Combined application of Pseudomonas chlororaphis strains and zinc suppresses Alternaria alternata leaf spot in chili pepper

ABSTRACT Chili pepper ( Capsicum annuum L.) is a vital vegetable crop susceptible to yield loss from Alternaria alternata (Fr.), Keissl., which causes leaf spot/blight and fruit rot. Harnessing beneficial rhizobacteria offers an eco-friendly strategy to enhance plant resilience to biotic stressors. In vitro screening identified Pseudomonas chlororaphis strains viz., P. chlororaphis subsp. aurantiaca (ARS-38) and P. chlororaphis subsp. chlororaphis (RP-4) as potent antifungal strains, inhibiting A. alternata growth by 62% and 64%, respectively. In a zinc (Zn)-amended medium, their antifungal activity increased further, reducing fungal growth by 70% with individual strains and 93% when combined. Microscopy revealed fungal growth inhibition linked to conidia and conidiophore disintegration. In planta under open-field pot conditions, A. alternata infection resulted in 100% disease incidence (DI) and altered histochemical, photosynthetic, and enzyme responses. Treatment with ARS-38 + RP-4 in Zn-amended soil enhanced plant resistance by increasing lignin and phenolic accumulation and reducing enzyme activities, resulting in a 96% reduction in DI. Zinc supplementation improved plant health and enhanced disease suppression when combined with the bacterial strains. Combining rhizobacteria with Zn (2 mg/kg) emerges as a promising strategy to mitigate Alternaria leaf spot disease and enhance biostress tolerance in chili plants. IMPORTANCE Biological control offers a sustainable approach for reducing fungal diseases while limiting reliance on chemical fungicides. This study shows that two Pseudomonas chlororaphis strains, ARS-38 and RP-4, combined with low-level zinc supplementation, strongly suppress Alternaria alternata , an important chili pepper pathogen. The combined treatment reduced fungal growth in vitro and substantially decreased disease development in plants while improving structural, physiological, and biochemical responses associated with plant health. The findings demonstrate the potential value of integrating beneficial rhizobacteria with micronutrient supplementation for sustainable disease management and provide a basis for further development of microbial–micronutrient strategies for crop protection.