University of Maryland Extension

Downy Mildew on Basil and Coleus

Downy Mildew on Basil

Downy mildew has become an emerging disease of basil. The first US detection of this disease was in Florida in 2007, and and in 2008 there were numerous reports throughout the eastern US in both greenhouse and field-grown basil. Dr. Meg McGrath, vegetable pathologist at Cornell University, has posted a very informative article, with several nice photos, on her website:

Symptoms of downy mildew on basil can be quite subtle, starting as a blotchy chlorosis on the upper leaf surface. Gray, fuzzy growth consisting of the spore structures of the pathogen can be seen on the underside of chlorotic leaves. As the disease progresses, the chlorotic areas on infected leaves become brown. Symptoms and signs of the pathogen are very similar to downy mildew of coleus. In fact, the basil strain was once thought to be the same as coleus downy mildew. New research has found that the two strains are different – basil downy mildew is known to affect only basil.

Dr. McGrath reports that two phosphorous acid fungicides, ProPhyt and K-Phite, were effective in managing the disease, and both have downy mildew listed under herbs on the current label. In addition, Actinovate AG is also labeled for suppression of foliar diseases, such as downy mildew, on herbs.

Downy Mildew on Coleus

Check coleus plants for downy mildew. If the coleus have brown areas on the leaves, leaves dropping off, and stunted seedling then you need to check for downy mildew. Some growers think that this disease is found only in vegetatively produced coleus. Both seed and vegetatively propagated types are susceptible. It appears that not all coleus cultivars may be affected by downy mildew. The amount of blighting and leaf drop that results may vary among the cultivars. Coleus cultivars also may differ in how many sporangia are produced on coleus leaves. This is an important characteristic because the sporangia are responsible for spread of the downy mildew. Wind currents or splashing water dislodge sporangia and make them available to infect nearby healthy plants. The brown or blighted areas on diseased foliage have an irregular shape and can cause the leaf to twist and drop. The fungus reproduces with spores called sporangia and can often be seen on the underside of the coleus leaves. In some instances, these sporangia may be few in number and very difficult to see without the help of a microscope. In the right weather conditions, the sporangia are produced in high numbers and form a fine carpet of grayish fuzz on the underside of the leaf that is obvious to the naked eye. It is best to look for these sporangia when the greenhouse environment is humid and damp.

Control options for downy mildew on coleus: Heritage (azoxystrobin), Stature (dimethomorph), and Subdue MAXX (mefenoxam), Pentathlon LF (mancozeb), Subdue MAXX EC, and Insignia (pyraclostrobin).

Author: Karen Rane, Plant Diagnostic Lab, University of Maryland
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