Chitosan Mitigates Phytophthora Blight in Chayote (Sechium edule) by Direct Pathogen Inhibition and Systemic Resistance Induction

Phytophthora blight, caused by Phytophthora capsici, is a destructive disease that significantly constrains the production of chayote (Sechium edule) in Mexico, leading to substantial yield and economic losses. The increasing ineffectiveness of synthetic fungicides and associated environmental concerns underscore the need for sustainable control alternatives. This study evaluated the antifungal efficacy of low molecular weight chitosan (75–85% deacetylation; Sigma-Aldrich) against P. capsici under in vitro and in vivo conditions. Chitosan solutions (0.1–3.0 g L−1) were tested for their ability to inhibit pathogen growth and suppress disease symptoms. In vitro assays demonstrated a concentration-dependent inhibition of mycelial growth, with the highest dose (3.0 g L−1) reducing radial expansion by 32.6%. In fruit inoculation experiments, treatment with 1.0 g L−1 chitosan decreased lesion size by 50.9%, while the same concentration reduced disease severity index (DSI) by 50% in whole plants. Notably, symptom suppression was observed in tissues not directly exposed to chitosan, suggesting the activation of systemic resistance. Although the underlying molecular mechanisms were not directly assessed, the results support the dual role of chitosan as a direct antifungal agent and a potential inducer of host defense responses. These findings highlight the potential of chitosan as a biodegradable, low-toxicity alternative to synthetic fungicides and support its integration into sustainable management strategies for Phytophthora blight in chayote production systems.