Distinction Between Sex and Compatibility in Ceratocystis Ulmi, as Shown by Unisexual Male Cultures

SUMMARY

Of 10 isolates of Ceratocystis ulmi collected in September, 1970, from eight trees of Ulmus X hollandica cl. ‘Belgica’ and two trees of U. X hollandica cl. ‘Commelin,’ seven were of compatibility type B and three, type A. Peeled, split, autoclaved elm twigs, dipped for 10 sec into a 3-da-old liquid shake-culture (Tchernoff medium) of a B isolate and, 3 da later, into a similar culture of an A, were compared in December, 1970, with similar twigs dipped first into A and 3 da later into B. Abundant, fertile perithecia formed in all but seven of the 42 possible combinations: an A isolate from Doorwerth, Netherlands, formed perithecia with each of the seven B isolates only if it was the second 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. Passage of the male culture through elms yielded symptoms typical of Dutch elm disease but did not change its unisexual nature. Of two aggressive cultures received from England (both compatibility type B), one reacted as hermaphrodite and the other as male in more limited tests.