Biological Control of Downy Mildew and Yield Enhancement of Cucumber Plants by Trichoderma harzianum and Bacillus subtilis (Ehrenberg) under Greenhouse Conditions

The downy mildew disease of cucurbits is considered the most economically damaging disease of Cucurbitaceae worldwide. The causal agent, Pseudoperonospora cubensis (Berkeley & Curtis), may cause complete crop losses of cucurbits. Few commercial cucurbit cultivars are resistant to this disease. Commercially, P. cubensis is controlled primarily with synthetic fungicides that inhibit or eliminate the pathogen. Several biological agents have also been identified that provide some level of control. In our study, foliar applications of three strains of Trichoderma harzianum and two native strains of Bacillus subtilis were evaluated for the control of the disease on cucumber plants grown under commercial greenhouse conditions. The study was conducted using a completely randomized design with six individual treatments during two production cycles: fall 2015 and spring 2016. The response variables included disease incidence and severity, plant height, total yield, fruit quality, and weight. B. subtilis provided the best control over the incidence and severity of the disease in both production cycles. Interestingly, while T. harzianum was less effective at controlling the disease, it enhanced plant growth and productivity, and produced a higher number of better-quality fruits per plot. This increased yield with higher quality fruits may result in higher profit for the growers.