Downy mildew is the name collectively given to a number of different, but symptomatically similar plant diseases. Often confused with gray mold (Botrytis) or powdery mildew, downy mildew often shows as a fuzzy, bluish (sometimes purplish) cast and occurs during cool (40-72° F; 5-23°C,) and humid (above 80 per cent) or foggy weather, as opposed to powdery mildew which occurs during warmer and humid weather. Wet foliage can also be a sufficient microclimate to produce spores and infect plants. It is unsightly and can become a systemic pathogen killing the entire plant, but fortunately is not necessarily fatal with proper prevention and treatment.
Symptoms and Causes of Downy Mildew
Downy mildew can be festering on your plants for some time before being noticed. That is because it usually first appears on the undersides of leaves. The leaf bottoms can appear to have white or bluish-white fuzzy or fluffy looking growths.
After some time, the tops of the leaves may develop small green or yellow translucent spots. If these spots remain untreated, they leave a gray colored lesion in their wake. Eventually whole leaves, then branches, even flowers and fruit will die. These deceased brown or bronze colored portions will continue up the plant, if not controlled and the climatic conditions do not change or are otherwise unaddressed.
Downy mildew is caused by several related, but host-specific fungal species of pathogenic water molds (oomycetes). This means the type that infects one kind of plant may not be the same species of fungus affecting another plant, though the symptoms may appear the same. Downy mildew spores are produced only on living plants and spread primarily through wind displacement and air movement, though they can be spread by insect pests moving from plant to plant. As spores land on a host plant, they can germinate and infect within as few as eight to 12 hours if the plant or foliage is wet. Some of the more commonly occurring pathogens for downy mildew are Peronospora spp., Plasmopara spp., and Pseudoperonospora spp. It should be noted, however, that finding downy mildew on one type of plant typically means that the conditions are favorable for the development of other downy mildews on other types of plants as well.
If unsure what kind of disease you have, take a large sample to your local cooperative extension service or larger garden center for identification where it can be viewed by a trained professional with a keen eye or viewed under a microscope. Downy mildew appears under the lens in a branched pattern whereas powdery mildew appears in a chain-like formation when view under extreme magnification.
Plants Commonly Affected by Downy Mildew
Downy mildew affects a wide range of plants. Host plants can be vegetables, fruits, herbs, annual flowers, perennials, and woody shrubs. Anywhere that cool, damp conditions exist, downy mildew can appear. The following list is by no means complete, but shows the range of hosts:
Herbs — Basil, rosemary, verbena
Fruit (botanically speaking any plant that produces a seed-bearing vessel) — Cucurbits (squashes, cucumbers, pumpkins, gourds, etc.), peas, tomatoes, grapes
Vegetables — potatoes, kale and other cole crops, spinach, lettuce, onions, garlic, asparagus, rhubarb
Annuals — alyssum, poppies, sunflowers, impatiens, pansies, coleus, snapdragons
Perennials — coreopsis, aster, perennial geraniums, daisies, Veronica
Woody shrubs — roses, viburnum
Other — hops
Read also: 7 Reasons for Yellowing Leaves
Treating Downy Mildew
The good news is downy mildew can usually be effectively controlled with a wide range of commercially available fungicides. Many products are formulated to control a range of plant diseases and downy mildew can often be treated with the same formulations available to combat other common plant diseases such as Pythium and Phytophthora. Fungicides formulated to combat or control true fungi, however, will not often be effective to use on downy mildew as the biology of each respective disease is quite different.
“Traditional” fungicides for downy mildew include many of the most commonly occurring chemicals on the market. These include (this list is not exhaustive, but representative) fungicides containing: chlorothalonil, copper sulfate and lime (commonly sold as “Bordeaux mix” and sometimes used in organic food production), fosetyl-Al, mancozeb, metalaxyl, mefenoxam, or trifloxystrobin. For those seeking less harsh alternatives, there are indeed some formulations approved for organic food production that can be used to treat downy mildew.
Read also: Fighting Fungi the Organic Way
Neem oil is a naturally occurring substance derived from the Azadirachta indica tree from India. Neem oil can also be used to control a wide range of insect pests organically, too. Additionally, biological control can be found by using a naturally occurring bacteria, Bacillus subtilis, which will prevent downy mildew from attaching to plant surfaces. Other fungicides available for the environmentally conscious include those containing baking soda and rhamnolipid (a type of fat) biosurfactancts that burst the cellular walls of mildew and other diseases naturally upon contact.
As with any pesticide, remember “the label is the law.” Use each fungicide in accordance with personal protective equipment (PPE), rates, and only on plants listed on the document included with each pesticide at the time of purchase. If you have lost the original label or it has become difficult to read, copies of product labels are required to be found on each manufacturer’s website.
Prevention of Downy Mildew
As with many diseases, botanical or otherwise, preventative measures are preferable to treatment. Whether plant selection or cultural practices, there are several strategies for avoiding downy mildews. While the weather cannot be controlled, the conditions favoring the formation of any mildew can be. Any planting area, new or existing, should be cleaned out to remove plant debris. Many types of spores can overwinter in field and garden debris and emerge from seemingly nowhere the following season. Some downy mildews can survive cold weather; others can be carried by wind and insects from warmer climates. This means removing frost-killed vegetables from the previous year’s garden before planting new and clearing out fallen branches and leaves from around the landscape.
Ask a Grower: What is the ideal humidity level for leafy greens?
New plants should be selected based on their resistance to downy mildew when such options exist. This information should be available on nursery tags at the garden or box store where they are being selected. Lists of downy mildew-resistant plants can be found from many reputable sources with a quick online search if there is no information on the tags. Certified root stock that is guaranteed to be free of downy mildew spores for onions, garlic, asparagus, and rhubarb should be selected if planning to grow these types of crops. Practicing crop rotation in the garden will help manage the spread of downy mildew as well. Observe a three-year minimum rotation cycle for root crops to help avoid contracting any types of downy mildew that may be lingering in the soil. Delaying planting until after the threat of cooler, moist weather is another strategy. Existing plantings can be managed to avoid downy mildew formation as well.
No matter how established your plants are, making sure there is enough airflow around them is critical to help avoid mildews and fungal problems of all kinds. For mature plants, make sure the lower branches and leaves are not laying on the ground. Keep all dead or damaged branches pruned out regardless of their location on the plant. Keep your plants trimmed back away from the house and other structures as well. For overly dense, but otherwise healthy plants, make some periodic thinning cuts to make sure there is good air circulation around them. This will help to reduce humidity around your plants and allow for better drying out of the foliage. Consider removing (or transplanting) mature landscape plants if they have grown too much into, or too close to their neighboring botanical friend. When planting new, consider spacing your selections based on the maximum spread listed on the tag so they do not grow into one another. Again, if this information is not readily available on the plant or nursery tag, there are hundreds to thousands of reputable plant databases online.
Read also: 10 Best Indoor Garden Watering Practices
Once the spacing and thinning issues have been addressed, consider watering practices. Too much moisture is one of the key causes of fungal issues including mildews. Since downy mildew develops in cool and moist environments, care should be taken when irrigating plants that may be susceptible to it. Watering should be done during the warmest part of the day or prior to it so that excess moisture has a chance to evaporate. When irrigating plants, try to keep all off the leaf surfaces, instead favoring the root zone and soil around the plants.
With such a range of practical and easily performed preventative strategies, downy mildew does not have to be a garden scourge. If you do, however, lose the battle on any of your plants, make sure to fully remove the plant and any fallen leaves or branches, and don’t allow them to decompose near your garden. Where possible, burn or otherwise dispose of plants that have been affected by downy mildew to help prevent its further spread or return.