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Variability in Virulence of Heterobasidion annosum Isolates from Ponderosa and Jeffrey Pine in Areas of High and Low Photochemical Air Pollution. R. L. James, Plant Pathologist, USDA Forest Service, Forest Pest Management, Missoula, MT 59801. F. W. Cobb, Jr., Associate Professor, Department of Plant Pathology, University of California, Berkeley 94720. Plant Dis. 66:835-837. Accepted for publication 4 December 1981. 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, 1982. DOI: 10.1094/PD-66-835.

Virulence of Heterobasidion annosum (= Fomes annosus) isolates from different geographic locations throughout California was evaluated by inoculation of ponderosa pine trees in the field and seedlings in the greenhouse. Tests were designed to compare isolates obtained from areas of chronic photochemical air pollution exposure with some from areas relatively free from pollution. Isolates displayed a wide range of virulence based on number of plants infected and rate of colonization. Percentage of infection varied from 4 to 74 in the greenhouse and from 0 to 50 in field tests. Colonization rates showed a 17-fold difference among isolates. However, relationships between geographic origin and level of virulence were not evident. Isolates from areas of relatively high, chronic air pollution exposure were generally as virulent as those from environments with little or no pollution.

Keyword(s): Pinus jeffreyi, P. ponderosa, root disease.