EFFECT OF DSRNA ASSOCIATED WITH ISOLATES OF CRYPHONECTRIA-PARASITICA FROM THE CENTRAL APPALACHIANS AND THEIR RELATEDNESS TO OTHER DSRNAS FROM NORTH-AMERICA AND EUROPE

A 12-kb segment of double-stranded (ds) RNA was associated with 25% of the isolates of Cryphonectria parasitica recovered from actively grow ing cankers in the central Appalachians. The relative virulence of the se dsRNA-containing isolates ranged from a level comparable to that of the dsRNA-containing hypovirulent control isolate GH-2 to a level sig nificantly greater than that of the dsRNA-free Virulent control isolat e 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 differe nces in cultural morphology were noted when dsRNA-free and dsRNA-conta ining isogenic isolates were compared. These studies indicate that the single 12-kb segment of dsRNA neither alters morphology nor confers h ypovirulence to the pathogen, and therefore it has little potential fo r biological control of chestnut blight. The relationships of the dsRN As found in these isolates to dsRNAs associated with hypovirulent isol ates 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, designa ted D2, contains two segments of dsRNA (5 and 12 kb); and the third ty pe, designated C-18, contains 11 segments of dsRNA (1-5 kb). Clones fr om isolates SR-2 (Maryland), D2 (Pennsylvania), and C-18 (West Virgini a) did not cross-hybridize, indicating that these dsRNAs have neither close affinities to one another nor to the other dsRNAs tested from Ne w Jersey (NB58) and Europe (Ep-713 and Ep-747). However, clones from i solate SR-2 cross-hybridized with the single 12-kb segment of dsRNA fr om 26 other isolates, indicating that dsRNAs of the SR-2 type are clos ely related.