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 ϫ 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-