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Ecology and Epidemiology

Ascospore Production, Release, Germination, and Infection of Populus by Mycosphaerella populorum. Christopher J. Luley, Department of Plant Pathology, Iowa State University, Ames 50011, Present address: Missouri Department of Conservation, P.O. Box 180, Jefferson City 65102; Harold S. McNabb, Jr., Department of Plant Pathology, Iowa State University, Ames 50011. Phytopathology 79:1013-1018. Accepted for publication 22 May 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-1013.

Production and release of ascospores from ascomata and infection of a susceptible clone (NE-1) and a moderately resistant clone (NE-19) of Populus by Mycosphaerella populorum were monitored in weekly sampling periods in a central Iowa plantation of Populus. Ascospores matured at approximately the time of bud swell of P. deltoides. Ascospore production, as measured by a liberation tunnel, and ascospore release, as detected by Vaseline-coated slide traps, peaked in mid- to late May and continued for 34 mo after initial release. Peak ascospore release in the field corresponded to peak periods of foliar and stem infection of the two clones of Populus. Stems of the moderately resistant clone were infected only in May, whereas foliar and stem infection of the susceptible clone occurred periodically throughout the growing season. Hourly ascospore release, monitored with a Burkard spore trap in 1985, was greatest after rains and during daylight hours. Levels of foliar and stem infection were also highly correlated with wetting periods associated with rain and daylight hours. Over 40% of ascospores germinated after 4 hr at 20, 25, and 30 C, and germ tube elongation after 16 hr was greatest at 25 C. These results show a strong relationship between the ascospore inoculum level and the degree of primary foliar and stem infection in a plantation of Populus.

Additional keywords: cottonwood, Septoria leaf spot and canker, Septoria musiva.