Wednesday, April 9, 2008

Greenhouse - Downy Mildew

Downy mildew is an emerging disease of greenhouse plants such as coleus. The following is an article on this disease from the University of Maryland.

Downy mildew is being found on coleus in some greenhouses. We first saw this new downy mildew species, in the genus Peronospora, back in 2006. Symptoms vary considerably and can include leaf curling, stippling, and angular necrotic lesions. The foliage may appear pale green or yellowish, emerging leaves can be small or discolored, plants can be stunted, and severely infected plants will drop their leaves.

The fungus can be inside leaves and stems that exhibit no symptoms. Downy mildew grows entirely inside the plant, with only the sporangiophores and Leaf curling and tan spores sporangia (spores) protruding outside the plant, and always on the lower leaf surface. Clusters of sporangia are tan in color. Gray, white fuzz on the undersides of the leaves is characteristic of downy mildew. Do not confuse these symptoms with Botrytis or powdery mildew which can also cause gray or white fuzzy growth on leaves.

Management: Downy mildew spreads rapidly under wet, humid conditions. Reduce leaf
wetness to less than 6 hours and keep the relative humidity below 85%. Do not place discarded plants in the compost pile. The fungus survives as resistant spores in soil and plant debris for years. Protectant fungicides such as mancozeb (Cleary’s Protect TO, Fore, Manzate) or chlorothalonil Angular necrotic leaf spots (Daconil Ultrex, Concorde, Spectro 90) are effective to protect uninfected plants, but will not be very useful once symptoms are seen. Once symptoms appear, a systemic fungicide should be applied. The fungicide Stature (mancozeb plus dimethomorph) is the best choice for cleaning up infected plants. Heritage (azoxystrobin), Compass (trifloxystrobin), and Quadris (azoxystrobin) are also registered and could be used in rotation with Stature.

Reprinted from the March 28, 2008 edition of the Greenhouse TPM/IPM Weekly Report from the University of Maryland Cooperative Extension.

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