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It is important to mention that organic Honeycrisp and Granny Smith have topped Red Delicious in term of acreage since 2013 which can make powdery mildew management challenging organically, especially under high disease pressure. Apple and Pear Powdery Mildew There are several species of powdery mildew that attack tree fruit. Primary infections can be controlled by removal of the primary inoculum sources (i.e., flower and shoot buds infected the previous year). The American Phytopathological Society (APS). appressorium > infection peg > haustorium) is repeated until susceptible tissue is no longer available. During the growing season, this fungal obligate parasite continuously produces asexual spores (conidia) on specialized short stalks called conidiophores (Figure 9). Growers may be tempted to relax spray programs during dry conditions when other apple diseases cannot develop, but mildew thrives in dry weather and protection needs to be maintained. It is generally a serious disease in the UK, particularly the eastern part but usually less of a problem in the west. Powdery mildew is a fungal disease that affects a wide range of plants. Powdery mildews rank among the most important diseases of food and ornamental plants. Under high disease pressure, the fungus can cause the fruit to russet (Figure 1E,F). The fungus causes whitish fuzzy lesions (Figure 1B,C) and causes leaves to curl (Figure 1C,D). (Courtesy K. Yoder and S. Marine), (control always needed when disease is present), (control usually needed when disease is present), (control needed only under high disease pressure). The Plant Health Instructor. It is one of the most predominant diseases in the more arid apple growing areas. Mildew colonies generally appear first on the lower leaf surface as white felt-like patches. Bulletin OILB, 20(9):101-104. As the fungus spreads it can cover both sides of the leaf. Mycological Research, 97(1):71-76. The word is derived from the vertical chasm that is formed during ascospore discharge. Podosphaera leucotricha is an ascomycete fungus in the Erysiphaceae family and is found in all apple-producing regions. The white powder consists of spores or 'conidia' which are spread on the breeze, infecting shoots, leaves and occasionally fruit during the summer. Before the 1940’s, mildew was considered a disease primarily of nursery stock and was of relatively minor importance to most commercial apple growers. Powdery mildew of apple. Therefore, enhanced disease management programs should be implemented. Look for powdery mildew from green tip to half inch green through to terminal growth set. Since cleistothecia in other groups of ascomycetes lack a hymenium (i.e., the asci are randomly scattered throughout the enclosed structure), the term chasmothecia has been recently introduced to distinguish powdery mildew ascocarps from other cleistothecia. If they land on susceptible tissue, they initiate infection and produce colonies of mycelium. Apple cultivars are available that demonstrate natural resistance to mildew and need control only under high disease pressure; these include Jonafree, Prima, and Enterprise, but they are not widely grown. 2010. 2010. Apple cultivar susceptibility to the powdery mildew fungus. Preventive applications and rotations of the different bio-pesticides from tight cluster to third cover spray should help keep powdery mildew under the economic threshold. YOU ARE REQUIRED BY LAW TO FOLLOW THE LABEL. At this time of the year, most shoots have already stopped growing and temperatures in central Washington become unfavorable for powdery mildew. Chemical control of powdery mildew is done in conjunction with controls for scab. Infected flower buds have a silver-gray appearance and open 5-8 days later than healthy ones, if at all. It looks almost like a fine coating of talcum powder. 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