Perithecial Material of Erysiphe and Microsphaera on Trifolium Pratense

Abstract

Powdery mildew of red clover, prior to 1921, was of extremely uncommon occurrence in the eastern United States. That year it suddenly became more abundant and in 1922 reached epiphytotic proportions in several states in the eastern half of the country. Massachusetts and Michigan reported it as common everywhere. Pennsylvania, Ohio, Minnesota, and Iowa reported seriously destructive outbreaks, local in some cases, statewide in others. Collectors, interested in learning the identity of the species, searched for perithecia and met with a surprising lack of success. Plant Disease Bulletin of the United States Department of Agriculture for July 1, 1922, states: The name of the mildew is not known definitely, for as yet the perithecial stage of the fungus does not seem to have been found. Efforts should be made to discover the perfect stage this year and thus settle the question of nomenclature. 2 Reporting for Pennsylvania, C. R. Orton stated that a careful search throughout the preceding season had failed to reveal perithecia. Reports from the other states in which the conidial stage was so abundant were also negative; and in the years that have followed, eastern plant pathologists have not reported finding any perithecial material. Plant Disease Bulletin for August 1922 stated, however, that the perfect stage of Erysiphe Polygoni DC. had meanwhile been reported as having been found on red clover in Washington,3 Oregon, and Idaho as early as 1915; and that B. F. Dana had re-