Bacillus Species Interfere with Infective Structures of Colletotrichum spp. and Reduce Symptoms of Apple Bitter Rot in Post-Harvest.

Colletotrichum species cause bitter rot (BR) in apples, with symptoms typically manifesting at harvest or during storage. The efficacy of chemical control is limited, and the use of fungicides may lead to the selection of resistant isolates. As a sustainable alternative, biological control methods, i.e., Bacillus spp., have been used to control diseases caused by Colletotrichum spp. This study evaluated the effects of B. subtilis, B. velezensis, and B. amyloliquefaciens on conidial germination (CG), mycelial growth (MG), and suppression of BR in apples. The impact of these bacterial strains on MG of five Colletotrichum species (C. chrysophilum, C. limetticola, C. melonis, C. nymphaeae and C. siamense) was assessed. For B. velezensis, currently under development as a biocontrol product, both the biomass and supernatant fractions were evaluated and compared with the commercial formulations Serenade (B. subtilis) and Duravel (B. amyloliquefaciens). For postharvest control of BR, 'Gala' apples were treated with bacterial suspensions or fungicides and subsequently inoculated with conidia from three Colletotrichum isolates. Captan, dithianon, and metiram+pyraclostrobin served as chemical standards. All the tested Bacillus strains significantly inhibited MG across the Colletotrichum species evaluated. Bacillus velezensis biomass reduced both CG and MG of C. nymphaeae and C. chrysophilum. Metiram+pyraclostrobin inhibited more than 98.2% of CG, while B. amyloliquefaciens achieved 63% inhibition of CG of C. chrysophilum. All fungicides reduced the incidence of BR by over 75% while B. amyloliquefaciens and B. subtilis gave variably reduced incidence of the disease depending on the isolate, experiment, and concentration.