Downy Mildew is a foliar disease that zeros in on both young and old leaves. It affects a wide variety of ornamental flowers including but not limited to roses, impatiens, salvia, asters, coreopsis and geraniums. This fungus also attacks fruits like grapes, anything in the cucurbits family including cucumbers, squash and melons. It also has a fondness for leafy greens.
Because it is a foliar condition, lettuce, spinach and delicate leafy greens are hardest hit. Downy mildew attributes to leaf destruction, which cuts back on the plant getting proper nutrients and results in stunted fruit and vegetable growth.
Selecting more tolerant varieties of plants is often the key. There are a few downy mildew resistant varieties like cucumbers carried under the Seminis® brand and basil in the lemon, lime or spice varieties.
Impatiens seem exceptionally susceptible to the fungus but New Guinea Impatiens are rarely bothered.
Zinna ‘State Fair’, Bee Balm ‘Jacob Kline’ and Garden Phlox ‘David’ are examples of varieties within a species that are less susceptible to downy mildew. Many growers offer disease resistance information on their tags.
Rugosa or bush roses seem to be better bred to handle mildew problems. Roses with rough or leathery foliage such as Peace and Playboy varieties fare better under wet, humid conditions to ward off mildew than their more delicate cousins.
One problem with controlling downy mildew is that the fungus species varies depending on which type of plant it infects. That means different fungal species favor slightly different conditions. To control it you need to identify it on your plants.
Luckily all downy mildew shares the same characteristics.
• Downy mildew grows and thrives in humid, damp conditions where average yearly temperatures are around 65 °F.
• This fungus is a hitchhiker, so it needs a stream of water or mist to get from leaf to leaf. Prolonged wet conditions are ideal for its growth.
• Check the top side of leaves for yellow lesions shaped like footballs. Undersides of leaves will have purple or grayish soft, fluffy spores. Powdery mildew spores differ from downy mildew spores. Signs of powdery mildew appear mainly on the top of leaves as a white dusty powder. Powdery mildew can coat the undersides of leaves with white powder as well. As a rule, powdery mildew thrives in hot, humid conditions around and above 80 °F.
Once you determine that your plants have downy mildew there are several measures you can take to curb this fungal disease.
• Dry up the area as best as you can. Water only in the early morning and avoid top watering plants. Try to drip irrigate or water at ground level to avoid getting leaves wet and/or splashing water from leaf to leaf.
• Try to insure proper air circulation between plants.
• Remove infected leaves and discard. Do not put them in your compost pile.
• Apply a fungicide at recommended intervals designed for a particular crop. Preventive fungicides are also available. Ask a nursery plant specialist for help with selecting the proper brand for your needs.
Since downy mildew outbreaks are unpredictable it is always a good idea to keep a close eye on developing plants.