyracantha à haute performance. Comptes Rendus de l’Académie d’Agriculture de France 79(3), 67–75. Donovan, A.M., Morgan, R., Valobra-Piagnani, C., Ridout, M.S., James, D.J. and Garrett, C.M.E. (1994) Assessment of somaclonal variation in apple. I. Resistance to the fire blight pathogen, Erwinia amylovora. Journal of Horticultural Science 69(1), 105–113. Fideghelli, C., Cobianchi, D., Rivalta, L., Della Strada, G. and Maas Geesteranus, H.P. (1984) Breeding program for fire blight resistance by the Istituto Sperimentale per la Frutticoltura. Acta Horticulturae 151, 283–289. Fischer, C. and Richter, K. (1996) Breeding for fire blight resistance within a multiple resistance – breeding programme in apples. Acta Horticulturae 411, 375–381. Fischer, C. and Schaefer, H.J. (1990) Vergleichende Untersuchungen der Resistenz von pfelsorten gegenüber Feuerbrand im Gewächshaus und im Freiland. Gartenbau 37, 299–300. Fischer, M. (1996) Results of resistance tests to Erwinia amylovora (Burrill) Winslow et al. of Malus and Pyrus progenies within the rootstock selection programme. Acta Horticulturae 411, 401–407. Fisher, E.G., Parker, K.G., Luepschen, N.S. and Kwong, S.S. (1959) The influence of phos- phorus, potassium, mulch, and soil drainage on fruit size, yield, and firmness of the ‘Bartlett’ pear and on development of the fire blight disease. Proceedings of the American Society for Horticultural Science 73, 78–90. Gardner, R.G., Cummins, J.N. and Aldwinckle, H.S. (1980a) Fire blight resistance in the Geneva apple rootstock breeding program. Journal of the American Society for Horticultural Science 105(6), 907–912. Gardner, R.G., Cummins, J.N. and Aldwinckle, H.S. (1980b) Inheritance of fire blight resistance in Malus in relation to rootstock breeding. Journal of the American Society for Horticultural Science 105(6), 912–916. Gwynne, D.C. (1984) Fire blight in perry pears and cider apples in the south west of England. Acta Horticulturae 151, 41–47. Hokanson, S.C., McFerson, J.R., Forsline, P.L., Lamboy, W.F., Luby, J.J., Djangaliev, A.D. and Aldwinckle, H.S. (1997) Collecting and managing wild Malus germplasm in its center of diversity. HortScience 32(2), 173–176. Breeding for Resistance to Fire Blight 271

Huet, J. and Michelesi, J.C. (1990) Sensibilité au feu bactérien des principaux porte-greffe du poirier et du pommier utilisés en Europe. In: Fire Blight of Pomoïdeae (Erwinia amylovora, Burrill, Winslow et al.). Applied Research in Europe (1978–1988), EUR 12601, ECSC-EEC-EAEC, Brussels-Luxembourg, ed., pp. 116–118. Hunter, D.M. (1993) Pear breeding for the 21st century – program and progress at Harrow. Acta Horticulturae 338, 377–383. Hunter, D.M., Pinsonneault, P., Kappel, F., Quamme, H.A., Bonn, W.G. and Layne, R.E.C. (1992) ‘Harrow Sweet’ pear. HortScience 27(12), 1331–1334. Keil, H.L. and van der Zwet, T. (1975) Fire blight susceptibility of dwarfing apple root- stocks. Fruit Varieties Journal 29(2), 30–33. Korban, S.S., Ries, S.M., Klopmeyer, M.J., Morrisey, J.F. and Hattermann, D.R. (1988) Genotypic responses of scab-resistant apple cultivars/selections to two strains of Erwinia amylovora and the inheritance of resistance to fire blight. Annals of Applied Biology, 113(1), 101–105. Layne, R.E.C. (1964) Pear breeding for resistance to fire blight. In: Research Report (1963–1964). Canadian Department of Agriculture, Harrow, Ontario, pp. 20, 23–24. Lecomte, P. (1993) Shoot and blossom susceptibility to fire blight (Erwinia amylovora) of Asian pear cultivars. Acta Horticulturae 338, 397–406. Lecomte, P. and Cadic, A. (1993) Further results on shoot susceptibility of Cotoneaster to fire blight. Acta Horticulturae 338, 407–412. Le Lezec, M., Thibault, B., Balavoine, P. and Paulin, J.P. (1985) Sensibilité variétale du pommier et du poirier au feu bactérien. Phytoma – Défense des Cultures 365, 37–44. Le Lezec, M., Bautrais, P. and Belouin, A. (1991) L’amélioration du poirier pour la résis- tance au feu bactérien. Arboriculture Fruitière 440, 29–37. Le Lezec, M., Lecomte, P., Laurens, F. and Michelesi, J.C. (1997) Sensibilité variétale au feu bactérien [3 parts]. Arboriculture Fruitière 503, 57–61; 504, 33–37; 505, 31–40. Lespinasse, Y. and Paulin, J.P. (1984) Apple breeding programme for fire blight resis- tance. Acta Horticulturae 151, 301–306. Lespinasse, Y. and Paulin, J.P. (1990) Apple breeding programme for fire blight resis- tance: strategy used and first results. Acta Horticulturae 273, 285–295. Lombard, P.B. and Westwood, M.N. (1987) Pear rootstocks. In: Rom, R.C. and Carlson, R.F. (eds) Rootstocks for Fruit Crops. Wiley, New York, pp. 145–183. Maas Geesteranus, H.P. and Heyting, J. (1978) Greenhouse tests on fire blight suscepti- bility of species and cultivars of ornamental Pomoideae. Acta Horticulturae 86, 41–44. Maroofi, A. and Mostafavi, M. (1996) Evaluation of the resistance of apple, pear and quince varieties to fire blight. Acta Horticulturae 411, 395–399. Norelli, J.L. (1986) Differential virulence in Erwinia amylovora. PhD thesis, Cornell University, USA. Norelli, J.L., Aldwinckle, H.S. and Beer, S.V. (1984) Differential host pathogen inter- actions among cultivars of apple and strains of Erwinia amylovora. Phytopathology 74(2), 136–139. Norelli, J.L., Aldwinckle, H.S. and Beer, S.V. (1986) Differential susceptibility of Malus spp. cultivars Robusta 5, Novole, and Ottawa 523 to Erwinia amylovora. Plant Disease 70(11), 1017–1019. Norelli, J.L., Aldwinckle, H.S., Beer, S.V. and Lamb, R.C. (1987) The effects of virulence of Erwinia amylovora on the evaluation of fire blight resistance in Malus. Phytopathology 77(11), 1551–1555. 272 Yves Lespinasse and Herb S. Aldwinckle

Oitto, W.A., van der Zwet, T. and Brooks, H.J. (1970) Rating of pear cultivars for resis- tance to fire blight. HortScience 5, 474–476. Paulin, J.P. and Cadic, A. (1997) Crataegus and fire blight: interest of some clones as orna- mentals. In: Proceedings, International Symposium on Urban Tree Health, Paris, September. 22–26. Paulin, J.P. and Lespinasse, Y. (1990) Pathogenicity of strains of Erwinia amylovora to some apple cultivars in the greenhouse. Acta Horticulturae 273, 319–326. Paulin, J.P., Chartier, R. and Boré, J.M. (1990) Sensibilité au feu bactérien de quelques variétés de pommiers à cidre. In: Fire Blight of Pomoïdae (Erwinia amylovora, Burrill, Winslow et al.). Applied Research in Europe (1978–1988), EUR 12601, ECSC-EEC-EAEC, Brussels-Luxembourg, ed., pp. 119–121. Paulin, J.P., Lachaud, G., Cadic, A. and Renoux, A. (1993a) Susceptibility of Crataegus species to fire blight. Acta Horticulturae 338, 421–425. Paulin, J.P., Chartier, R. and Boré, J.M. (1993b) Blossom susceptibility to fire blight of cider apple. Acta Horticulturae 338, 427–430. Persiel, F. and Zeller, W. (1981) Some progress in breeding Cotoneaster for resistance to fire blight, Erwinia amylovora (Burr.) Winslow et al. Acta Horticulturae 117, 83–87. Persiel, F. and Zeller, W. (1990) Breeding upright growing types of Cotoneaster for resis- tance to fire blight, Erwinia amylovora (Burr.) Winslow et al. Acta Horticulturae 273, 297–301. Pinet-Leblay, C., Turpin, F.X. and Chevreau, E. (1992) Effect of gamma and ultraviolet irradiation on adventitious regeneration from in vitro cultured pear leaves. Euphytica 62, 225–233. Pinet-Leblay, C., Brisset, M.N., Chevreau, E. and Paulin, J.P. (1996) In vitro screening for resistance to fire blight using a pathogenicity mutant of Erwinia amylovora. Acta Horticulturae 411, 415. Quamme, H.A. (1977) Resistance to naturally and artificially induced fire blight in the Harrow pear collection. Canadian Plant Disease Survey 57, 9–12. Quamme, H.A. and Bonn, W.G. (1981) Virulence of Erwinia amylovora and its influence on the determination of fire blight resistance of pear cultivars and seedlings. Canadian Journal of Plant Pathology 3(4), 187–190. Quamme, H.A. and Spearman, G.A. (1983) ‘Harvest Queen’ and ‘Harrow Delight’ pear. HortScience 18(5), 770–772. Quamme, H.A., van der Zwet, T. and Dirks, V. (1976) Relationship of fire blight resistance of young pear seedlings inoculated in the greenhouse to mature seedling trees nat- urally infected in the field. Plant Disease Reporter 60, 660–664. Quamme, H.A., Kappel, F. and Hall, J.W. (1990) Efficacy of early selection for fire blight resistance and the analysis of combining ability for fire blight resistance in several pear progenies. Canadian Journal of Plant Science 70(3), 905–913. Quarta, R. (1990) The Italian pear breeding programme for fire blight resistance. In: Fire Blight of Pomoïdae (Erwinia amylovora, Burrill, Winslow et al.). Applied Research in Europe (1978–1988), EUR 12601, ECSC-EEC-EAEC, Brussels-Luxembourg, ed., pp. 136–143. Robinson, T.L., Cummins, J.N., Hoying, S.A. and Smith, W. (1997) Performance of the new Cornell-Geneva apple rootstocks. Compact Fruit Tree 30, 1–5. Shaw, L. (1934) Studies on resistance of apple and other rosaceous plants to fire blight. Journal of Agricultural Research 49, 283–313. Thibault, B. (1981) Pear breeding for fire blight resistance: program and first studies in France. Acta Horticulturae 117, 63–69. Breeding for Resistance to Fire Blight 273

Thibault, B. (1990) Le programme français de création de poirier résistant au feu bac- térien. In: Fire Blight of Pomoïdae (Erwinia amylovora, Burrill, Winslow et al.). Applied Research in Europe (1978–1988), EUR 12601, ECSC-EEC-EAEC, Brussels- Luxembourg, ed., pp. 123–129. Thibault, B. and Le Lezec, M. (1990) Sensibilité au feu bactérien des principales variétés de pommier et de poirier utilisées en Europe. In: Fire Blight of Pomoïdeae (Erwinia amylovora, Burrill, Winslow et al.). Applied Research in Europe (1978–1988), EUR 12601, ECSC-EEC-EAEC, Brussels-Luxembourg, ed., pp. 96–109. Thibault, B. and Maas Geesteranus, H.P. (1984) Amélioration du poirier: transmission de la résistance. Acta Horticulturae 151, 293–300. Thibault, B., Welcker, C. and Lespinasse, Y. (1983) Heritability of the character ‘sec- ondary blooming’ on pear (Pyrus communis). Acta Horticulturae 139, 181–193. Thibault, B., Lecomte, P., Hermann, L. and Belouin, A. (1987) Comparison between two methods of selection for resistance to Erwinia amylovora in young seedlings of pear. Acta Horticulturae 217, 265–272. Thompson, J.M. (1972) Fire blight rating, bloom dates, and precocity of apple varieties tested in the southeast. Fruit Varieties and Horticultural Digest 26, 84–97. Thompson, J.M., Zimmerman, R.H. and van der Zwet, T. (1975) Inheritance of fire blight in Pyrus. I. A dominant gene, Se, causing sensitivity. Journal of Heredity 66, 259–264. van der Zwet, T. (1975) Comparative sensitivity and response of various pyrus tissues to infection by Erwinia amylovora. In: Kiraly, Z. and Klement, Z. (eds) Current Topics in Plant Pathology. Hungarian Academy of Sciences, Budapest, pp. 263–276. van der Zwet, T. and Bell, R.L. (1990) Fire blight susceptibility in Pyrus germplasm from Eastern Europe. HortScience 25, 566–568. van der Zwet, T. and Bell, R.L. (1995) Response of central European Pyrus germplasm to natural fire blight infection and artificial inoculation. HortScience 30, 1287–1291. van der Zwet, T. and Keil, H.L. (1979) Fire Blight: a Bacterial Disease of Rosaceous Plants. Handbook 510, US Department of Agriculture, 200 pp. van der Zwet, T., Oitto, W.A. and Brooks, H.J. (1970) Scoring system for rating the sever- ity of fire blight in pear. Plant Disease Reporter 54, 835–839. van der Zwet, T., Bell, R.L. and Blake, R.C. (1981) Some factors affecting selection for fire blight resistance in pear. Acta Horticulturae 117, 55–61. van der Zwet, T., Bell, R.L. and Blake, R.C. (1984) Comparative evaluation of the degree of fire blight resistance in various pear cultivars and selections. Acta Horticulturae 151, 267–275. Viseur, J. (1990) Evaluation of fire blight resistance of somaclonal variants obtained from the pear cultivar ‘Durondeau’. Acta Horticulturae 273, 275–284. Zeller, W. (1978) Field trials on the resistance of pear and apple varieties to fire blight Erwinia amylovora (natural and artificial infection). Acta Horticulturae 86, 15–23. Zeller, W. (1979) Resistance and resistance breeding in ornamentals. EPPO Bulletin 9(1), 35–44. Zeller, W. (1990) Test of pome fruit susceptibility to fire blight in the federal republic of Germany. In: Fire Blight of Pomoïdae (Erwinia amylovora, Burrill, Winslow et al.). Applied Research in Europe (1978–1988), ECSC-EEC-EAEC, Brussels-Luxembourg, ed., pp. 110–115.