Virulence in the chestnut blight pathogen, Cryphonectria parasitica can be altered by the invasion of hypoviruses into the pathogen population.  Hypovirus infection reduces pathogen growth rate, which allows the chestnut host, Castanea dentata, to limit canker expansion and prevent girdling and death of stems.  In consequence, chestnut populations can recover when hypoviruses spread throughout the C. parasitica population.  However, spread and persistence of hypoviruses is limited by vegetative incompatibility (vic) within the pathogen population.  In C. parasitica, vegetative incompatibility is governed by at least six vic loci, with each locus having two alleles.  Mismatches of alleles at any vic locus cause apoptosis at the point of hyphal fusion, which inhibits the spread of hypoviruses that reside in the fungal cytoplasm.  We characterize vic diversity within seven C. parasitica populations in Michigan:  Two populations have long-standing blight epidemics without hypoviruses, three populations have been invaded by hypoviruses, and the two remaining populations have blight epidemics that were initiated in the last decade.  Thirty to fifty single-spore isolates from each population were paired on petri dishes containing chestnut pieces and allowed to grow together.  Formation of a barrage line at the point of hyphal fusion was evidence of vegetative incompatibility. 

Results/Conclusions

Each population contains two common vegetative compatibility (vc) groups which collectively represent greater than 45% of the total C. parasitica population.  Stivers, a population with a long-standing epidemic had a total of 18 vc groups, which is an increase from nine vc groups in 1996 when the population was initially sampled.  In contrast, the other population with a long-standing epidemic, Missaukee diseased, had only four vc groups.  One newly infected population near Missaukee diseased, Missaukee healthy, also appears to have four vc groups.  Leelanau, another newly infected population has up to 11 vc groups.  The vc structure at the three populations where hypovirus has spread and trees are recovering, County Line, Frankfort, and Roscommon, are all dominated by one or two vc groups with an aggregate frequency greater than 90%, however, the common vc group at each site differs.  In addition, there has been little change in vc structure between 1996 and 2009 at these three sites.  The low diversity and stability at hypovirus sites is probably due to the fact that hypoviruses prevent sexual reproduction in C. parasitica.