Ultrastructural insights of infection and biological control of Alternaria alternata Kl3 mediated leaf spot disease in kohlrabi.

Rapid shifts in agro-climatic conditions have accelerated the emergence and severity of fungal plant diseases necessitating continuous pathogen monitoring and development of sustainable control strategies. Consequently, plant-beneficial bacteria-based biological control is one such prime and widely used strategy. Leaf spot diseases caused by Alternaria spp. pose a serious threat to Brassica cultivation, yet etiological and mechanistic insights into kohlrabi (Brassica oleracea var. gongylodes) leaf spot disease remian limited. In this comprehensive study a dominant fungal isolate Kl3, was recovered from diseased leaves of kohlrabi in Santiniketan, India. Based on combined micromorphological features and multilocus phylogeny (ITS-5.8S, gapdh, tef1, alt a1); Kl3 was identified as Alternaria alternata Kl3; which successfully produced disease symptoms in detached kohlrabi leaves ensuring its phytopathogenic potential. Considering the increasing risks of fungicide resistance, the biocontrol potential of plant growth promoting rhizobacteria (PGPR); including Paenibacillus sp. DRP1 and Enterobacter sp. DRP2 was evaluated. Both strains significantly inhibited conidial germination and induced severe mycelial damage to Kl3; which was observed using calcofluor white-based fluorescence microscopy and FE-SEM analysis. Chitinase and protease were the predominant antifungal compounds. Furthermore, FE-SEM analysis provided unique ultrastructural insights and established a timeline for Kl3 infection, starting from conidial germination, direct and indirect invasion by Kl3 mycelia, colonization in mesophyll tissue and conidiogenesis on kohlrabi leaves. Concurrently, PGPR inoculation resulted in significant disease reduction, elicitation of plant defense response and improved kohlrabi yield even in the presence of Kl3 in vivo.