Distribution of chestnut blight and diversity of Cryphonectria parasitica in chestnut forests in Bulgaria

Summary We surveyed chestnut stands at 18 sites in 11 locations in Bulgaria in 2005 and 2007 for the presence of chestnut blight. We found chestnut blight in seven locations (Belogradchik, Berkovitsa, Brezhani, Barziya, Govezhda, Petrich and Petrovo) but not in four others (Tsaparevo, Kresna, Dupnica and Botevgrad). We successfully isolated Cryphonectria parasitica from cankers on 606 trees with symptoms of chestnut blight and assayed them for vegetative compatibility (vc) types and mating type. Three vc types were identified among the 606 isolates; all three were among the European vc types with known vegetative incompatibility (vic) genotypes. Vc type EU-12 was the most common, representing 80% of the isolates, and was found at all locations with blight, with the exception of Belogradchik in north-west Bulgaria, where all isolates were vc type EU-2. Only one population (Barziya) had more than one vc type, with a combination of EU-12 and EU-10 in almost equal frequencies. Similarly, the diversity of mating types was very low. All but three of 536 isolates assayed were in mating-type MAT-1; MAT-2 was only found in one population in the north-west (Berkovitsa). We inspected 671 bark samples from chestnut blight cankers with stromata of C. parasitica and found perithecia in only 33, of which 28 were from Berkovitsa where MAT-2 was present. We did not detect hypoviruses in any of the 270 isolates screened using the standard double-stranded RNA extraction protocol. Similar to results from previous studies in south-eastern Europe, the diversity of vc types and mating type of C. parasitica in Bulgaria is low, and reproduction of the fungus is mainly asexual. Unfortunately, naturally occurring hypovirulence was not detected. Nevertheless, we observed a small number of superficial cankers typical of those caused by C. parasitica isolates infected with a hypovirus.