Oversummer survival of Monilinia vaccinii-corymbosi in relation to pseudosclerotial maturity and soil surface environment

Pseudosclerotia (infected, mummified fruit) on the orchard floor act as ove rsummering and overwintering structures and the sole source of primary inoc ulum of Monilinia vaccinii-corymbosi, the causal agent of mummy berry disea se of blueberry. Survival of pseudosclerotia may be affected by their matur ity (degree of stromatization), which can vary considerably at the time of fruit abscission in early summer, and by variations in the soil surface env ironment. From July through October in 2 years, survival of pseudosclerotia of varying initial maturity (expressed as the proportion of fruit containi ng mature, melanized entostromata; immature, nonmelanized entostromata; or undifferentiated mycelia) was investigated in the laboratory relative to so il surface temperature and soil moisture content and in the field in relati on to shading (full sun versus 50% shade) and ground cover (bare soil versu s grass). In the laboratory, oversummer survival, expressed as the percenta ge of intact pseudosclerotia at the end of the experiment, was higher for c ool soil temperatures (approximately 15 degreesC), soils drier than field c apacity, and pseudosclerotia containing mature entostromata. In the field, survival was related solely to initial maturity of pseudosclerotia and was highest for pseudosclerotia containing mature entostromata. Shading or gras s ground cover did not significantly (P > 0.05) affect oversummer survival, presumably because they did not greatly modify soil temperature or soil mo isture. When individual, intact pseudosclerotia were tested for viability u sing fluorescein diacetate staining, a linear relationship (r = 0.982, P < 0.0001, n = 90) between viable and intact pseudosclerotia was observed, sup porting the use of the percentage of intact pseudosclerotia as a measure of oversummer survival.