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Molecular Plant Pathology

Effect of dsRNA Associated with Isolates of Cryphonectria parasitica from the Central Appalachians and Their Relatedness to Other dsRNAs from North America and Europe. S. A. Enebak, West Virginia University, Division of Plant and Soil Sciences, Morgantown, Current address: University of Minnesota, North Central Experiment Station, 1861 Hwy. 169 East, Grand Rapids 55744; W. L. MacDonald(2), and B. I. Hillman(3). (2)West Virginia University, Division of Plant and Soil Sciences, Morgantown; (3)Rutgers University, Cook College, Department of Plant Pathology, New Brunswick, NJ 08903. Phytopathology 84:528-534. Accepted for publication 4 February 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-528.

A 12-kb segment of double-stranded (ds) RNA was associated with 25% of the isolates of Cryphonectria parasitica recovered from actively growing cankers in the central Appalachians. The relative virulence of these dsRNA-containing isolates ranged from a level comparable to that of the dsRNA-containing hypovirulent control isolate GH-2 to a level significantly greater than that of the dsRNA-free virulent control isolate Ep-155. When dsRNA-containing isolates and their dsRNA-free progeny were inoculated into Golden Delicious apples, excised dormant chestnut stems, or American chestnut sprouts, none of the lesions produced by the dsRNA-free progeny was significantly larger than those produced by the dsRNA-containing isolate from which they were derived. No differences in cultural morphology were noted when dsRNA-free and dsRNA-containing isogenic isolates were compared. These studies indicate that the single 12-kb segment of dsRNA neither alters morphology nor confers hypovirulence to the pathogen, and therefore it has little potential for biological control of chestnut blight. The relationships of the dsRNAs found in these isolates to dsRNAs associated with hypovirulent isolates from Europe and North America were examined by using recombinant cDNA libraries from three different dsRNAs. The first type, designated SR-2, contains one segment of dsRNA (12 kb); the second type, designated D2, contains two segments of dsRNA (5 and 12 kb); and the third type, designated C-18, contains 11 segments of dsRNA (1-5 kb). Clones from isolates SR-2 (Maryland), D2 (Pennsylvania), and C-18 (West Virginia) did not cross-hybridize, indicating that these dsRNAs have neither close affinities to one another nor to the other dsRNAs tested from New Jersey (NB58) and Europe (Ep-713 and Ep-747). However, clones from isolate SR-2 cross-hybridized with the single 12-kb segment of dsRNA from 26 other isolates, indicating that dsRNAs of the SR-2 type are closely related.