Effect of biocontrol agents and potassium phosphite against Phytophthora crown rot, caused by Phytophthora capsici, on zucchini in a closed soilless system

Five biocontrol agents and potassium phosphite, used at various concentrations and at a different number of applications, have been tested to establish their ability to control Phytophthora capsici on hydroponically grown zucchini plants. In a first set of trials, various experimental biocontrol agents (Trichoderma sp. TW2, a mixture of Pseudomonas FC7B, FC8B, FC9B, Fusarium solani FUS25 and Pseudomonas sp. PB26) and a commercial formulation of Trichoderma gamsii +T. asperellum (Remedier) were applied at the artificial infestation with the pathogen of a peat substrate, 5-7 days before planting the zucchini seedlings, and later at 5-day-intervals. BCAs were compared with a potassium phosphite-based fertiliser. In a second set of trials, the potassium phosphite fertiliser was applied directly to the growing media or via a nutrient solution every 6 days, starting at the infestation with the pathogen and 5-7 days before planting, in order to select the optimal rate, type and number of applications. Potassium phosphite reduced Phytophthora crown rot of zucchini by 62-94%, providing more consistant disease severity reduction than those achieved using the experimental BCAs, alone or in mixture, and the Trichodenna gamsii + T. apserellum formulated mixture (29-47% reduction in disease severity). One application of potassium phosphite, at the highest tested concentration, was less effective than three applications. Potassium phosphite consistently reduced the severity of Phytophthora crown rot under different disease pressure (by 48-79%) when applied via a treated peat growing media or via a nutrient solution with 3-6 applications, thereby offering growers an important opportunity to control P. capsici on soilless grown zucchini.