Organic Control of Basil Downy Mildew

By Joan Allen

Basil growers in the northeast have been battling downy mildew since it was first reported in the region in 2008.  While the pathogen, Pernospora belbahrii, does not overwinter in our area, wind-borne spores move in from the south over the course of the growing season with infection typically being confirmed by late July to early August in Connecticut.  Symptoms include chlorosis or yellowing delimited by the veins followed by the death of the leaves.  Spores (sporangia) are produced on the undersides of the leaves in association with the chlorosis, giving it a dirty appearance and reducing the marketability of the basil for fresh market sale.

Sweet basil (Ocimum basilicum) varieties tend to be the most susceptible as shown in a trial done at Rutgers University (Wyenandt, 2010).  The citrus and spice basil varieties tended to be less susceptible.

IN a study at two locations in Connecticut in 2011, five organic control products were evaluated for control of basil downy mildew.  Sweet basil “Genovese’ and lemon basil were grown in plots at one site and only ‘Genovese’ was grown at the other.  Based on first detection dates for disease in previous seasons, products were applied preventatively beginning in late July.  Disease was first detected in all ‘Genovese’ plots at both locations by early August.

Products tested included Milstop, OxiDate, Trilogy, Serenade MAX and Actinovate AG.  Treatments were applied weekly at recommended rates using a CO₂ pressurized backpack sprayer.  The boom was equipped with three nozzles to facilitate thorough coverage of both upper and lower leaf surfaces.  After disease was confirmed in the field, OxiDate was applied twice weekly (weather permitting) and the spray volume was doubled by going over each plot twice per the request of the manufacturer.  Disease was confirmed during the third week of treatments.

Treatments were applied for five weeks.  Disease was evaluated during the sixth week by rating the plants as follows:  0 = no symptoms or sporulation, 1 = symptoms with 0-10% leaf area with sporulation, 2 = 10-50% leaf areas with sporulation; and 3 = >50% leaf area with sporulation.  Weather was favorable for sporulation and spread of the pathogen with regular rain events at both locations.  Results varied between the two sites.

Plants treated with OxiDate and Milstop had significantly lower disease ratings than the control at both sites.  Actinovate, Serenade MAX and Trilogy resulted in a lower average disease rating at one site but not both.  Only ‘Genovese’ plots were evaluated because the lemon basil showed very few symptoms and no sporulation was observed, demonstrating its resistance to downy mildew.

This study was conducted by Joan Allen, Department of Plant Science and Landscape Architecture at UConn, and was funded by a NE SARE Partnership Grant.