Abstract
Inoculation during 1974 of natural clones of Populus tremuloides in the field with single-ascospore isolates of H. 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 Aug. 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 [USA]. 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. P. grandidentata became infected with H. mammatum after inoculation, but some clones showed resistance by profuse callus production.

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