Bitter Rot of Apples: Recent Changes in What We Know and Implications for Disease Management

Apple growers, private consultants, and extension specialists have all noted that bitter rot is increasingly common and is causing sporadic but economically significant losses throughout the northeastern and north central apple growing regions of North America. Forty years ago, bitter rot was considered a “southern disease” and apples with bitter rot were rarely observed in northern production regions. Four factors have probably contributed to the increasing incidence of bitter rot in these regions. First, as a result of global warming, we have more days during summer with warm wetting events that are essential for initiating bitter rot infections and perhaps for increasing inoculum within orchards before the bitter rot pathogens move to apple fruit. Second, some new cultivars (e.g., Honeycrisp) are very susceptible to infection. Third, we are also growing more late-maturing cultivars such as Cripps Pink that may be picked in early November, and these cultivars may need additional fungicide sprays during September and/or early October if they are to be fully protected from bitter rot. Finally, mancozeb fungicides are very effective against Colletotrichum species, and the season-long use of mancozeb may have suppressed Colletotrichum populations in apple orchards prior to 1990 when the mancozeb labels were changed to prohibit applications during summer (i.e., within 77 days of harvest). During the same time that bitter rot was expanding into more northern production regions, scientific breakthroughs enabled us to better identify the species that cause both bitter rot of pome fruit and the associated apple leaf spot disease in various regions around the world. This new information helps to explain some of the regional differences that were observed many years earlier, but the new information also leads to new questions on how best to manage this disease.