Fruit Pathology
University of Wisconsin - Madison 
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CottonballGeneral DescriptionCottonball is the most important disease affecting cranberry during the growing season in Wisconsin, and can also be economically important in British Columbia. While the tip blight phase is normally economically insignificant, fruit rot incidence can become epidemic in limited areas, affecting more than 90% of the berries. Losses also result from increased screening costs, as sorting lines must be run at slower speeds to cull out the infected fruit. For more information visit the Plant Pathology 300 Cottonball Webpage.
SymptomsTip blight symptoms appear about three weeks after budbreak and include tan to brown discoloration and wilting or crooking of succulent shoot tips, and a V-shaped pattern of tan discoloration at the base of leaves along the midvein. Just before or coinciding with bloom, a white spore-bearing fungal mantle appears on the crooked portions of infected shoots and flower peduncles. Tip blight occurrence is typically spotty in beds and can be overlooked as infected shoots are overgrown by healthy shoots.Infected fruit fail to ripen normally. Brown, necrotic bands appear lengthwise on the berry and spread until the entire berry is yellowish-brown. The interior of the berry is filled with white, cottony fungal mycelia, but the berry remains firm (hard rot). Some infected berries turn dark brown and shriveled, and are referred to as mummies. Disease CycleThe causal organism, Monilinia oxycocci overwinters as pseudosclerotia (hardened masses of fungal and plant tissue) in mummified fruit in or on the soil, or attached to the upright. In the spring, sclerotia which have been at least partially buried, germinate and form apothecia, which in turn bear ascospores (primary spores). Mature apothecia are urn-shaped, brown, 1/2" or less in diameter, and born on slender stalks ranging from 1/2-2" in length. Ascospores are released for a period of 25-35 days, corresponding to the period between budbreak and bloom. Ascospore dispersal is maximal during a 10- to 14-day period and peaks when the majority of shoots are 1/2"-1" long and very susceptible to infection. Shoot infections give rise to conidia (secondary spores), which are spread by wind during a 10- to 14-day period during full bloom. Conidia infect open flowers, and eventually fungal mycelia fill the infected berries. Sclerotia develop in 25-50% of the infected berries; berries without sclerotia decompose during the winter and spring. Need more detail? Check out the cottonball disease cycle page.ControlCranberry beds should be clean picked to avoid leaving in the field mummified fruit in which the cottonball fungus overwinters. Currently, sterol demethylation inhibitor fungicides are used to control cottonball. Check annual pest recommendation bulletins for currently registered fungicides. Because development of M. oxycocci and cranberry are closely associated, the timing of fungicide applications is based on the growth stage of the crop. Typically, four applications are made: 1) at budbreak; 2) 7-10 days after the first spray; 3) at early bloom; and 4) 7-10 days after the third spray. Refer to a current product label for up-to-date information on rates, methods of application, and appropriate safety precautions.
Questions or comments? Please contact Dr. Patty McManus.
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