Cause An oomycete, Peronospora belbahrii, that can be spread through contaminated seed and produces airborne spores, which can disperse and spread disease. The first United States report (October 2007) was from the Southeastern United States. Basil downy mildew was found in many states, including California, during 2009 and by 2011, it was discovered in Oregon. It can be a problem in both field- and greenhouse-grown basil crops, as well as in home gardens. Downy mildew is promoted by leaf wetness as well as high relative humidity, and has a wide temperature range.
Symptoms Leaves have a yellowing, which resembles a nutritional deficiency, and on the undersurface of an affected leaf, purplish-brown sporulation can be observed.
- Plant seed not infested with the basil downy mildew pathogen (MT McGrath of Cornell University reports that a seed testing procedure is being developed).
- Time irrigation and plant spacing to promote aeration and minimize leaf wetness and reduce relative humidity.
- Control greenhouse humidity with fans, lights, and increasing temperature.
- Bury or destroy infected plants after the final harvest, to reduce spores in the air. Disturbed spores can be killed by the UV radiation of bright sunlight.
- Cultivar evaluations to date have shown that sweet basil (Ocimum basilicum) is very susceptible while Ocimum citriodorum and Ocimum americanum are more resistant. Plant a less susceptible variety:
Susceptible O. basilicum varieties: 'Amethyst Imp', 'Aroma 2', 'Cinnamon', 'Gecofure', 'Genovese', 'Genoveser Martina', 'Italian Large Leaf', 'Magical Michael', 'Mariden', 'Nufar', 'Opal Purple Variegated', 'Poppy Joe's', 'Queentette', 'Rubin' and 'Superbo'.
Moderately Resistant O. basilicum varieties: 'Red Leaf', 'Red Rubin', and 'Sweet Aden'; O. citriodorum varieties: 'Lemon', 'Lemon Mrs. Burns', and 'Lemona'; O. sp variety: 'Lime'.
Highly Resistant O. basilicum variety: 'Blue Spice Fl'; O. sp variety: 'Spice'; O. basilicum x americanum variety 'Blue Spice'.
Chemical control Fungicides are a primary management practice for commercial production; however, few fungicides are labeled at this time. A number of fungicide studies have been proposed by the IR-4 program for 2012, recognizing the urgent need for basil producers.
- Oxidate at 40 to 128 fl oz/100 gal water on 5-day intervals. Efficacy unknown. 1-hr reentry.
- Phosphonates formulations (Group 33) are very effective.
- Fosphite at 1 to 3 quarts/100 gal water/A on 14- to 21-day intervals. 4-hr reentry.
- K-Phite 7LP at 1 to 3 quarts/100 gal water/A on 14- to 21-day intervals. 4-hr reentry.
- Helena ProPhyt at 3 to 4 pints/A on 7-day intervals. 4-hr reentry.
- Quadris is labeled for use on basil for other diseases but will only suppress downy mildew. Not for use in the greenhouse.
- Trilogy as a 1% solution on 7- to 14-day intervals before infection. Do not use above 90°F or when plants are under heat or moisture stress. Do not use when foliage is wet as good coverage is essential. 4-hr reentry. O
Biological control Efficacy unknown in the Pacific Northwest.
- Actinovate AG at 3 to 12 oz/A on 7- to 14-day intervals will suppress downy mildew. 1-hr reentry. O
References Blomquist, C.L., Rooney-Latham, S., and Nolan, P.A. 2009. First Report of Downy Mildew on Field-Grown Sweet Basil Caused by a Peronospora sp. in San Diego County, California. Plant Disease 93:968.
Wyenandt, C.A., Simon, J.E., McGrath, M.T., and Ward, D.L. 2010. Susceptibility of Basil Cultivars and Breeding Lines to Downy Mildew (Peronospora belbahrii) Hortscience 45(9):1416-1419.