| Summary: | Phytophthora blight of pepper caused by <i>Phytophthora capsici</i> is a major constraint to bell pepper (<i>Capsicum annuum</i>) production. The long-term effectiveness of chemicals currently in use against <i>P. capsici</i> is uncertain due to the development of fungicide resistance by this pathogen. Hence, the efficacy of alternative chemicals such as phosphorous-acid-containing products was evaluated in this study. In in vitro tests, ProPhyt, K-Phite, Lexx-A-Phos, Agri-Fos, and Nutri-Phite were less effective in inhibiting mycelial growth (EC<sub>50</sub> = 50.5 to 324.4 µg mL<sup>−1</sup>) and sporangium formation (EC<sub>50</sub> = 6.1 to 225.7 µg mL<sup>−1</sup>) of two <i>P. capsici</i> isolates, but more effective against zoospore germination compared with mefenoxam. Among phosphorous-acid-containing products tested, Nutri-Phite was most effective in inhibiting mycelial growth of both <i>P. capsici</i> isolates. In greenhouse studies, Nutri-Phite was effective against Phytophthora blight used as drench. The use of Nutri-Phite, Agri-Fos, ProPhyt, and K-Phite could induce systemic resistance against foliar blight when applied to the root and potting mix. The results indicated that some phosphorous-acid-containing products have the potential to lower disease occurrence and delay Phytophthora blight of bell pepper without phytotoxic effects. The utility of the systemic protection induced by these products is promising in Phytophthora blight management.
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