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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), and fruit infection is most often seen in orchards with high incidence and severity of foliar infections (Figure 1). Fruit infections are the most severe problem, reducing fruit quality and leading to potential financial losses for the grower.
Figure 1. Left: Sweet cherry shoot infected with powdery mildew. Middle: Sweet cherry fruit infected with powdery mildew. Right: 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 2) 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
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.