Biology and control of chestnut blight disease caused by Cryphonectria parasitica (Murr.) Barr in Massachusetts

One hundred and two virulent (V) strains of Cryphonectria parasitica were isolated from chestnut blight cankers on American chestnut trees (Castanea dentata) in western Massachusetts, and vegetative compatibility was investigated. The 102 strains fell into 54 vegetative compatibility groups (VCGs): 10 groups contained more than 2 strains in each, 6 groups had 2 strains per VCG, and 38 VCGs contained only one strain each. A wide diversity of VCGs may be due to the presence of C. parasitica in Massachusetts for at least 80 years. A representative strain from each of the 10 VCGs with more than two strains, was paired with 22 hypovirulent (H) strains not native to Massachusetts (4 strains with French double-stranded RNA (dsRNA), 17 strains with Italian dsRNA, and 1 strain with American dsRNA). Some of the V strain could be converted to hypovirulent by the French and/or Italian H strains, but none were converted by the American H strain. Ten V strains from different VCGs were compared with 25 H strains, and with 29 strains converted to hypovirulence in cultural characteristics, pathogenicity, and phenol oxidase activity. Fungal growth, colony color, and pycnidial development were compared on various media. Fungal growth could be accentuated by the addition of ground bark or wood tissue to the media. Cultural characteristics were consistent among the V strains, but much variability was noted with the H strains. A rapid method (bark/wood test) for testing the virulence was developed, and compared with living tree inoculations and excised stem sections. The bark/wood test gave the fastest and most consistent results. Phenol oxidase activity of V strains was much stronger than those of H strains on the modified Bavendamm\u27s medium. Several H strains were cured by the treatments of glucose and cyclic nucleotides (cAMP, cGMP). When the H strains were cured, they took on the cultural characteristics of normal V strains and virulence was restored. DsRNA bands were not detectable by gel electrophoresis from the cured strains, while uncured strains retained dsRNAs. It is assumed that RNA-dependant RNA polymerase gene, which regulates RNA replications, was depressed by cyclic nucleotides. Subsequently, RNA replication was inhibited and virulence returns as all genes can be expressed