Evaluation of systemic acquired resistance inducers for control of Phytophthora capsici on squash

Phytophthora blight induced by Phytophthora capsici is a major constraint in vegetable production worldwide. Limited information is available regarding potential systemic acquired resistance (SAR) inducers that may provide protection of squash (Cucurbita pepo) plants against the disease and the direct effect of the products on the pathogen. In this study, the effect of DL-3-aminobutyric acid (BABA), 2,6-dichloroisonicotinic acid (INA), Saver (a.i. salicylic acid), Syrup (nutrient supplement), and acibenzolar-S-methyl (ASM) on mycelial growth, zoospore germination and sporangium production of P. capsici was evaluated. The products were tested in in vitro studies at concentrations ranging from 25 to 2000 μg ml−1. Mycelial growth and zoospore germination were generally not significantly affected by BABA and ASM and sporangium production was not significantly affected by BABA. INA and Saver reduced mycelial growth and sporangium production significantly at 100 μg ml−1 or higher concentrations and zoospore germination at 500 and 1000 μg ml−1. In greenhouse studies, all the products applied as a soil drench or foliar spray at 25 or 50 μg ml−1 significantly reduced disease severity on squash, compared with the pathogen-only control, and zoospores at a concentration of 103 spores ml−1 were used to inoculate the leaves. INA, BABA, and ASM also reduced disease significantly when zoospores at 103 spores ml−1 were used to inoculate the root. The results indicated that most of the SAR inducers did not inhibit the growth of the pathogen at concentrations generally recommended for use but had the potential to suppress the disease on squash significantly.