Resistance to Fire Blight Among Flowering Pears and Quince
PLANT PATHOLOGY
HORTSCIENCE 40(2):413–415. 2004. damaged or killed by fi re blight in both nurs- ery and landscape plantings. The economic importance and history of pears as a fruit crop Resistance to Fire Blight among has resulted in numerous attempts to document resistance among species and cultivars. van Flowering Pears and Quince der Zwet and Keil (1979) summarized relative 1 2 3 susceptibility of fi ve important species of pears Andrew C. Bell, Thomas G. Ranney, and Thomas A. Eaker to fi re blight. Furthermore, they provided fi re Department of Horticultural Science, Mountain Horticultural Crops Research and blight resistance ratings for >400 cultivars of Extension Center, North Carolina State University, Fletcher, NC 28732-9244 pear fruit trees based on an extensive survey of the literature. 2 Turner B. Sutton There have been few attempts to document Department of Plant Pathology, North Carolina State University, Raleigh, resistance to fi re blight among fl owering pears. NC 27695-7616 Fare et al. (1991) and McNeil et al. (1986) re- ported resistance ratings for selected cultivars Additional index words. Chaenomeles spp., Erwinia amylovora, Pyrus spp., disease of Pyrus calleryana. Both studies reported resistance, host plant resistance ‘Redspire’ and ‘Aristocrat’ as being more sus- ceptible than ‘Bradford’, ‘Capital’, ‘Fauriei’, Abstract. Fire blight, caused by the bacterium Erwinia amylovora (Burrill) Winslow et and ‘Whitehouse’. Because the incidence of al., is one of the most destructive diseases of plants in the Rosaceae subfamily Maloideae. fi re blight can be sporadic and vary from region Artifi cial inoculations, using E. amylovora strain E2002a, were conducted to determine to region and year to year, observations and levels of resistance to fi re blight among taxa of fl owering pears (Pyrus L. spp.) and quince results from fi eld surveys can be inconsistent (Chaenomeles Lindl. spp.). The level of resistance was measured as the length of the fi re and highly variable. Plants that initially ap- blight lesion as a percentage of overall shoot length. Considerable variation in resistance pear to be resistant may later be found to be was observed among both pears and quince. Pyrus ussuriensis Maxim. ‘Prairie Gem’ susceptible when conditions are favorable for was highly resistant with a lesion length of 1% of the total shoot length. Pyrus calleryana disease development. In the aforementioned Decne. ‘Bradford’ was intermediate with a 50% lesion length while P. calleryana ‘Chan- studies, artifi cial inoculations were not used ticleer’ was signifi cantly more resistant with a lesion length of 31%. Nine pear taxa were and neither study documented which strain(s) highly susceptible and did not differ signifi cantly from 100% disease severity (total shoot of E. amylovora were present. death). Chaenomeles speciosa (Sweet) Nak. ‘Contorta’ was highly resistant with a lesion There is limited information on resistance length of 15%. Six quince taxa, including C. x superba (Frahm) Rehd. ‘Cameo’, ‘Texas to fi re blight among taxa of fl owering quince. Scarlet’, and ‘Jet Trail’ were highly susceptible while nine other taxa showed intermedi- Based on their survey of the literature, van ate resistance. der Zwet and Keil (1979) listed only one spe- cies, Chaenomeles japonica (Thunb.) Spach., Flowering pears (Pyrus L. spp.) and fl ower- are frequently used as landscape plants. For and reported that it was very susceptible to ing quince (Chaenomeles Lindl. spp.) are two example, the callery pear (Pyrus calleryana blossom blight and moderately susceptible genera in the Rosaceae subfamily Maloideae Decne.) is one of the most common fl owering to twig blight. that include many species and hybrids with trees used in landscape horticulture. The most Despite commercial importance to land- signifi cant economic importance as nursery notable and widely planted cultivar of callery scape horticulture, extensive information on and landscape plants. Despite their popularity, pear is ‘Bradford’, which was introduced by fi re blight resistance among fl owering pears cultivation of these genera can be diffi cult due the U.S. Department of Agriculture in the early and quince is not readily available. Ornamental to their susceptibility to fi re blight. 1960s. ‘Bradford’ and other cultivars of callery plant breeding programs could benefi t greatly The genus Pyrus consists of ≈22 species pear offer many desirable landscape qualities from screening studies that use specifi c clones whose native ranges extend from Europe and including early spring fl owering, broad adapt- and artifi cial inoculations. Artifi cial inocula- North Africa to across the Asian continent ability, excellent resistance to arthropod pests, tions of actively growing shoots with virulent (Bell and Hough, 1986). In addition to being and outstanding fall color. The popularity and strains of E. amylovora provide an effective cultivated as a fruit crop, some species of pears use of fl owering pears as landscape plants has and consistent means for evaluating fi re blight increased signifi cantly in recent decades. resistance (Aldwinckle and Preczewski, 1979). Received for publication 5 Jan. 2004. Accepted The genus Chaenomeles consists of three Therefore, the objective of this study was to for publication 1 Apr. 2004. This research was species, two of which are native to China while determine levels of resistance to fi re blight funded, in part, by the North Carolina Agricultural the third is restricted to Japan. The fl owering among a variety of fl owering pear and quince Research Service (NCARS), Raleigh, U.S. Depart- quince is closely related to pears, apples (Malus taxa using artifi cial inoculations. ment of Agriculture, and the J. Frank Schmidt Mill. spp.), and the true quince (Cydonia Mill. Family Charitable Foundation. The use of trade spp.). Unlike its close relatives, fl owering Materials and Methods names in this publication does not imply endorse- quince is not commonly cultivated as a fruit ment by the NCARS of the products named nor crop. However, Weber (1964) reported that the Pears. Twenty-seven taxa (Table 1) of criticism of similar ones not mentioned. Technical assistance of Sharon Williamson and staff of the fl owering quince has been cultivated for >400 containerized pears, arranged in a randomized Mountain Horticultural Crops Research Station is years and that >500 cultivars have been named complete block design with 4 to 12 replica- greatly appreciated. The authors wish to thank Don and propagated. Today, numerous cultivars of tions, were screened for fi re blight resistance Shadow, Ken Tilt, the Arnold Arboretum, and the fl owering quince are readily available in the using artifi cial inoculations at the Mountain Landscape Plant Development Center for providing horticultural industry. The fl owering quince is Horticultural Crops Research Station, Fletcher, plant material and Jay Norelli and Herb Aldwinckle highly prized as an early fl owering shrub and N.C. This study included cultivars, clonal selec- who generously provided the isolate of Erwinia is commonly used in landscapes. tions, and hybrids of P. amygdaliformis Vill., amylovora used for inoculum. From a dissertation Fire blight is a highly destructive disease P. betulifolia Bunge., P. calleryana, P. elaea- submitted by Andrew Bell in partial fulfi llment of caused by the bacterium Erwinia amylovora grifolia Pall., P. fauriei Schneid., P. koehnei the requirements for the PhD degree. 1Corresponding author and former graduate teaching (Burrill) Winslow et al. This disease can be Schneid., P. nivalis Jacq., P. pyrifolia (Burm.) assistant. Currently: Associate director, North Caro- especially problematic in regions where envi- Nak., P. regelii Rehd., P. salicifolia Pall., and lina Botanical Garden, University of North Carolina, ronmental conditions for disease development P. ussuriensis Maxim. All taxa were 1-year Chapel Hill, N.C.; e-mail [email protected]. are favorable, specifi cally where springtime budded plants on P. calleryana rootstocks 2Professor. weather is warm and wet (van der Zwet and grown in 14.2-L containers. Container substrate 3Research specialist. Keil, 1979). Susceptible plants can be severely was 5 pine bark : 1 peat (by volume) amended
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AAprilHSBook.indbprilHSBook.indb 441313 22/9/05/9/05 44:04:30:04:30 PPMM with 3 kg·m–3 micronutrients (Esmigran, The Table 1. Fire blight ratings for taxa of fl owering pears (Pyrus spp.) based on artifi cial inoculation. Scotts Co., Marysville, Ohio) and 5.3 kg·m–3 Pyrus Disease severity dolomitic limestone. Plants were placed in a taxa (% lesion length)z lath house under 50% shade in April 2000 and P. salicifolia ‘Pendula’ 100 a fertilized with 50 g controlled-release fertilizer 93-8-5y (P. fauriei x P. salicifolia ‘Pendula’) 98 a (Osmocote Plus, 15–9–12, The Scotts Co., P. nivalis 97 ab Marysville, Ohio). Irrigation was applied as P. koehnei 97 ab P. pyrifolia 95 abc necessary. y The isolate of E. amylovora used in this 93-32-4 (P. salicifolia ‘Pendula’ x P. ussuriensis) 94 abc 911010y (P. ussuriensis x P. nivalis) 92 abc study was strain (E2002a). This strain has P. elaeagrifolia ‘Turkish Mist’ 91 abc previously been categorized and used to screen P. fauriei ‘Korean Sun’ 89 abc advanced selections of pears for resistance to 93-2-2y ((P. calleryana x P. fauriei) x P. nivalis) 87 abc fi re blight (R.L. Bell, 1999, unpublished data 93-17-3y (P. elaeagrifolia x P. amygdaliformis) 81 cd and personal communication). The inoculum P. calleryana ‘Red Spire’ 69 de was prepared from 24 h cultures grown on P. calleryana ‘Aristocrat’ 65 ef nutrient agar plates at 26 °C. The cultures 91-53-1y (P. calleryana ‘Chantcleer’ x P. betulifolia) 63 ef were rinsed with distilled water and bacte- P. calleryana ‘Whitehouse’ 62 efg rial suspension collected. The inoculum was P. calleryana ‘Bradford’ 50 fgh 7 P. calleryana ‘Fauriei’ 46 ghi adjusted to a concentration of ≈1 × 10 colony 93-15-1y (P. elaeagrifolia x P. ussuriensis) 44 hij forming units (cfu)/mL. One to two actively 911014y (P. usseriensis x P. regelii) 42 hij growing shoots (subsamples) per tree were 91-42-1y (P. amydaliformis x P. regelii) 38 hij inoculated in May 2000. The two youngest 93-61-1y (P. amygdaliformis x P. calleryana ‘Chanticleer’) 32 ijk leaves were bisected with a pair of scissors P. calleryana ‘Chanticleer’ (syn. ‘Cleveland Select’) 31 jkl that had been dipped into the inoculum prior P. regelii 22 kl to each cut. The disease lesion length and 93-70-2y (P. calleryana ‘Chanticleer’ x P. elaeagrifolia) 22 kl total length of the current season’s growth of P. betulifolia ‘Dancer’ 16 lm the inoculated shoot were measured 40 d fol- P. ussuriensis ‘Prairie Gem’ 3 m 950104y (P. calleryana x P. betulifolia) 1 m lowing inoculation. The severity of infection y was expressed as the length of the fi re blight Interspecifi c hybrid taxa. zPercentage lesion length = [length of fi re blight lesion (cm)] / [total shoot length (cm)]. Percent lesion lesion as a percentage of overall shoot length. lengths followed by same letter did not differ signifi cantly (P < 0.05) according to Waller-Duncan’s test All data were subjected to analysis of variance on arc-sin transformed data. procedures after arc-sin transformation. Means for percent lesion length were separated by the the 12 taxa were highly susceptible with percent Table 2. Fire blight ratings for taxa of fl owering Waller-Duncan test. lesion length >85%. Five taxa were interme- quince (Chaenomeles spp.) based on artifi cial Quince. Twelve taxa (Table 2) of fl owering diate in susceptibility; C. speciosa ‘Nivalis’ inoculation. quince were screened for fi re blight resistance was the least susceptible of this group while Chaenomeles Disease severity in May 2001. This study included taxa of C. C. speciosa ‘Mandarin’ the most with lesion taxa (% lesion length)z cathayensis (Hemsl.) Schneid., C. japonica, length of 37% and 68% respectively. C. x superba ‘Jet Trail’ 98 a C. speciosa (Sweet) Nak., and C. x superba C. speciosa ‘Toyo-Nishiki’ 96 a (Frahm) Rehd. Plant material consisted of Discussion C. x superba ‘Texas Scarlet’ 95 a 1-year-old container-grown rooted cuttings. C. x superba ‘Cameo’ 91 a Growing conditions, plant care, experimen- C. aponica ‘Sargentii’ 89 a Pears. Cultivars of P. calleryana differed C. speciosa ‘Spitfi re’ 88 a tal design, inoculation procedure, and data signifi cantly in their susceptibility to fi re C. speciosa ‘Mandarin’ 68 b analysis were conducted as described above. blight with P. calleryana ‘Chanticleer’ (syn. C. cathayensis 62 bc Inoculum for this study was E. amylovora ‘Cleveland Select’) being more resistant to C. speciosa ‘Dragon’s Blood’ 56 bcd strain E2002a (≈2 × 108 (cfu)/mL). fi re blight than P. calleryana ‘Bradford’, C. x superba ‘Crimson and Gold’ 40 cd ‘Whitehouse’, ‘Aristocrat’, and ‘Red Spire’. C. speciosa ‘Nivalis’ 37 de Results Our results were in general agreement with C. speciosa ‘Contorta’ 15 e studies by Fare et al. (1991) and McNeil et zPercentage lesion length = [length of fi re blight Pears. The taxa included in this study al. (1986) in which ‘Bradford’, ‘Fauriei’, and lesion (cm)]/[total shoot length (cm)]. Percent le- showed considerable variation in resistance to ‘Whitehouse’ showed greater resistance than sion lengths followed by same letter did not differ fi re blight with the severity of infection rang- ‘Aristocrat’ and ‘Redspire’ under conditions signifi cantly (P < 0.05) according to Waller-Duncan’s test on arc-sin transformed data. ing from 1% to 100% of the current season’s of natural infection. However, in this study, shoot growth (Table 1). Ten taxa were highly ‘Whitehouse’ was not signifi cantly more re- fi re blight resistance (van der Zwet and Keil, susceptible with a percent length >85%. The sistant than either ‘Aristocrat’ or ‘Red Spire’. 1979). The study reported herein included susceptible taxa included specifi c clones of P. Overall, ‘Chanticleer’ was the most resistant clonal selections of these species. One clone, elaeagrifolia, P. fauriei, P. koehnei, P. nivalis, P. commercially available cultivar of callery P. ussuriensis ‘Prairie Gem’, was found to be pyrifolia, and P. salicifolia as well as a number pear included in this study. Pyrus betulifolia very resistant. of advanced hybrid selections. At the other ‘Dancer’ and a clone of P. regelii exhibited The hybrid taxa included in this study were extreme, two taxa, P. ussuriensis ‘Prairie Gem’ a level of resistance similar to that of P. cal- provided by the Landscape Plant Develop- and Pyrus 950104, a clone derived from open leryana ‘Chanticleer’. ment Center, Chanhassen, Minn. The Center’s pollination of a P. calleryana × P. betulifolia The genetic diversity that exists within the ornamental pear breeding program has been hybrid, were highly resistant with minimal genus Pyrus has been exploited for fruit tree successful in obtaining many new hybrid plants disease development. Fifteen other taxa were breeding; however, many of these taxa also by using a very diverse group of species as intermediate with lesion length ranging from possess desirable ornamental characteristics. breeding parents (Hummel, 2000). Promising 16% to 81% of the annual shoot growth. Some species vary in fl owering time, heat selections have been identifi ed based on desir- Quince. The taxa included varied sig- and cold tolerance, leaf shape, tree form, able ornamental characteristics, but no data nifi cantly in resistance to E. amylovora strain and disease resistance. While considerable regarding their resistance to fi re blight have E2002a (Table 2). Among the fl owering quince variation in resistance to fi re blight exists in been recorded. One hybrid, Pyrus 950104, from taxa, C. speciosa ‘Contorta’ was highly resis- all species of pears, P. ussuriensis and P. cal- this program was found to be very resistant tant with a mean lesion length of 15%. Six of leryana have been reported to exhibit the most to this virulent strain of E. amylovora. Pyrus
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AAprilHSBook.indbprilHSBook.indb 441414 22/9/05/9/05 44:04:33:04:33 PPMM 93-70-2 and 93-61-1 were signifi cantly more (blossom blight) due to their fl owering habit. Literature Cited resistant than ‘Bradford’, the industry standard Secondary infection (twig and leaf blight) Aldwinckle, H.S. and J.L. Preczewski. 1979. Re- for resistance. and blossom infection on sporadic blooms action of terminal shoots of apple cultivars to Cultivars of P. calleryana are among the typically occurs later in the growing season invasion by Erwinia amylovora. Phytopathology most widely planted fl owering trees in much when environmental conditions are favorable 66:1439–1444. of the United States. Despite their popular- for disease development. Bell, R.L. and L.F. Hough. 1986. Interspecifi c and ity, these plants are often short lived in the This study included a number of cultivars intergeneric hybridization of Pyrus. HortScience landscape. Some cultivars are very weak of fl owering quince that are common and 21:62–64. wooded, have poor branch arrangement, and important to the horticultural industry. Only Fare, D.C., C.H. Gilliam, and H.G. Ponder. 1991. are susceptible to fi re blight. Genetic diversity one cultivar, C. speciosa ‘Contorta’ was highly Fire blight susceptibility, growth, and other characteristics in ornamental pears in Alabama. and interspecifi c crossability in the genus Py- resistant. Chaenomeles speciosa ‘Nivalis’ J. Arboricult. 17:257–260. rus has lead to the development of many new and C. x superba ‘Crimson and Gold’ were Hummel, R. 2000. Update on the landscape plant hybrid plants. The success of new hybrids is, moderately resistant (<50% lesion length). development center Pyrus breeding project: in part, dependent on their resistance to fi re Although a number of taxa in this study are Promising selections. Proc. 11th METRIA blight. Results from this study document the very susceptible, they may escape infection Conf. 19 Feb. 2003.
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