Sweet Pepper cv. Lai Lai Ripeness Stage Influences Susceptibility to Mycotoxinogenic Alternaria alternata Causing Black Mold

Sweet pepper (Capsicum annuum) is a highly nutritious and economically important vegetable grown worldwide. Black mold, caused by mycotoxin-producing Alternaria spp., is a common postharvest disease during cold storage and transport, leading to significant produce losses. A better understanding of the infection process is essential for improving disease control. This study examined Alternaria alternata isolates infecting green (mature unripe) and red (ripe) pepper fruit. Findings indicate that black mold can infect fruit at both ripening stages, with differences in symptom progression, growth rates, and sporulation. Disease development was influenced by fruit ripeness in a temperature-dependent manner. At 7°C, lesion size and sporulation were similar on green and red fruit, but at 22°C, lesions were significantly larger on red fruit (P < 0.05). Microscopic studies revealed comparable conidial germination on both fruit stages; however, appressoria formation was less frequent on green fruit early in infection. Fungal penetration into the pericarp occurred 8 hours post-inoculation through cuticle wounds, with hyphae growing intercellularly among pericarp walls. By 24 hours post-inoculation, cell contents were disorganized, and cell walls had dissolved. In red fruit, vascular bundles were destroyed, whereas in green fruit, they remained intact. At 22°C, high levels of the mycotoxins altenuene, alternariol, and alternariol monomethyl ether were detected in both green and red infected fruit. The susceptibility of mature green fruit to black mold highlights the need for effective field treatments to prevent pathogen establishment and reduce postharvest disease.