Double-stranded RNA and associated virulence in South African isolates of Sphaeropsis sapinea

Abstract

Sphaeropsis sapinea (Fr.) Dyko & Sutton is an opportunistic pathogen of various Pinus spp., causing severe shoot blight and dieback. Some isolates of S. sapinea display characteristics such as reduced virulence, reduced growth rate, lack of pigmentation, altered colony morphology, and suppressed conidiation. South African isolates of S. sapinea displaying a range of growth patterns, including reduced growth rate and atypical morphology, were screened for the presence of double-stranded RNA (dsRNA). They were also tested for relative virulence in pathogenicity tests. Double-stranded RNA was isolated by means of phenol extraction and cellulose chromatographic purification. A single species of dsRNA (± 4.3 kilobase pairs in size) was obtained from two slow-growing isolates and two isolates with more regular growth. The virulence of these dsRNA-containing isolates was tested on mature Pinus patula Schlecht. et Cham. trees. Although reduced virulence was positively correlated with slower growth in vitro, the presence of dsRNA could not be linked to either of these characteristics.Key words: double-stranded RNA, hypovirulence, Sphaeropsis sapinea.