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Epidemiology of the Black Knot Disease of Plums. D. H. Smith, Former Graduate Assistant, The Pennsylvania State University, now Assistant Plant Pathologist, University of Georgia, College of Agriculture Experiment Stations, Georgia Station, Experiment, Georgia 30212; F. H. Lewis(2), and S. H. Wainwright(3). (2)(3)Scientist-in-Charge; and Graduate Assistant; The Pennsylvania State University Fruit Research Laboratory, Arendtsville 17303. Phytopathology 60:1441-1444. Accepted for publication 20 April 1970. DOI: 10.1094/Phyto-60-1441.

Dibotryon morbosum ascospores were ejected for distances as great as 45 mm shortly after ascocarps were moistened in the laboratory. In field experiments, ascospore ejection occurred only on days with measurable rainfall. The initial ejection period of the season seemed to be related to ascospore maturity. During air-sampling periods (April, May, and June) with a Hirst spore trap in 1965 and 1967, ascospores were trapped on 14 days during each year. In both years, initial catches occurred in late April, with final catches in late June. The time between the onset of rainfall and the deposition of ascospores on Hirst trap slides ranged from 0.3 to 3 hr. Ascospores germinated on agar media, but rarely did germ tubes continue to elongate and produce the imperfect stage. Of the temp studied, 24 C was most favorable for germination. In greenhouse pathogenicity tests, severe infection of Prunus domestica L. 'Stanley' trees occurred with postinoculation moisture periods as brief as 6 hr at 21 C. Apparently, ascospores constitute the only significant source of inoculum. In several years of field tests with Stanley trees, it was shown that most natural infection occurred after petal-fall during May and June in southern Pennsylvania.