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.