Identification of Alternaria Species Causing Spike‐Stalk Brown Spot in ‘Shine Muscat’ Grapes and Its Biocontrol by Bacillus velezensis LJa5

ABSTRACT ‘Shine Muscat’ grapes are economically high‐value table grapes, prized for their rose‐like fragrance and superior edible quality. Spike‐stalk brown spot has recently emerged as a major disease affecting yield and fruit quality in Sichuan Province, yet its causal agents and sustainable management strategies remain poorly understood. In this study, we isolated two causal organisms of spike‐stalk brown spot in ‘Shine Muscat’ grapes from infected rachises and fruits and morphological and multi‐gene phylogenetic analyses (ITS, RPB2 , LSU , Alt a 1 , GAPDH ) identified them as Alternaria tenuissima and A. alternata . Strain LJa5 was isolated from healthy plants and rhizosphere soil, and dual culture screening identified it as highly inhibitory against both Alternaria species (76.03% and 82.42% inhibition). Furthermore, its cell‐free supernatant inhibited the two Alternaria strains. Morphological, biochemical and phylogenetic analyses (16S rRNA, gyrA ) confirmed LJa5 as Bacillus velezensis . The strain exhibited broad‐spectrum antifungal activity against 14 pathogens (59.24%–81.83% inhibition), produced extracellular hydrolases and possessed motility and biofilm formation capacity. In vitro fruit assays demonstrated that LJa5 suspension at 1.0 × 10 9 CFU/mL achieved control efficacies of 49.96% and 51.15% against diseases caused by A. tenuissima and A. alternata , respectively. This study represents the first report identifying the causal agents of mixed infection responsible for spike‐stalk brown spot in ‘Shine Muscat’ grapes, and demonstrates the potential of B. velezensis LJa5 as a safe and effective biological control agent. These findings provide a foundation for sustainable, environmentally friendly disease management strategies in grape production.