Diversity, Pathogenicity, and Fungicide Sensitivity of Fungal Species Associated with Late-Season Rots of Wine Grape in the Mid-Atlantic United States

Late-season bunch rots cause major losses in grape production every year in the Mid-Atlantic United States, but the causal agents are not well characterized. In this study, 265 fungal isolates were collected from rotten grapes from 2014 to 2020 and identified to the genus level according to internal transcribed spacer sequences. The most prevalent of the 15 genera were Botrytis, Colletotrichum, Aspergillus, Alternaria, Pestalotiopsis, and Neopestalotiopsis. Of these, isolates within three prevalent, yet understudied, genera were identified to be Aspergillus uvarum, Alternaria alternata, and Neopestalotiopsis rosae. The pathogenicity of these three fungal species was evaluated in two field trials by artificially inoculating wounded and nonwounded grapes (Vitis vinifera) of four cultivars at the phenological stages of bloom, véraison, and preharvest. Upon ripening, fruit were weighed and assessed for severity of multiple diseases. On nonwounded fruit, A. uvarum caused significantly higher disease severity than the control in both seasons. On wounded fruit, each inocula caused significantly higher disease than the respective controls in the first season, but only A. uvarum and Botrytis cinerea caused this in the second season. Also, wounding was found to have a detrimental effect on cluster weight, which was significantly influenced by inoculation timing and cultivar. Lastly, A. uvarum and N. rosae were tested for sensitivity to azoxystrobin, boscalid, and difenoconazole. The A. uvarum isolates were found to be more sensitive to boscalid and difenoconazole in general, with varying sensitivity to azoxystrobin. N. rosae isolates were resistant to boscalid and azoxystrobin but displayed much higher sensitivity to difenoconazole. Evidence from the isolate collection and field trials demonstrates that A. uvarum could be a significant pathogen of wine grapes in the Mid-Atlantic United States. Results from this study will be useful for the identification and management of the understudied Alternaria, Aspergillus, and Neopestalotiopsis fruit rots of wine grapes.