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

Plant Dis. 2023 Feb 1. doi: 10.1094/PDIS-10-22-2424-RE. Online ahead of print.

ABSTRACT

Downy mildew is a common, widespread disease affecting the leaves of sweet basil, which is the herb grown the most globally. No tolerance for disease symptoms, especially on basil leaves marketed 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, as well as products in development for this use. Most fungicides tested are biopesticides. Products tested that currently are 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). Seven other products were tested. A conventional fungicide, Revus (mandipropamid), was included in most experiments serving as a positive control for comparison. Applications were made weekly to foliage with a backpack sprayer starting before symptoms were seen in the plots, except in 2013 when disease onset was early and in 2015 when a twice weekly application schedule was used. Organic treatments tested in 2013 started with a soil drench application around the base of plants two days after they were transplanted. Fungicide efficacy was assessed based on incidence of symptomatic leaves rather than disease severity which is stringent but realistic because there is no tolerance for disease on fresh-market herbs. None were effective based on weekly severity assessments or AUDPC values, which confirms results obtained by other researchers that downy mildew cannot be effectively managed with organic fungicides applied to susceptible cultivars of basil.

PMID:36724027 | DOI:10.1094/PDIS-10-22-2424-RE