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Environmental Factors Regulating Ascospore Discharge by Mycosphaerella ligulicola. R. E. McCoy, Former graduate student, Department of Plant Pathology, Cornell University, Ithaca, N.Y. 14850, Present address of senior author: Agricultural Research Center, University, of Florida, Fort Lauderdale 33314; A. W. Dimock, Professor, Department of Plant Pathology, Cornell University, Ithaca, N.Y. 14850. Phytopathology 63:586-589. Accepted for publication 17 November 1972. DOI: 10.1094/Phyto-63-586.

Ascospore discharge by Mycosphaerella ligulicola is regulated by moisture and, depending on isolate, by light, at a constant air temperature of 20 C. Explosive discharge of ascospores is effected by sprinkling water or accumulating heavy dew upon perithecium-bearing chrysanthemum tissue. Approximately 0.1 mm of moisture accumulation is necessary to initiate such discharge. Light prevented or stopped discharge, even in the presence of adequate moisture, in three of four isolates tested. These three isolates discharged ascospores only in darkness, whereas ascospore discharge by the fourth isolate occurred in either light or darkness. Ascospore trapping during an outbreak of Mycosphaerella blight of field-grown chrysanthemums revealed peak aerial concentrations just after an evening thundershower; much lower concentrations were present during nighttime dew periods.