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Maturation of Thyriothecia of Schizothyrium pomi on the Reservoir Host Rubus allegheniensis

Flyspeck disease caused by the Ascomycete fungus Schizothyrium pomi is a major component of a summer disease complex that has become an increasing problem on apple in the northeastern United States. This study describes the development of thyriothecia of S. pomi on one of its major reservoir hosts, wild blackberry, and suggests that temperature drives thyriothecium maturation. An empirical model relating thyriothecium maturity to degree-days from apple leaf budbreak in Massachusetts had a high coefficient of determination, R² = 0.90. Ninety percent of thyriothecia matured between 540 and 1,625 degree-days from the budbreak biofix. S. pomi produced one generation of asci per year on blackberry. This discrete maturation period occurred well before signs of disease developed on apple fruit and before production of conidia. Considering that fungicide applications for scab management in commercial apple orchards also will protect fruit from flyspeck during early fruit development, it is unlikely that ascospores produced on reservoir hosts directly infect fruit. Instead, they probably initiate infections in reservoir hosts adjacent to commercial orchards, which produce conidia that are the inoculum which infects apple fruit in commercial production.