Issue link: http://read.dmtmag.com/i/675334
42 MAY 15, 2016 Good Fruit Grower www.goodfruit.com presence of the disease at harvest and to see if resistance and susceptibility vary at different developmental stages of fruit. Environmental factors, such as temperature and relative humidity, and inoculum concentration thresh- olds also were taken into consideration to see how many spores were necessary to have the disease show at harvest. Probst presented the latest findings on this powdery mildew research at several winter meetings, including November's Northwest Cherry Research Review and the Washington State Tree Fruit Association's annual meet- ing in December. Seeking spores The flowering stage has been of particular interest to researchers because, for a lot of commodities, flow- ers are highly susceptible to infection, Probst said. For instance, strawberry blooms are highly susceptible to infection, but as soon as the fruit reaches the white or pink stage, generally within about two weeks, it becomes fully resistant. "This has major implications for disease control," Probst said. "This is called ontogenic resistance. It's seen in grape berries at about four to six weeks." The good news: Cherries have a long period of resis- tance, which is fabulous, she said. However, cherries' resistance to mildew declines over time, which means they have no ontogenic resistance. Additionally, the period from pre-bloom to fruit set in cherries was not a critical period for establishment of powdery mildew infections, the research showed. "Flowering precedes natural onset of disease on leaves," she said. "Airborne spores are detectable, but at very low concentrations, and cherries covered at fruit set were not infected at harvest, so the timing between bloom and fruit set is not critical for diseases that present at harvest." Generally, Probst said, the more spores the more dis- ease; however, few spores are needed to cause significant damage. There could be as many as 10,000 spores during the season, but cherries only need about 500 spores to see significant infection at harvest. Other factors Researchers also examined the effects of temperature and humidity on disease incidence and severity at vari- ous stages of fruit development. Probst called these the most "frustrating experiments" she had ever done. "We processed thousands of these cherries, and we just couldn't get disease to develop. The only time we got disease to develop was on fruit harvested in mid-June," she said. Disease can initiate year-round on susceptible leaves, with an optimal temperature, relative humidity and