A Photographic Diagnostic Guide for Identification of the Principal Cranberry Fruit Rot Pathogens

The large American cranberry (Vaccinium macrocarpon Aiton) is native to parts of eastern North America and is economically important in Massachusetts, Michigan, New Jersey, and Wisconsin, as well as several eastern Canadian provinces and the North American Pacific Northwest. The farm gate values of cranberry ranged from $298 to $344 million and from $70 to $88 million in the United States and Canada, respectively from 2010 to 2011 (14,16). When cranberry cultivation first began in the early nineteenth century, growers were able to produce cranberries for 5 to 10 years until the fruit succumbed to the "the rot" (10). Modern cultural methods and fungicides allow plantings to remain productive for several decades, but cranberry fruit rot remains the most devastating disease problem, especially in warmer growing regions where crop losses approach 100% if fruit rot is left unchecked (10,11). As many as 15 fungal species contribute to the cranberry fruit rot disease complex (9,10,11), although the most significant economic losses can be attributed to a subset of five to seven species, depending on geographic region. Although rotten cranberry fruit take on various symptoms (Fig. 1), determining the pathogen(s) involved is not possible by viewing symptoms alone. Rather, plant disease diagnosticians and fruit pathology laboratories culture rotten cranberries on different microbiological media to isolate and identify the cranberry fruit rot pathogens. It is very common to isolate more than one pathogen from a rotten berry, but in such cases the role of each pathogen in causing rot is not known. These pathogens differ in colony appearance when grown on different types of microbiological media, making it difficult for diagnosticians to share descriptions of colony morphology. Moreover, many university-based laboratories undergo frequent personnel changes which make training difficult. Thus, the objective of this report is to provide a photographic identification guide for each of the seven principal fruit rot pathogens when grown on different microbiological media. Our intent is that the photographs will supplement, but not replace, descriptions of pathogens provided in the Compendium of Blueberry and Cranberry Diseases (7).