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Resistance

Resistance Responses in Half-Sib Loblolly Pine Progenies after Inoculation with Cronartium quercuum f. sp. fusiforme. E. G. Kuhlman, Principal, USDA Forest Service, Southeastern Forest Experiment Station, Athens, GA 30602; H. R. Powers, Jr., Chief plant pathologist, USDA Forest Service, Southeastern Forest Experiment Station, Athens, GA 30602. Phytopathology 78:484-487. Accepted for publication 19 October 1987. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1988. DOI: 10.1094/Phyto-78-484.

Open-pollinated progeny from 156 loblolly pine trees in the USFS-Georgia Forestry Commission orchards were inoculated with Cronartium quercuum f. sp. fusiforme. Nine months after inoculation, the incidence of galls among the 156 families ranged from 2 to 97%. The susceptible control family had significantly more galls than 116 of the 156 other families tested according to the 0.70 disease ratio computation (percent galls test family/percent galls susceptible control = ≤ 0.70). The 0.70 disease ratio was more conservative than a chi-square test of independence in separating resistant and susceptible families. Responses to infection were compared between resistant and susceptible families to determine the relative frequency of resistant responses. The absence of stem symptoms was the most common resistant response in many resistant families. The third-generation progeny derived from resistant selections 2318, 10-5, and 42R had 50% or more seedlings with this response. Needle spots were early symptoms of infection, but the frequency of seedlings with needle spots at 3 and 6 mo was not correlated with gall frequency at 9 mo. Some third-generation progeny had high frequencies of stem spots without swellings (symnos) at 3, 6, and 9 mo after inoculation as a resistant response. Stem spots present at 6 mo but no longer visible at 9 mo were present in other resistant selections. The ratio of short, medium, and long galls among 90 families was 10, 29, and 62%; however, gall length varied significantly among families. Galls on seedlings in certain resistant families were more frequently long than were galls on seedlings in susceptible families. Progeny from some resistant families had twice as many short galls as the average.