Downy Mildew and Alternaria of Cole Crops

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Symptoms of downy mildew on cole crops include purple to yellowish-brown spots on upper leaf surfaces. A grayish-white spore mass will develop and cover the underside of leaves under ideal temperatures (night temperatures of 46 – 61°F and day temperatures below 75°F). Downy mildew can kill young plants. Heavily infected leaves may drop providing entry points for bacterial infections (black rot and soft rot).

Symptoms of Alternaria on infected leaves include small, expanding circular lesions with concentric rings that may have a ‘shot-hole’ appearance as lesions age. Heavily infected seedlings may result in damping-off.

Control of downy mildew and Alternaria begins with preventative fungicide applications. Use one of the following at the first sign of disease and continue every 7 to 10 days: (Please refer to the pesticide table on page F21 of the 2009 Delaware Commercial Vegetable Production Recommendations to determine which fungicide is labeled for each specific crop.)
● Quadris (azoxystrobin, 11) at 6.0 to 15.5 fl oz 2.08F/A
● chlorothalonil (M5) at 1.5 pt 6F/A or OLF
● Cabrio (pyraclostrobin, 11) at 12.0 to 16.0 oz 20EG/A
● Endura (boscalid, 7) at 6.0 to 9.0 oz 70WG/A
● maneb (M3) at 1.5 to 2.0 lb 75DF/A or OLF
● Ridomil Gold Bravo (mefenoxam + chlorothalonil, 4 + M5) at 1.5 lb 76.5WP/A (14-day schedule)

For Alternaria only, apply:
Switch (cyprodinil, 9) at 11.0 to 14.0 oz 62.5WG/A

For downy mildew only, apply:
Actigard (acibenzolar-S-methyl, P1) at 1.0 oz 50WG/A (Begin applications 7-10 days after thinning and re-apply every 7 days for a total of 4 applications per season.)
or
Aliette (fosetyl Al, 33) at 3.0 to 5.0 lb 80WDG/A (on a 14-day schedule).

For more information please see the 2009 Delaware Commercial Vegetable Production Recommendations.

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