Models for Apothecial Emergence and Ascospore Discharge of Monilinia vaccinii-corymbosi in Michigan Blueberry Fields

Monilinia vaccinii-corymbosi ( Mvc) is the causal agent of mummy berry in highbush blueberries. The primary inoculum of Mvc consists of ascospores that are discharged from apothecia on overwintered mummified blueberry fruits on the ground below blueberry bushes. Models currently exist in southern climates to predict the emergence of apothecia and ascospore discharge based on growing degree days. To better understand the release of primary inoculum in northern climates, apothecial emergence and ascospore discharge were studied in relation to weather variables. A Burkard spore trap was used to collect ascospores in blueberry fields with a history of the disease in 2002, 2003, 2004, and 2009. Peak ascospore discharge was inversely correlated with daily relative humidity, and the majority of ascospores were initially discharged between 61 and 291 growing degree days (using 5.5°C base) and after 1,549 to 1,924 chill hours (above freezing) were attained. The chilling requirement of pseudosclerotia was studied by removing them at different times during the winter and allowing them to germinate in the laboratory. Additionally, the optimal average apothecial cup diameter in the field was investigated in relation to ascospore discharge (between 3 and 7 mm). Finally, we proposed a simplified degree-day model to predict apothecia emergence and ascospore discharge with apothecia emergence timings at several locations from 2007 to 2010. This information can be used to predict mummy berry apothecium emergence and ascospore discharge and could be incorporated into web-based risk modeling applications for providing growers with useful tools to improve fungicide spray timing.