Efficacy of Conventional Fungicides for Downy Mildew in Field-Grown Sweet Basil in the United States

Application of fungicides has been the main management practice for Peronospora belbahrii, which is the most important pathogen of sweet basil in the United States. Six replicated experiments were conducted between 2010 and 2016 with field-grown basil of a susceptible cultivar exposed to naturally occurring wind-dispersed sporangiospores of P. belbahrii to evaluate conventional fungicides registered for basil downy mildew in the United States and in development for this use. This project revealed the importance for successful management of using a preventive fungicide application schedule, maintaining a 7-day application interval, and using application equipment designed to provide thorough spray coverage to plants (drop nozzles). Fungicide efficacy was assessed based on incidence of symptomatic leaves rather than disease severity, which is stringent but realistic because there is zero tolerance for disease on fresh-market herbs. Most fungicides were tested as the formulated product marketed in the United States. Oxathiapiprolin was tested as experimental formulations. Its trade name is Orondis. Overall best control was achieved in 2016. Excellent control (99% based on AUDPC values) was obtained with four fungicide programs with oxathiapiprolin, Revus, and ProPhyt, indicating this combination of chemistry was more important than specific timing for each fungicide. Ranman applied in alternation with Revus plus K-Phite was not quite as effective (89% control); this treatment was ineffective in 2015 when the 7-day spray interval was not maintained. Best treatment in 2015 was Quadris applied in alternation with Revus plus oxathiapiprolin for two of three Revus applications. Two different alternations of these fungicides also were effective. But Quadris alternated with Revus was ineffective. When tested singly, the most effective fungicides in 2013 (listed in order based on AUDPC values) were Zampro, Revus, oxathiapiprolin, and Ranman. ProPhyt was effective in 2013 but not in 2012, when another phosphorous acid fungicide, K-Phite, also was ineffective. Only oxathiapiprolin and Zampro were effective in the 2012 experiment; Revus and Ranman were ineffective. Presidio was ineffective both years. Based on the results from this study, Orondis is the most effective fungicide among those evaluated for managing basil downy mildew, and Zampro is second. Neither were labeled for this use on field-grown basil as of June 2020. Ranman applied in alternation with Revus plus K-Phite, a commonly recommended program of labeled fungicides, provided very good control.

Keywords: Ocimum basilicum; Peronospora belbahrii; chemical control.