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 whether fungicides affect development of leaf lesions or fruiting bodies by Stagonosporopsis citrulli. Cyprodinil plus fludioxonil (Switch), cyprodinil plus 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 area under the disease progress curve (AUDPC) in field plots compared with water. Cyprodinil plus fludioxonil and cyprodinil plus 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 with water (P < 0.05). Cyprodinil plus fludioxonil and cyprodinil plus 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 with 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.