Pathogenicity of ten Phaeoacremonium species associated with esca and Petri disease of grapevine

Abstract

Nineteen species of Phaeoacremonium have been associated with grapevines in South Africa, of which only six species have been confirmed as pathogens through pathogenicity tests conducted on field-grown grapevines. This study determined the pathogenic status of ten Phaeoacremonium spp. recently found for the first time on South African grapevines. These were: Pm. australiense, Pm. austroafricanum, Pm. fraxinopennsylvanicum, Pm. griseo-olivaceum, Pm. griseorubrum, Pm. iranianum, Pm. italicum, Pm. prunicolum, Pm. scolyti and Pm. sicilianum. In the pathogenicity tests, Ph. parasiticum was used as the positive control, and sterile water as the negative control. Up to three isolates were used per species, depending on isolate availability. Freshly cut pruning wounds in a 9-year-old Cabernet Sauvignon vineyard in Stellenbosch, South Africa, were inoculated with 200conidia of each fungus per wound. Inoculated pruning wounds were removed after 18 months, cut longitudinally and lesion lengths were measured. Re-isolation proportions were determined by conducting isolations from inoculated spurs. All the inoculated isolates successfully colonized pruning wounds, and caused lesions that were significantly different from the negative control. All isolates were re-isolated at proportions varying from 28.6 to 85.7%. Phaeoacremonium griseo-olivaceum STE-U 7859 produced the longest lesions (mean = 79.5 mm) and Pm. iranianum STE-U 6998 the shortest (62.0 mm). No statistically significant differences in mean lesion lengths were observed between the inoculated species.  There were also no significant differences between isolates of the same species, except in Pm. prunicolum where isolate STE-U 5968 produced longer lesions (mean = 77.3 mm) than STE-U 7857 (62.3 mm). This study confirmed the capabilities of all the tested Phaeoacremonium spp. to infect grapevine pruning wounds and cause lesions. The study also confirmed the importance of pruning wounds as ports of entry by these pathogens into host plants.