# Synergistic protective effects of rhizobacterial culture filtrate and zinc oxide nanoparticles against Pantoea leaf spot in cucumber

## TL;DR

This study shows that combining zinc oxide nanoparticles and Bacillus culture filtrate can effectively control bacterial leaf spot in cucumbers while improving plant health and yield.

## Contribution

The study demonstrates a novel synergistic approach using ZnO-NPs and Bacillus culture filtrate for sustainable disease management in cucumber crops.

## Key findings

- The co-application of ZnO-NPs and Bacillus culture filtrate reduced leaf spot severity to a symptom score of 1.1 on a 1–7 scale.
- The combined treatment increased chlorophyll content, phenolic content, and nutrient levels in cucumber plants.
- The treatment maximized productivity with 14.2 fruits per plant and a total yield of 2.42 kg/plant.

## Abstract

Global food security is increasingly threatened by plant diseases and the excessive use of chemical pesticides and fertilizers, highlighting the urgent need for sustainable and environmentally friendly alternatives. Nanotechnology and biological agents such as plant-growth-promoting rhizobacteria (PGPR), particularly Bacillus spp., offer cutting-edge strategies for sustainable crop protection. This study investigates the individual and synergistic effects of Bacillus culture filtrate (BCF) and zinc oxide nanoparticles (ZnO-NPs) in controlling Pantoea sp., the causal agent of bacterial leaf spot (BLS) in cucumber, and enhancing plant performance under greenhouse conditions. Antimicrobial activity was assessed in vitro, Bacillus-derived secondary metabolites were identified via gas chromatography–mass spectrometry (GC–MS), and plant health indicators—including disease severity, physiological traits, and fruit yield—were evaluated under greenhouse conditions. The in vitro assays revealed that ZnO-NPs exhibited a concentration-dependent antibacterial effect, with inhibition zones increasing from 3 mm at 0 ppm to 38 mm at 200 ppm. Meanwhile, Bacillus culture filtrate alone produced a significant inhibition zone of 40 mm against Pantoea sp. GC–MS analysis of the methanolic extract of Bacillus sp. identified twenty bioactive compounds, many of which are known for their antimicrobial and plant growth-promoting properties, likely contributing to the observed effects. Under greenhouse conditions challenged with Pantoea sp., the co-application of 200 ppm ZnO-NPs and Bacillus culture filtrate significantly reduced bacterial leaf spot severity in cucumber plants, lowering the symptom score to 1.1 on a 1–7 scale. This combined treatment also significantly enhanced physiological traits, including leaf chlorophyll content (SPAD value: 59.0), total phenolic content (1.76 mg GAE g−1 FW), and leaf macro- and micronutrient levels, while maximizing productivity—achieving the highest fruit number per plant (14.2) and total yield (2.42 kg/plant). These findings support the integration of ZnO-NPs and Bacillus-based biostimulants as a promising eco-friendly strategy for disease management and productivity enhancement in cucumber cultivation.