<i>Bacillus</i> Species Interfere with Infective Structures of <i>Colletotrichum</i> spp. and Reduce Symptoms of Apple Bitter Rot in Postharvest

<i>Colletotrichum</i> 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 using <i>Bacillus</i> spp. has been used for diseases caused by <i>Colletotrichum</i> spp. This study evaluated the effects of <i>B. subtilis</i>, <i>B. velezensis</i>, and <i>B. amyloliquefaciens</i> on conidial germination (CG), mycelial growth (MG), and suppression of BR in apples. The impact of these bacterial strains on MG of five <i>Colletotrichum</i> species (<i>C. chrysophilum</i>, <i>C. limetticola, C. melonis</i>, <i>C. nymphaeae</i>, and <i>C. siamense</i>) was assessed. For <i>B. velezensis</i>, currently under development as a biocontrol product, both the biomass and supernatant fractions were evaluated and compared with the commercial formulations Serenade (<i>B. subtilis</i>) and Duravel (<i>B. amyloliquefaciens</i>). For postharvest control of BR, 'Gala' apples were treated with bacterial suspensions or fungicides and subsequently inoculated with conidia from three <i>Colletotrichum</i> isolates. Captan, dithianon, and metiram + pyraclostrobin served as chemical standards. All the tested <i>Bacillus</i> strains significantly inhibited MG across the <i>Colletotrichum</i> species evaluated. <i>B</i>. <i>velezensis</i> biomass reduced both CG and MG of <i>C. nymphaeae</i> and <i>C. chrysophilum</i>. Metiram + pyraclostrobin inhibited more than 98.2% of CG, while <i>B. amyloliquefaciens</i> achieved 63% inhibition of CG of <i>C. chrysophilum</i>. All fungicides reduced the incidence of BR by over 75% while <i>B. amyloliquefaciens</i> and <i>B. subtilis</i> variably reduced incidence of the disease depending on the isolate, experiment, and concentration.