Vegetable Disease Updates – August 5, 2011

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

Basil Downy Mildew
Basil downy mildew has been seen in nearby NJ. Any sweet basil growers should be scouting for this disease. Phosphite fungicides such as Prophyte have shown the best efficacy for controlling basil downy mildew.

Cucurbit Downy Mildew
Cucurbit downy mildew continues to be seen at low levels in commercial cucumber fields. The dry hot weather has been helping the fungicides to keep it in check. So far we have not seen it move into other cucurbits such as pumpkin. Keep scouting and check the 2011 Commercial Vegetable Productions Recommendations for fungicide suggestions. Once the cooler weather returns, and hopefully some rainfall, look for this disease to increase. Keep up with preventative fungicide applications.

Late Blight
We just received a confirmed report of late blight from Ann Arundel County in MD and in New Brunswick, Canada. Keep on the lookout for this disease on tomato and potato.

Watermelon
Cercospora leaf spot was diagnosed on watermelon last week. Cercospora leaf spot symptoms occur primarily on foliage, but petiole and stem lesions can develop when conditions are highly favorable for disease development. Fruit lesions are not known to occur. On older leaves, small, circular to irregular circular spots with tan to light brown lesions appear. The number and size of lesions increases, and eventually they coalesce and cause entire leaves to become diseased.

Lesion margins may appear dark purple or black, and may have yellow halos surrounding them. Severely infected leaves turn yellow, senesce, and fall off. On watermelon, lesions often form on younger rather than older foliage. Cercospora leaf spot can reduce fruit size and quality, but economic losses are rarely severe. Fungicides such as chlorothalonil (Bravo) and mancozeb including Gavel, as well as the triazole fungicides such as Inspire Super and strobilurins (Cabrio and Quadris) should provide good control of Cercospora leaf spot. As wilthall vine crops be sure to apply in enough water to get good coverage, usually a minimum of 15 gal/A.

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