Distinction between Sex and Compatibility in Ceratocystis ulmi, as Shown by Unisexual Male Cultures

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Abstract
Of isolates of C. ulmi [Dutch elm disease pathogen] collected in Sept. 1970, from 8 trees of Ulmus .times. hollandica cl. ''Belgica'' and 2 trees of U. .times. hollandica cl. ''Commelin'', 7 were of compatibility type B and 3 of type A. Peeled, split, autoclaved elm twigs, dipped for 10 s into a 3 day old liquid shake-culture (Tchernoff medium) of a B isolate and 3 days later, into a similar culture of an A, were compared in Dec. 1970, with similar twigs dipped 1st into A and 3 days later into B. Abundant, fertile perithecia formed in all but 7 of the 42 possible combinations. An A isolate from Doorwerth, Netherlands, formed perithecia with each of the 7 B isolates only if it was the 2nd inoculum. It thus acted as a male: able to serve as effector but not as receptor. These tests were made with 3 mo old isolates. Reisolation from the same branch of the original tree in April, 1971, again yielded type A, but hermaphroditic rather than male. Pasage of the male culture through elms yielded symptoms typical of Dutch elm disease but did not change its unisexual nature. Of 2 aggressive cultures received from England (both compatibility type B), 1 reacted as hermaphrodite and the other as male in more limited tests.