Sulfur promotes biocontrol of purple blotch disease via Trichoderma spp. and enhances the growth, yield and quality of onion

Onion is one of the most important export crops in Egypt; its productivity and exportation is restricted mainly by purple blotch disease. In vitro and field experiments during two growing seasons were conducted aiming to improve control of Alternaria porri (the pathogen causing purple blotch disease) by the application of combinations of elemental sulfur (S0) and Trichoderma spp. (T. harzianum and T. viride). Fungicide (Ridomil Gold M Z-68% WG) was applied as pesticide treatment control. In vitro experiments revealed that supplementation of S0 at 2 g L−1 to the growth medium that was utilized to culture Trichoderma spp. inhibited (100%) the growth of Alternaria porri. Trichoderma viride showed higher efficiency in controlling purple blotch than T. harzianum, and resulted in disease controlling rates that were similar to those by fungicide (Ridomil Gold). Disease incidences decreased by 63.9% in onion plants that were inoculated with T. viride, compared to 66.3% when fungicide was applied. Also, antioxidant activity of catalase and peroxidase was the highest when S0 was applied at 240 kg ha−1 and the onion plants were treated with T. harzianum and/or T. viride. Application of S0 at 240 kg ha−1, in combination with inoculating onion seedlings with T. harzianum, led to enhanced onion growth and yield over fungicide application (traditional practice of farmers) in both seasons. As a result, green bulb yield (t ha−1) and total soluble solids (TSS, %) were 41.9 and 21.7% higher, respectively; harvested dry bulb (t ha−1), marketable bulb yield and TSS were 33, 35 and 13% higher, respectively; while nonmarketable bulb yield was 21.5% lower than those under fungicide application.