Efficacy of Organic Fungicides for Downy Mildew in Field-Grown Sweet Basil.

Downy mildew is a common, widespread disease affecting basil leaves. No tolerance for disease symptoms, especially on leaves for fresh consumption, necessitates management. 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 Peronospora belbahrii to evaluate fungicides approved for use on organically produced crops and products in development. Products tested currently registered for use on basil in the U.S. and labeled for downy mildew were Actinovate (Streptomyces lydicus), Companion (Bacillus subtilis), Cueva (copper octanoate), Double Nickel (Bacillus amyloliquefaciens), Forticept EP #1 (thyme oil), Milagrum Plus (Bacillus subtilis), Organocide (sesame oil), Oso (polyoxin D zinc salt), OxiDate (hydrogen dioxide), Procidic (citric acid), Regalia (Reynoutria sachalinensis extract), Stargus (Bacillus amyloliquefaciens), and Trilogy (neem oil). Most are biopesticides. A conventional fungicide, Revus (mandipropamid), was included in most experiments as a positive control. Applications were made weekly to foliage with a backpack sprayer starting before symptoms were seen, except in 2013 when disease onset was early and 2015 when applications were made twice weekly. Organic treatments tested in 2013 started with a soil drench application around the base of plants two days after transplantation. Fungicide efficacy was assessed based on incidence of symptomatic leaves rather than disease severity because there is no tolerance for disease on fresh-market herbs. None were effective based on weekly severity assessments or AUDPC values, confirming results from other researchers that downy mildew cannot be effectively managed with organic fungicides applied to susceptible basil cultivars.

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.

Resistance to Triadimefon and Benomyl: Dynamics and Impact on Managing Cucurbit Powdery Mildew.

Frequency of fungicide-resistant strains of Podosphaera xanthii on pumpkins in New York before treatment varied from 3 to 80% for the demethylation inhibiting (DMI) fungicide triadimefon and from 0 to 48% for the benzimidazole fungicide benomyl between 1993 and 1996. When the initial frequency of triadimefon-resistant strains was less than 55%, one application of triadimefon plus chlorothalonil was effective. This application was made after reaching the action threshold of one leaf with powdery mildew symptoms per 50 old leaves (defined as the oldest third of the foliage). The frequency of triadimefon-resistant strains increased from 3 to 71% by 20 days after the first fungicide application in 1993. Triadimefon in the second application did not contribute to control. Loss of efficacy was due to resistance because, compared with triadimefon-treated pumpkins, pumpkins treated with other systemic fungicides were less severely infected by powdery mildew on abaxial leaf surfaces where the companion multi-site contact fungicide contributes little to control. Triadimefon was not effective in 1995 when 80% of the pathogen population was resistant before treatment. Benomyl was effective in 1995, but not in 1996 when 48% of the isolates tested were resistant to both benomyl and triadimefon before treatment. An in-field seedling assay was developed to determine local occurrence of resistant strains before the first treatment was needed. Although sensitivity of the pathogen population to the DMI fungicides myclobutanil and propiconazole also decreased after they were applied, these fungicides were more effective than triadimefon.