Comparison between Rosellinia necatrix isolates from soil and diseased roots in terms of hypovirulence.

The white root rot fungus, Rosellinia necatrix, is a devastating soil-borne pathogen of many plant species. Biocontrol with the hypovirulence factor is promising, but disease symptoms, signs or culture morphology of the pathogen cannot be reliably used as markers for hypovirulence in this fungus. We attempted to obtain hypovirulent isolates from soil rather than from diseased roots, based on the hypothesis that hypovirulent isolates were more likely to persist in soil as saprobes. Sixteen isolates, belonging to eight mycelial compatibility groups (MCGs), were obtained from soil in two active and one abandoned Japanese pear orchards. Comparison of these isolates based on clonality revealed that six MCGs were commonly recovered from both diseased roots and soil and two MCGs exclusively from soil. No MCG was found in more than one orchard. With two exceptions, isolates within the same MCG were similar in virulence, competitive saprophytic ability (CSA) and mycelial growth rate whether or not they carried dsRNA. The two exceptional isolates recovered from soil had multiple dsRNA segments that caused hypovirulence, weakened CSA and restricted mycelial growth on nutrient-rich media. They belonged to different MCGs, each including dsRNA-free isolates. Isolates from soil contained various dsRNAs (44%), including the hypovirulence factor, more frequently than isolates from diseased roots in the same fields (25%), which is much higher than the proportion of isolates with dsRNA from diseased roots (19%) in a total of 424 isolates from Japan examined so far. These results suggest that isolation of R. necatrix from soil is an effective method to obtain isolates with dsRNAs, including the hypovirulence factor.