Premix Fungicides that Reduce Development of Fruiting Bodies but not Leaf Lesions by Stagonosporopsis citrulli on Watermelon Leaves in the Field
Fungicide applications are the main method to manage gummy stem blight on watermelon (Citrullus lanatus) and other cucurbits, but it is unknown if fungicides affect development of leaf lesions or fruiting bodies by Stagonosporopsis citrulli. Cyprodinil + fludioxonil (Switch), cyprodinil + difenoconazole (Inspire Super), cyprodinil (Vangard), fludioxonil (Cannonball), and difenoconazole (Inspire) were applied to watermelon in rotation with chlorothalonil (Bravo) in fall 2017, 2018, and 2019. Water and chlorothalonil applied weekly served as control treatments. All fungicides reduced disease severity (percentage of leaf area diseased) and AUDPC in field plots compared to water. Cyprodinil + fludioxonil and cyprodinil + difenoconazole reduced disease severity and AUDPC more than chlorothalonil. Fungicides did not affect the number, diameter, expansion, or area of lesions. All fungicides reduced the number of lesions with fruiting bodies of S. citrulli compared to water (P < 0.05). Cyprodinil + fludioxonil and cyprodinil + difenoconazole reduced the percentage of leaf lesions with fruiting bodies, and the diameter and area of the portions of leaf lesions covered with fruiting bodies, compared to water and chlorothalonil. Premix fungicides containing cyprodinil reduced fruiting body formation by S. citrulli, which may partially explain their efficacy in managing gummy stem blight.