Downy Mildew of Basil in Illinois: Occurrence and Management

Downy mildew of basil, caused by Peronospora belbahrii, was first diagnosed in Illinois in 2009. Since then, this disease has occurred in Illinois every year. P. belbahrii infects leaves, rapidly multiplies and spreads, and can cause total crop loss. Infection of basil by P. belbahriiin Illinois can occur from late April until about the middle of October, but it is more common during August–October. Trials were conducted during 2009–12 in commercial basil fields near Momence (Kankakee County, IL) to evaluate efficacy of 14 fungicides for conventional production and eight compounds for organic productions of vegetables for control of downy mildew of basil. The plots were sprayed using 60 gallons of water/A at a weekly schedule.  None of the compounds for organic production provided effective control of downy mildew in basil plots. In contrast, most of the fungicides applied, either alone, combined together, or combined with spreader–sticker Induce 90 (0.125% at v:v) provided effective control of downy mildew.  The most effective fungicides were: chlorothalonil (Bravo Weather Stik 6F, 2 pt/A); dimethomorph (Forum 4.16SC, 6 fl oz/A); mancozeb + zoxamide (Gavel 75DF, 2 lb/A); fluopicolide (Presidio 4SC, 4 fl oz/A); azoxystrobin (Quadris 2.08SC, 15.5 fl oz/A); cyazofamid (Ranman 400SC, 2.75 fl oz/A); mandipropamid (Revus 2.09SC, 8 fl oz/A);  famoxadone + cymoxanil (Tanos 50DWG, 10 oz/A), ametoctradin + dimethomorph (Zampro 525SC, 14 fl oz/A), experimental fungicide-I (38.6 fl oz/A); and experimental fungicide-II (38.6 fl oz/A. The severity of downy mildew in the plots that received any of above-mentioned fungicides combined with phosphonate (ProPhyt 4L, 3 pt/A) and Induce 90 did not exceed 5%; while the severity of the disease in control plots and plots that received compounds for organic production was 85% and greater than 75%, respectively.  Currently, basil growers in Illinois apply Quadris plus ProPhyt alternated with Ranman plus ProPhyt alternated with Revus plus ProPhyt under special permit of section 18.