Integrated Biological Control of Alternaria Leaf Spot in Spinach through Trichoderma harzianum and Bacillus subtilis Mediated Disease Suppression and Growth Promotion

Abstract

Spinach (Spinacia oleracea L.) is a cool-season annual plant native to countries in the southwestern and central regions of Asia. Alternaria alternata, the causal agent of Alternaria leaf spot, is a major constraint to spinach production worldwide. The objective of this study was to determine the relative efficacy of biological control agents (BCAs) under lab and field conditions. In vitro testing showed that Bacillus subtilis provided the greatest early inhibition (50.0% inhibition at 8 days). Trichoderma harzianum showed the most consistent antagonistic activity (29.67% inhibition at 12 days) when compared to the other BCAs. Based on consistent in vitro antagonistic performance, T. harzianum and B. subtilis were selected for evaluation under field conditions using a Randomized Complete Block Design (RCBD). The field trial showed that the treatment involving BCAs (T. harzianum + B. subtilis) resulted in significantly lower severity scores of 12.0%, representing a 65.7% reduction compared to the untreated control (35.2%). The T. harzianum + B. subtilis treatment also provided the greatest vigour in plant growth with higher fresh weight (27.30 g) and total dry weight (2.25 g). These improvements are likely associated with the known mycoparasitic activity of T. harzianum and the induction of plant defence responses by B. subtilis. The data support the idea that a combination of BCAs could be a viable, eco-friendly option for integrated management of Alternaria leaf spot in spinach production.