Powdery mildew is a common fungal pathogen of sweet cherries. Although yield losses have not been reported, powdery mildew is considered the most serious pre-harvest disease of cherries in the Pacific Northwest. Powdery mildew is commonly seen on tree foliage during the growing season (Figures 1,3), and fruit infection is most often seen in orchards with high incidence and severity of foliar infections (Figure 2). Fruit infections are the most severe problem, reducing fruit quality and leading to potential financial losses for the grower.

Figure 1. Sweet cherry shoot infected with powdery mildew.	Figure 2. Sweet cherry fruit infected with powdery mildew.

Figure 3. Close-up of powdery mildew infection on a sweet cherry leaf.

      Powdery mildew is currently controlled by multiple fungicide applications. However, recent reports have documented reduced effectiveness of certain fungicides due to fungal resistance. These findings, coupled with increased grower and consumer awareness of chemical applications in the environment, led to the initiation of a project at Washington State University's Irrigated Agriculture Research and Extension Center (WSU-IAREC) to investigate the potential development of powdery mildew resistant sweet cherry varieties.

      In the spring of 1998, a series of cross-pollinations (Figure 4) were performed between PMR-1 (Powdery Mildew Resistant - 1) and three commercial varieties (Bing, Van, and Rainier). PMR-1 is a cherry variety identified by Dr. Tom Toyama, the former stone fruit breeder from WSU-IAREC. PMR-1 is completely resistant to powdery mildew infection, but fruit quality does not meet commercial standards. However, PMR-1 appeared to be an ideal candidate for use as a powdery mildew resistance donor parent in a breeding program.

Figure 4. Hand pollination was done in the field, and seeds were germinated and grown in the greenhouse after stratification.

      While the seeds collected from the crosses were being cold stratified, a laboratory screening test was developed to speed the identification of resistant varieties. Small leaf disks were used to conserve space, and a growth chamber ensured environmental conditions were constant. A group of 14 varieties, all previously rated for powdery mildew susceptibility in the field, were used to test the leaf disk screening method. Results showed that use of the leaf disk screening method were not significantly different than field rating. Interestingly, there was a wide range in susceptibility to powdery mildew shown between just 14 varieties, and three other potential sources of complete resistance were identified (Figure 5).

Figure 5. Variation in susceptibility to powdery mildew in 14 varieties tested using a leaf disk screening method.

      Each individual seedling from the crosses between PMR-1 and Bing, Van, and Rainier were screened for powdery mildew resistance using the leaf disk assay and subsequently field rated (Figure 6). In each population, roughly half of the seedlings were resistant, and the other half susceptible to powdery mildew. A genetic ratio such as this indicates that this type of powdery mildew resistance is controlled by a single gene in PMR-1, and that gene is easily transferred to subsequent progeny through traditional breeding methods.

      The future of these seedlings will be evaluated in the coming years. With several hundred seedlings a good possibility exists that the powdery mildew resistance will be combined with excellent commercial qualities. In addition, future breeding priorities are to combine powdery mildew resistance with self fertility and other useful horticultural traits, as well as utilize other potential sources of powdery mildew resistance identified.

     In 2000, crosses were made between Chelan™ and Moreau (two other potentially resistant cultivars identified by screening) and the susceptible cultivar Bing. Segregation for powdery mildew resistance within the progeny populations for these crosses also indicated a single gene was responsible for the resistance. Whether this gene is the same as the one found in PMR-1 remains to be determined.