Plant Mildew Treatment

The best offense for dealing with plant mildew is a good defense. Use plants resistant to the common mildew types such as downy mildew, powdery mildew and gray mold (Botrytis blight). Then follow mildew-suppressing gardening practices. Whether you have mildew on your plants or not, there are treatments that will control them and keep them away.

Location

Mildew thrives when the conditions are right, and some forms of mildew, such as powdery mildew, are specific to particular plants, according to Colorado State University Extension. Downy mildew and gray mold need humidity to grow, while powdery mildew likes dry weather. When a plant gets mildew over and over, then it is time to consider its location as a prime suspect. You can't change the weather, but you can alter the environment the plant grows in, although this may only be practical when the plant is relatively small, or newly planted. Move an affected plant to a place where the conditions don't favor the pest.

Pruning

Improved air circulation helps to treat plants that have mildew problems. When you can't move the plant, or don't want to, then pay attention to pruning. By first pruning out dead, diseased and dying wood, you will open the plant for more circulation. Next, prune judiciously to thin out the interior branches. Finally, prune neighboring plants that might be restricting air flow.

Cultural Practices

To control powdery mildew, don't fertilize late in the summer, avoid watering systems that wet the leaves and remove and destroy infected plants and infected plant parts. When controlling downy mildew, focus on enhancing the plant's vigor by carefully controlling its water and its fertilizer. Too much or too little of either weakens the plant's resistance to disease and pests. Control gray mold by removing infected plants and maintaining a balanced feeding program. Increasing potash reduces the problem for berries and fruits, according to Garden.ie.

Chemical

When your plant is losing leaves, changing colors and looking ill, it is probably time to go to stronger treatments. Products that contain sulfur, neem oil or triforine are labeled for use on powdery mildew. Neem oil is also listed for use on downy mildew. Products with sulfur may also be labeled for use on gray mold. Read the label, since these controls may limit the types of plants they can be used on.

Alternative Controls

Use baking soda mixed with a lightweight horticultural oil like Sunspray on powdery mildew. According to the Colorado State University Extension, this is still in the experimental stage, so use it at your own risk. The mix is 1 tbsp. baking soda to 2.5 tbsp. horticultural oil in 1 gallon of water. A biological named Bacillus subtilis is labeled for use on vegetables to control downy mildew. There is anecdotal evidence that hydrogen peroxide and bicarbonates are also effective against downy mildew. Gray mold responds best to good cultural practices, removing infected plants and burning them, and keeping the plants vigorous.

References

• Colorado State University Extension: Powdery Mildews
• National Sustainable Agriculture Information Service: Downy Mildew Control on Cucurbits
• Garden.ie: List of Garden Plant Diseases: Grey Mould
• "The Complete Book of Gardening"; Michael Wright; 1983
• The Garden of Eaden Blog: Organic Control of Grey Mould on Tomato Plants