Genetic Control of Resistance to Hypoxylon Infection and Canker Development in Populus tremuloides

The responses of 24 families of P . tremuloides (6 groups of 4 maternal half-sibling families each) to 4 sources of Hypoxylon mammatum were observed. Three mechanisms of resistance to the disease were studied: (1) callus formation, (2) branch death, and (3) resistance through retardation of canker growth. Resistance by callous formation is due to a hypersensitive response of the host to the pathogen. Little variation exists in the nature or time of the host response, but the pathogen's ability to elicit the host response, expressed as incidence per inoculum, varies from about 3 to 35 percent. Evidence suggests that a few major genes control this trait and are the basis for Mendelian ratios within families and discrete differences between groups of half-sibling families. Resistance by branch death occurs at a low incidence (8.3 percent) and death is due to the canker encircling the branch. Heritability estimates are low, 0.075 or less in response to the four inocula and 0.027 for all data. These are probably underestimates because all potentially resistant phenotypes most likely have not been expressed 4 months after inoculation. The third form of resistance, retardation of the spread of the pathogen, is measured as canker length. h 2 is low for three inocula (= 0.074), but reasonably high (0.254) for the fourth source.

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Author(s): Frederick A. Valentine, Paul D. Manion, Kathleen E. Moore

Publication: Tree Improvement and Genetics - Lake States Forest Tree Improvement Conference - 1975