Abstract
The impact of chestnut blight, caused by the fungal pathogen Cryphonectria parasitica, has diminished in Europe due to a natural biological control caused by hypovirus infection. Hypovirulence-mediated biological control has been far less successful in North America meriting further evaluation of field isolates that have the ability to produce non-lethal cankers, generate hypovirulent inoculum, and exhibit a greater ecological fitness in forest systems. In this study, Cryphonectria hypoviruses (CHV) CHV3-County Line, CHV1-Euro7, and CHV1-Ep713 were evaluated in five different isolates of C. parasitica. One hundred and eighty cankers representing each treatment combination were initiated on American chestnut sprouts in the Monongahela National Forest, West Virginia, USA. The size of cankers, the persistence of hypovirulent (HV) isolates, stroma production, and hypovirus transmission to conidia were assessed four and 12 months after canker expansion. CHV3-County Line infected isolates produced significantly smaller cankers than the isolates infected with either CHV1-type. With regard to CHV1-Euro7 isolates, the fungal genome appeared to contribute to the differences in canker size. After four months, HV isolates harboring either CHV1-type (30%) were retrieved at a significantly higher rate than isolates containing CHV3-County Line (14%). After 12 months, the HV recovery was similar among the three hypoviruses indicating smaller cankers will maintain their HV status after one year. Very few stroma were produced after one year in the field from HV isolates. In vitro, CHV3-County Line (49%) had a significantly lower rate of hypovirus transmission to conidia when compared to CHV1-Euro7 (87%) and CHV1-Ep713 (80%). Significant differences existed among the five different isolates indicating HV transmission is dependent on the fungal genome. This research provided additional evidence that each hypovirus interacts with its host differently and certain isolate/hypovirus combinations have better biological control potential than others.
Highlights
The impact of chestnut blight, caused by the fungal pathogen Cryphonectria parasitica, has diminished in Europe presumably due to a natural biological control caused by hypovirus infection [1] [2]
HV Bockenhauer and Schomberg isolates showed a greater variability in hypovirus effect, again illustrating significantly smaller cankers by CHV3-County Line isolates when compared to their CHV1 counterparts (P = 0.001)
The most important message consequential to this study is that certain isolate/hypovirus combinations have better biological control potential than others
Summary
The impact of chestnut blight, caused by the fungal pathogen Cryphonectria parasitica, has diminished in Europe presumably due to a natural biological control caused by hypovirus infection [1] [2]. When compared to CHV1-Euro, another European CHV1-type isolate, the level of nucleotide identity for the entire 5’ non-coding domain is 93%; the CHV1-Euro genome is 11 nucleotides shorter than the CHV1-Ep713 genome [14]. Despite the similarity, these two isolates have phenotypic and morphological traits that differ considerably [5]. CHV1-Ep713 is a severely debilitating hypovirus isolate in vitro and produces small cankers and few asexual spores in vivo. CHV1-Euro infection has a milder impact on fungal virulence; this isolate grows rapidly in vitro and produces larger cankers in vivo with moderate asexual sporulation [14]
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