Reassessing the Etiology of Aspergillus Vine Canker and Summer Bunch Rot of Table Grapes in California

Abstract

Fungal taxonomy is in constant flux and the advent of reliable DNA barcodes has enabled the enhancement of plant pathogen identification accuracy. In California, Aspergillus Vine Canker (AVC) and Summer Bunch Rot (SBR) are economically important diseases that affect the wood and fruit of grapevines, respectively, and their causal agents are primarily species of black aspergilli (Aspergillus section Nigri). During the last decade, the taxonomy of this fungal group has been rearranged several times using morphological, physiological and genetic analyses, resulting in the incorporation of multiple cryptic species that are difficult to distinguish. Therefore, in this study, we aimed to reassess the etiology of AVC and SBR using a combination of morphological observations with phylogenetic reconstructions based on nucleotide sequences of the calmodulin (CaM) gene. Results revealed that the isolates causing AVC from recent isolations corresponded to A. tubingensis, whereas the isolates obtained from initial surveys when the disease was discovered were confirmed as A. niger and A. carbonarius. Similarly, the isolates obtained from table grapes with SBR symptoms and from spore traps placed in those vineyards were identified primarily as A. tubingensis, followed by A. niger and A. carbonarius. Notably, the A. niger isolates formed a subclade with strains previously known as A. welwitschiae, a species that was recently synonymized with A. niger. Overall, the most prevalent species was A. tubingensis, associated with both AVC and SBR, and representative isolates recovered from AVC-symptomatic wood, berries with SBR symptoms, and spore traps were equally pathogenic in healthy wood and berries of ‘Red Globe’ grapevines. This study also constitutes the first report of A. tubingensis causing Aspergillus Vine Canker and Summer Bunch Rot of grapes in California and in the United States.