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Variation in Resistance of Trembling Aspen to Hypoxylon mammatum Identified by Inoculating Naturally Occurring Clones. John R. French, Graduate Student, Department of Botany and Plant Pathology, Michigan State University, East Lansing, MI 48824, Present address of senior author: FMC Corporation, 100 Niagara Street, Middleport, NY 14105; John H. Hart, professor, Department of Botany and Plant Pathology, Michigan State University, East Lansing, MI 48824. Phytopathology 68:485-489. Accepted for publication 27 September 1977. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-485.

Inoculation during 1974 of natural clones of Populus tremuloides in the field with single-ascospore isolates of Hypoxylon mammatum produced significant interclonal differences in length of cankers after 70 days. More and larger cankers were produced from June inoculations than from those in July or August. The two most pathogenic isolates were selected and used during 1975 to test for differences in susceptibility among 100 clones of P. tremuloides and among 13 clones of P. grandidentata, occurring in 12 geographic areas of Michigan. Clones of P. tremuloides in northern areas had significantly shorter cankers than those in southern areas. Significant interclonal variability in canker development occurred in 10 of 12 areas. The amount of natural infection in each clone was not correlated with the length of artifically-induced cankers. Populus grandidentata became infected with H. mammatum after inoculation, but some clones showed resistance by profuse callus production.

Additional keywords: hypoxylon canker, tree improvement, selection.