Persistence of Cryphonectria hypoviruses after their release for biological control of chestnut blight in West Virginia forests

Abstract

SummaryAlthough Cryphonectria hypoviruses have been relatively successful as biological control agents of chestnut blight in Europe, their success in North America has been limited. Experimental releases of hypoviruses were made in 1978–82 at two sites in West Virginia forests with high densities of regenerating chestnut trees. Cryphonectria hypovirus 1 (CHV‐1) from Europe, as well as American isolates of Cryphonectria parasitica containing CHV‐3, were used for these releases. Although most trees died during the 5‐year release period, it was not known if the hypoviruses persisted in the C. parasitica population at the two sites. When the experimental plots were revisited in 1994, few chestnut trees were found. The exception was one plot containing coppice sprouts that had grown from the root collars of the original trees. The authors intensively sampled C. parasitica from experimental plots and screened recovered isolates for double‐stranded RNA (dsRNA). None of the isolates contained CHV‐1; only six isolates contained CHV‐3, all from the plots with the coppice sprouts. CHV‐4, which occurs naturally in West Virginia forests and in two released isolates, hybridized to dsRNA from the isolates containing CHV‐3, indicating mixed infections. CHV‐4 also hybridized to dsRNA from other isolates sampled inside and outside the treated plots. In contrast to CHV‐1 and CHV‐3, however, CHV‐4 has little effect on the growth or phenotype of C. parasitica. The limited persistence of CHV‐1 and CHV‐3 may have resulted when the C. parasitica population was reduced in size due to the failure of chestnut trees to resprout because of competition from other hardwood species.