Abstract
The genus Cotoneaster is composed of ≈400 species with a wide variety of growth habits and forms. These hardy landscape shrubs used to be commonplace because of their low maintenance and landscape functionality. However, the interest in and sales of cotoneaster have decreased for a variety of reasons, with the greatest being its susceptibility to a bacterial disease fire blight caused by Erwinia amylovora. The resistances of 15 different genotypes of Cotoneaster to a wild-type strain of Erwinia amylovora (Ea153) and a strain LA635 that has a natural mutation in avrRpt2 that encodes for a type III secretion effector were tested separately by inoculating leaves. Fire blight resistance was assessed by calculating the percent shoot necrosis (PSN) [PSN = 100 × (lesion length ÷ total branch length)] at 6 to 8 weeks after inoculation. Across all experiments, Cotoneaster genotypes H2011-01-002 and C. ×suecicus ‘Emerald Sprite’ consistently had the lowest PSN values when inoculated with either strain. Cotoneaster ×suecicus ‘Emerald Beauty’ was significantly more resistant to Ea153 than to LA635, whereas C. splendens was significantly more susceptible to Ea153 than to LA635.
Cotoneaster is a genus of hardy, ornamental shrubs in the Maloideae of Rosaceae that is composed of ≈400 species that are widely distributed throughout the northern hemisphere (Dickoré and Kasperk, 2010; Fryer and Hylmö, 2009). The genus is divided into two subgenera: Chaenopetalum, which blooms all at once with white spreading petals, and subgenus Cotoneaster, which blooms successively over a long period of time with cup-like pink flowers (Fryer and Hylmö, 2009). The subgenera are divided further into series based on botanical characteristics and geographic origins (Fryer and Hylmö, 2009).
Cotoneasters are ideal urban plants because of their ability to tolerate pollution, grow in poor soils, and withstand harsh pruning. Many species have multi-season interest in the form of autumn color or a dense, consistent green foliage and fruits that persist through the winter. The size, color, and texture of leaves vary widely among species and cultivars. The fruit are mostly bright orange–red, but yellow and black-fruited species exist. The fruits attract wildlife, including birds such as robins and waxwings, which have spread cotoneasters from cultivation into the wild. Cotoneasters are easy to grow and are commonly used as foundation plants, as hedges, in parking lots, or in a mass planting along roadsides. Although cotoneasters often are attractive and utilitarian for landscaping, most available cultivars are susceptible to the bacterial plant disease fire blight caused by Erwinia amylovora.
Bacterial cells that overwinter in fire blight cankers likely serve as primary inoculum in the spring (Khan et al., 2012). Stem cankers produce bacterial ooze that is then disseminated by insects, wind, and rain to flowers or wounds (Vogt et al., 2013; Wöhner et al., 2018). After tissue invasion, the pathogen produces effector proteins that cause necrosis and can eventually kill the plant (Oh and Beer, 2005).
Fire blight periodically causes heavy losses for apple (Malus ×domestica) and pear (Pyrus communis) production. The estimated annual expenses for fire blight management and tree losses in the United States are approximately US$100 million and upward of US$9 million in Switzerland, thus illustrating why fire blight is one of the most economically devastating diseases in pome fruit production worldwide (Norelli et al., 2003; Vogt et al., 2013). Fire blight also affects landscape plants such as Cotoneaster, which has led to substantial decrease in sales. Copper and streptomycin are the only registered chemicals for fire blight control on Cotoneaster (Vanneste, 2000). In some regions, streptomycin-resistant strains of the pathogen are present; furthermore, in many countries, streptomycin is not permitted for fire blight control on Cotoneaster (McGhee et al., 2011; Peil et al., 2019; Vanneste, 2000; Vogt et al., 2013). With few materials for disease control, host plant resistance would be the most effective way to control the disease (Norelli et al., 2003). Crossing sensitive plant species with those that show resistance can be an effective method of introducing resistance genes and potentially creating new cultivars. Without new, disease-resistant cultivars, fire blight management on Cotoneaster will remain challenging.
The cotoneasters currently being planted as ornamentals are not representative of the whole genus, as illustrated by Dirr (2009), who listed only 14 species, thus omitting the major diversity of the genus. Neither subgenus is underrepresented among cultivated material, and there are relatively popular plants from both, including C. apiculatus and C. horizontalis from subgenus Cotoneaster and C. dammeri and C. ×suecicus ‘Coral Beauty’ from subgenus Chaenopetalum. Ongoing research by our group has assessed resistance among species to fire blight in inoculated greenhouse trials (Rothleutner et al., 2014). Resistant taxa were crossed with species of horticultural interest to develop new cultivars with increased resistance to the bacterial disease. The Oregon State University Ornamental Plant Breeding Laboratory has developed a number of experimental Cotoneaster, including genotypes H2011-01-002, H2011-02-001, H2011-02-005, and H2017-005-01, that have a wide variety of foliar and floral characteristics and growth habits. Two of these genotypes, H2011-02-001 and H2011-02-005, exhibited tolerance to fire blight in pathogen-inoculated greenhouse experiments and were released as the cultivars Emerald Sprite PP31,719 and Emerald Beauty PP32,308, respectively, by the Oregon Agricultural Experiment Station.
Breeding plants for disease resistance often becomes a multistep process as more germplasm sources carrying desirable traits are identified and/or if pathogens evolve and adapt to overcome resistance in plants. This latter case has been observed in an apple breeding program focused on fire blight resistance. Wild apple species have been explored as sources of resistance to fire blight (Aldwinkle and Preczewski, 1979; Peil et al., 2007, 2009; van der Zwet and Keil, 1979; Vogt et al., 2013), with one of importance being Malus ×robusta 5 (Mr5) (Peil et al., 2007). Mr5 has a major quantitative trait locus (QTL) for resistance to fire blight on linkage group 3 (Peil et al., 2007, 2009, 2011). Wöhner et al. (2014) reported that there were major resistance genes on linkage group 3 of Mr5 and suggested that there is likely a gene-for-gene relationship in the Mr5–E. amylovora pathosystem (Emeriewen et al., 2019; Fahrentrapp et al., 2013; Wöhner et al., 2014, 2018). This resistance has been of particular interest to breeders and has been used to introduce fire blight resistance to many newly released apple cultivars (Durel et al., 2009). Unfortunately, Vogt et al. (2013) found that strains of E. amylovora lacking the type III secretion effector AvrRpt2 or harboring avrRpt2 with a mutation were able to overcome the resistance of Mr5. The mutation in avrRpt2, among strains able to cause disease on Mr5, was a naturally occurring single-nucleotide polymorphism (SNP) that resulted in the substitution of cysteine with serine at codon 156 (Emeriewen et al., 2019; Smits et al., 2014; Vogt et al., 2013; Wöhner et al., 2014).
The objectives of this study were to 1) evaluate fire blight resistance of Cotoneaster genotypes in repeated experiments under controlled conditions of growth chambers and 2) evaluate the severity of fire blight on Cotoneaster genotypes inoculated with a wild-type strain of the pathogen compared with a pathogen with an avrRpt2C156S mutation.
Materials and Methods
Plant material.
Cotoneaster species, cultivars, and hybrids were screened for resistance to fire blight in five experiments (Table 1). Plants for resistance testing were clonally propagated from stem cuttings. For growth chamber experiments, plants were grown in 10-cm square pots in a potting mix with 2 perlite : 1 peat (by volume). For the greenhouse experiment, plants were potted in unaged douglas fir bark (Pseudotsuga menziesii; Lane Forest Products, Eugene, OR) in 2.5-L containers. Plants were hand-watered as needed and fertilized weekly using liquid soluble fertilizer 20N–8.74P–16.6K (Jack’s Professional General Purpose; J.R. Peters Laboratory, Allentown, PA). All plants were actively growing at the time of all experiments.
Experiments screening Cotoneaster taxa for fire blight resistance.
Bacterial strains.
Two strains of Erwinia amylovora were used to assess fire blight resistance. A wild-type strain of the pathogen Ea153 was isolated from fire blight infection on ‘Gala’ apple in Oregon. Ea153 has been used in field trials and is sensitive to streptomycin (Johnson et al., 1993; Stockwell et al., 1998). Pathogen strain LA635 has the avrRpt2C156S mutation and causes fire blight disease on Malus ×robusta 5 (Wöhner et al., 2018). LA635 is resistant to streptomycin because of the commonly detected rpsLK43R mutation; the mutations in rpsL and avrRpt2 were confirmed by a genome sequence analysis (Smits et al., 2014). The strains were stored at −80 °C in nutrient broth (Difco Laboratories, Franklin Lakes, NJ) amended with 15% glycerol until use.
Preparation of freeze-dried inoculum.
Strain Ea153 was grown on Pseudomonas agar F (Difco Laboratories) amended with cycloheximide (50 µg/mL) (hereafter referred to as PAF). Strain LA635 was grown on Pseudomonas agar F amended with cycloheximide (50 µg/mL) and streptomycin (100 µg/mL) (hereafter referred to as PAF-Sm). After 5 d, bacterial lawns were harvested, mixed with a skim milk cryoprotectant (Stockwell et al., 1998), frozen at −80 °C, and lyophilized with a freeze dryer (FreeZone 6 L; Labconco Co., Kansas City, MO). Freeze-dried bacterial preparations were stored in sealed tubes at −80 °C. The titer of the stored freeze-dried cell preparations of each strain was checked before each experiment by rehydrating weighed samples of the freeze-dried bacteria and dilution plating on PAF and PAF-Sm. Colonies were counted after 3 d and converted to colony-forming units (cfu) per milligram.
Inoculation using leaf bisection and data collection.
Experimental design.
Growth chamber experiments were conducted using Percival model LED-30HL1 (Percival Scientific Inc., Perry, IA) growth chambers with an IntellusUltra real-time controller. The growth chambers had a woodless design, programmable temperature, light-emitting diode (LED) lighting, and relative humidity. Growth chambers were programmed for all four experiments with a ramping cool white LED light for a 16-h photoperiod (8 h of 50% light and 8 h of 100% light), with daytime temperatures of 25 °C (±0.5 °C), nighttime temperatures of 20 °C (±0.5 °C), and constant relative humidity of 70% to 80% (±10%). The glasshouse conditions were set at temperatures of 24 °C day/17 °C night with a 14-h photoperiod for experiments supplied by 400-W high-pressure sodium lamps.
All experiments were arranged in a randomized complete block design. In growth chamber experiments, each chamber represented a block. In Expts. 1, 3, and 4, there were four blocks and in Expt. 2, there were three blocks. In all five experiments, each block had three subsamples (plants) per genotype and a water-inoculated control. In each growth chamber experiment, one shoot was inoculated. In the glasshouse experiment, the three blocks were spatially separated and located distal to the cooling fans. Three shoots were inoculated on each plant and used to calculate the mean percent shoot necrosis for each genotype. In all experiments, the total shoot length and lesion length on inoculated shoots were measured in millimeters weekly for 2 months.
Statistical analysis.
Data were analyzed using an analysis of variance (ANOVA) and PROC GLM (SAS version 9.4; SAS Institute Inc., Cary, NC) to generate a general linear model ANOVA. Initially, the full model that included treatment (control that was inoculated with water) was analyzed. The treatment was then removed, and the model was analyzed using block and taxa as explanatory variables. When appropriate, means were separated using Tukey’s honestly significant difference (α < 0.05). Because of the prevalence of 0% PSN datapoints, the data violated the assumption of normality as assessed using PROC UNIVARIATE (SAS version 9.4; SAS Institute Inc.). Specifically, tests for normality yielded the following results: Expt. 1, W = 0.78, P < 0.0001; Expt. 2, W = 0.62, P < 0.0001; Expt. 3, W = 0.91, P = 0.01; Expt. 4, W = 0.9, P = 0.01; and Expt. 5, W = 0.90, P = 0.001. Transformation did not improve normality; therefore, we used a mixed model analysis. Additionally, the normality plots deviated from normality solely because of the presence of numerous data points of 0; otherwise, they fit a normal distribution.
Results
Wild-type and avrRpt2 mutant of E. amylovora caused disease on Cotoneaster.
Symptoms of fire blight were observed on susceptible Cotoneaster genotypes in each experiment. The wild-type strain of E. amylovora, Ea153, was reisolated consistently from lesion borders on plants inoculated with that isolate (Fig. 1). The morphology of colonies recovered from Ea153-inoculated plants was typical of E. amylovora on CCT and PAF, and no colonies were observed on PAF-Sm. Similarly, LA635 was reisolated from plants inoculated with the avrRpt2 mutant. Colony morphology of bacteria recovered from LA635-inoculated plants were typical of E. amylovora on CCT, PAF, and PAF-Sm.
Fire blight symptoms in Cotoneaster. (A) Fire blight causing necrosis of stems below the inoculation site (marked by red tape) showing the common shepherd’s crook symptom. (B) Necrosis of vascular tissue in the lesion border of Cotoneaster dammeri.
Citation: HortScience horts 56, 7; 10.21273/HORTSCI15872-21
Variable susceptibility among genotypes observed for both the wild-type and avrRpt2 mutant of E. amylovora during 2018 growth chamber experiments.
In Expt. 1, the PSN of plants inoculated with LA635 ranged from 0% to 21% (Table 2). Using the full model, genotype (P < 0.0001) and treatment (P = 0.0003) were significant, but block was not (P = 0.2373). When the treatment was removed, genotype (P < 0.0001) remained significant but block was not (P = 0.2084). Three genotypes, C. splendens (09-0024), H2011-01-002, and ‘Emerald Sprite’, had less than 5% shoot necrosis, whereas Cotoneaster ×suecicus ‘Coral Beauty’ exhibited the highest PSN (21%). Some plants appeared to exhibit quantitative resistance between these extremes; small lesions were observed but there was limited spread.
Percent shoot necrosis of Cotoneaster genotypes inoculated in growth chambers with an avrRpt2 mutant of Erwinia amylovora strain LA635 (Expt. 1) or a wild-type strain Ea153 (Expt. 2), both at a concentration of 108 cfu/mL, in 2018 with a foliar bisection assay.
In Expt. 2, the PSN of plants inoculated with wild-type Ea153 ranged from 0 to 26%. Genotype (P = 0.05) and treatment (P = 0.0136) were significant, but block was not (P = 0.8825). When the treatment was removed, the model was not significant at α = 0.05 (0.0579); therefore, mean separation was not possible, and the results are presented and discussed in relative terms such as means ± se but not statistical differences. There were large variations among plants within blocks, including 0% and 66% PSN within block 1 for C. acutifolius and 0% and 100% PSN within block 2 of C. frigidus (data not shown). Cotoneaster acutifolius (10-0126) had the highest PSN (26%). Seven different genotypes had less than 5% shoot necrosis: C. daliensis (10-0129); C. dielsianus (09-0013); C. sikangensis (11-0057); C. ×suecicus ‘Coral Beauty’; H2011-01-002; ‘Emerald Sprite’; and ‘Emerald Beauty’ (Table 2). In both experiments, plants inoculated with two different strains of E. amylovora, H2011-01-002 showed complete resistance, with a PSN of 0%. Control plants showed no symptoms of stem necrosis after leaf bisection with scissors dipped in sterile deionized water.
Most genotypes exhibited similar responses to the wild-type and avrRpt2 mutant of E. amylovora during 2019 growth chamber experiments.
In Expts. 3 and 4, the severity of symptoms ranged from 3% to 59% (Table 3). Genotype and treatment were both significant (P < 0.0001), but block was not (P > 0.1). Taxa remained highly significant in both experiments with the treatment removed (P < 0.0001), but block was still not significant in Expt. 3 (P = 0.9196) and Expt. 4 (P = 0.8818). Only two genotypes exhibited different responses to the two different bacterial strains: C. splendens (09-0024) and ‘Emerald Beauty’. Cotoneaster splendens (09-0024) was resistant to LA635, with only 4% shoot necrosis; this species was sensitive to the wild-type strain Ea153, with 33% shoot necrosis. ‘Emerald Beauty’ exhibited greater resistance to the wild-type strain Ea153, with 5% shoot necrosis, and exhibited sensitivity to LA635, with 36% shoot necrosis. H2011-01-002 had the lowest PSN in Expts. 3 and 4, with 3% shoot necrosis when inoculated with LA635 and 3% shoot necrosis when inoculated with the wild-type strain Ea153. ‘Emerald Sprite’ showed relatively high rates of resistance to both strains, with 8% shoot necrosis when inoculated with LA635 and 3% shoot necrosis when inoculated with the wild-type strain Ea153. Cotoneaster dammeri (19-0036) in Expts. 3 and 4 was significantly less resistant than all other genotypes tested, with the highest PSN in both experiments. The H2017-005-01 hybrid, a C. dammeri (19-0036) progeny, had a significantly lower PSN than the C. dammeri (19-0036) parent, but the PSN was significantly greater than those of five genotypes inoculated with LA635 and four other genotypes inoculated with Ea153 (Table 3). Control plants showed no symptoms of stem necrosis after leaf bisection with scissors dipped in sterile deionized water. There appeared to be a difference in the progression of disease symptoms of the two strains of the pathogen during the experiment for ‘Emerald Beauty’ and C. splendens (09-0024) but not for genotype H2017-005-01, which had a similar AUDPC in both experiments (Table 4; Fig. 2).
Percent shoot necrosis of Cotoneaster genotypes inoculated in growth chambers with an avrRpt2 mutant of Erwinia amylovora (strain LA635) (Expt. 3) or wild-type pathogen Ea153 (Expt. 4), with both strains at a concentration of 109 cfu/mL, with a foliar bisection assay in 2019.
Area under the disease progress curve (AUDPC) for Cotoneaster genotypes evaluated for fire blight resistance to a wild-type strain (Ea153) or an avrRpt2 mutant (LA635) of Erwinia amylovora.
Area under the disease progress curve (AUDPC) of three different genotypes from the experiments performed in growth chambers (Expts. 3 and 4). ‘Emerald Beauty’ PP32,308 (H2011-02-005) and C. splendens displayed different percent shoot necrosis when inoculated with the wild-type strain Ea153 or the avrRpt2 mutant LA635. Genotype H2017-005-01 was susceptible to both strains of the pathogen.
Citation: HortScience horts 56, 7; 10.21273/HORTSCI15872-21
Genotypes ranged from asymptomatic to nearly complete mortality when inoculated with wild-type E. amylovora in a glasshouse during 2019.
In Expt. 5, there was a range of sensitivity to fire blight among the genotypes ranging from very high levels of resistance (0% PSN) to almost complete mortality (97% PSN; Table 5). Genotype (P < 0.0001), treatment (P < 0.0001), and block were significant (P = 0.043). With the treatment removed from the model, taxa (P < 0.0001) and block (0.0001) remained significant. Cotoneaster sikangensis (11-0057) and ‘Emerald Beauty’ were asymptomatic after inoculation (Table 5; Figs. 3 and 4). Genotypes H2011-01-002 and ‘Emerald Sprite’ exhibited relatively high resistance, with 4% and 3% shoot necrosis, respectively (Table 5; Figs. 3 and 4). Several taxa showed high susceptibility, with more than 50% shoot necrosis, including C. acutifolius (10-0126), C. daliensis (10-0129), C. dammeri (19-0036), C. dielsianus (09-0013), C. salicifolius var. floccosus (09-0022), and C. ×suecicus ‘Coral Beauty’. Genotype H2017-05-001 showed a PSN of 49%, which was intermediate of its two parents, C. dammeri (19-0036; 73%) and C. apiculatus (16-0027; 43%). Cotoneaster splendens (09-0024) exhibited 11% shoot necrosis, which was greater than that of its progeny H2011-01-002 (4%). Control plants showed no symptoms of stem necrosis after leaf bisection with scissors dipped in sterile deionized water.
Cotoneaster genotypes H2011-01-002, ‘Emerald Sprite’ PP31,719, and ‘Emerald Beauty’ PP32,308 at 8 weeks after inoculation with wild-type strain Ea153 in the glasshouse. The inoculation site is marked with red tape (Expt. 5).
Citation: HortScience horts 56, 7; 10.21273/HORTSCI15872-21
‘Emerald Beauty’ PP32,308 and Cotoneaster ×suecicus ‘Coral Beauty’ 8 weeks after inoculation with wild-type strain Ea153 in the glasshouse. The inoculation site is marked with red tape (Expt. 5).
Citation: HortScience horts 56, 7; 10.21273/HORTSCI15872-21
Percent shoot necrosis of Cotoneaster genotypes inoculated in greenhouse with Ea153, a wild-type strain of Erwinia amylovora, at 109 cfu/mL in 2019 with a foliar bisection assay (Expt. 5).
Discussion
In this series of experiments, we characterized the resistance of 15 genotypes of Cotoneaster to Erwinia amylovora. This is the third study performed at Oregon State University to rate disease resistance among Cotoneaster genotypes in the program; however, it is the first to examine sensitivity to an avrRpt2 mutant strain of this bacterial pathogen. This study was a continuation to determine the level of resistance of these genotypes to the same strain tested before (Rothleutner et al., 2014) and an avrRpt2 mutant strain that overcame resistance in the prominent wild apple rootstock Mr5.
Trees and shrubs under “natural” field conditions are typically infected by E. amylovora through flowers. Even though artificial shoot inoculation through leaf bisection represents a different path of infection, it is a proven method of studying host plant resistance to fire blight (Harshman et al., 2017; Peil et al., 2019; Persiel and Zeller, 1981). As reported by Rothleutner et al. (2014), a threshold tolerance of 5% can be used as a selection tool for breeding purposes because of the relatively low threshold for damage in ornamental plants. However, even 5% necrosis may be unacceptable, depending on specific situations. The titer of our inoculum used in this study varied from 108 cfu/mL in 2018 to 109 cfu/mL in 2019, but disease was observed during both years. These inocula concentrations are within the range of concentrations used by other studies (106–109 cfu/mL) (Bellenot-Kapusta et al., 2002; Persiel and Zeller, 1978). The relatively small size of Cotoneaster makes it a good plant to use in growth chambers, where we can have consistent environmental conditions, which is an improvement over some previous studies involving environmental conditions that likely impacted results (Rothleutner et al., 2014).
According to our results collected in the greenhouse experiment, the pattern of fire blight sensitivity was similar to that reported by Lecomte and Cadic (1993) for C. apiculatus, C. dammeri, C. dielsianus, C. salicifolius var. floccosus, and C. ×suecicus ‘Coral Beauty’. There often is a high degree of variability in resistance within species between years on the same plants, and this was clear according to our results as well as to those of others (Persiel and Zeller, 1978; Rothleutner et al., 2014), thus justifying this series of five experiments over the course of years and in two environments. For example, Rothleutner et al. (2014) reported that C. salicifolius var. floccosus was highly susceptible during one year but asymptomatic during the next year. This could have been because of a variety of reasons, such as inoculation titer, temperature of the greenhouses, or plant placement in the greenhouses. The confounding results also could have occurred because of sexual and apomictic reproduction from seed (Persiel and Zeller, 1981, 1990); however, Rothleutner et al. (2014) used clonal plants produced from stem cuttings in all experiments. Our results apply only to the particular genotypes (accessions) for species used in the study and should not be considered species-wide levels of resistance. Furthermore, the rather dramatic variation seen within a single block of Expt. 2, which was conducted in a growth chamber, illustrates the sometimes inexplicable variation when dealing with the complex interaction of plants and pathogens.
Apple breeders have incorporated fire blight-resistant wild apple material with sensitive cultivars as a way to develop resistant cultivars. The resistance found in Mr5 has been studied, and a major QTL has been found on linkage group 3. There is a gene-for-gene interaction in the host–pathogen system of Mr5 and E. amylovora whereby a SNP in the apple and a SNP in the pathogen allow an avrRpt2 mutant strain of the pathogen to overcome the resistance that the genotype once offered (Emeriewen et al., 2019; Vogt et al., 2013; Wöhner et al., 2018). The same strain of the pathogen used by Wöhner et al. (2018) was used in this project to determine if Cotoneaster genotypes resistant to the wild-type pathogen are able to maintain resistance against naturally occurring avrRpt2 mutant strains of the pathogen (Smits et al., 2014). However, because we have already observed a mutant strain overcome what was previously a reliable source of resistance in apple, future work involving breeding cotoneaster and other rosaceous crops should attempt to pyramid resistance genes to prevent or at least slow current and future mutant strains overcoming single sources of resistance.
There were only two genotypes that had different PSN when inoculated with the wild-type and avrRpt2 mutant. ‘Emerald Beauty’ exhibited high resistance to the wild-type strain Ea153 and increased disease severity with the avrRpt2 mutant LA635. The interesting result was that C. splendens (09-0024), in 2019, exhibited high resistance to LA635, but it was susceptible to wild-type Ea153. The disease progressed rapidly through the stem during the first few weeks after inoculation with Ea153 or, conversely, was stalled for the first 2 weeks after inoculation with LA635 (Fig. 2). These results suggest that the different genotypes may possess different mechanisms for disease resistance. In all experiments, C. dammeri (19-0036) was more susceptible to fire blight than all other genotypes tested, indicating that it is a poor parent to use in crosses. This was shown through one of its progeny, H2017-005-01, which had more than 30% shoot necrosis, suggesting that susceptibility was inherited. Cotoneasters, however, are largely apomictic tetraploids; therefore, diploids that produce sexual progeny such as C. dammeri often are used out of necessity, regardless of susceptibility. Diploid cultivars Emerald Sprite USPP31,719 and Emerald Beauty PP32,308 will be useful for future breeding because of their increased resistance to fire blight compared with C. dammeri (19-0036) and ‘Coral Beauty’.
Our study found a varying range of susceptibility of Cotoneaster to the wild-type and a mutant strain of the fire blight pathogen. Several genotypes were observed to have consistently high levels of resistance to both wild-type and mutant strains of the pathogen, which bodes well for releasing more reliably resistant cultivars. This study and previous fire blight disease resistance studies have provided insight regarding potential sources of resistance to develop fire blight-resistant cultivars of Cotoneaster.
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