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Etiology

Decay of Douglas-Fir by Sparassis radicata in Arizona. K. J. Martin, Research Assistant, Department of Plant Pathology, University of Arizona, Tucson, AZ 85721, Present address of senior author: Arizona Commission of Agriculture and Horticulture, Phoenix, AZ 85007; R. L. Gilbertson, professor, Department of Plant Pathology, University of Arizona, Tucson, AZ 85721. Phytopathology 68:149-154. Accepted for publication 30 June 1977. Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-149.

Sparassis radicata is one of the major causes of a brown root and butt rot of Douglas-fir in the Santa Catalina Mountains of Arizona. Of 50 recently fallen Douglas-firs with a brown root and butt rot, decay was attributed to S. radicata in 30% of the trees, to Phaeolus schweinitzii in 62% of the trees, and to both fungus species together in 8% of the trees. Agar-block decay tests showed that both dikaryotic and homokaryotic isolates of S. radicata cause a brown cubical decay in Douglas-fir and ponderosa pine wood comparable to that found in naturally decayed roots and butts. Results of genetic analysis of mating-type factors suggest that S. radicata spreads from tree to tree through root grafts or root contacts.