Not to be confused with powdery mildew, downy mildew is a different infection – though both are fungi. The difference is the conditions in which they occur and thus the different management requirements.
Downy Mildew Host Plants
Downy mildew is most often associated with greenhouses, though it can occur outdoors and even in indoor, potted plants if conditions are right. Hosts include many types of flowers such as snapdragon, geranium, alyssum, pansy, rose, rosemary, viola and other ornamentals. It is also seen in vegetable crops like tomatoes, squash, cucumbers and melon plants.
Symptoms of Downy Mildew
This blight happens mainly in cool, wet weather and can onset very rapidly if conditions are ripe. Often, the symptoms are confused with those of other fungi and blight. Leaves may become mottled, yellow, or show signs often associated with nutritional deficiency. Other symptoms similar to those from nematodes may be present. The hallmark of downy mildew (which gives it its name) is the fluffy gray, brown or purple fungal sporulations that develop on the underside of infected leaves.
Plants will often show signs of other infections, as noted, and can even suffer from those other problems due to the weakening caused by the mildew. For detailed photographs to identify this plant disease, see these images of downy mildew.
Downy Mildew Life Cycle
Downy mildew is an obligate parasite, so it can only grow on live plant tissue. It abandons dead tissues as it spreads. The infection is actually inside the leaf of the plant, though the manifestations and spores are outside. Different fungi actually cause downy mildew, mostly of the Peronospora family while some specialist strains like P. lamiii affect specific plants or species. All of these fungi are closely related to the Pythium and Phytophthora families of water molds.
They develop during cooler temperature times (spring, fall) when conditions are wet and humidity is relatively high. For spore germination and infection to begin, a thin film of water must be present on the underside of the leaf. Spores are robust and can be spread by wind, splashing water, and even insects via physical transfer.
Downy Mildew Impact on Plants
Stunted growth, susceptibility to other diseases, and loss of flower or fruit are common results with a downy mildew infestation. Nearly all plants will become weak and most will eventually die.
How to Prevent Downy Mildew
Prevention is about keeping down the wetness required to propagate this mildew. Watering from the base, controlling moisture, and allowing space and aeration to take place to prevent moisture buildup are key to all fungal prevention. Preventive fungicides are also an option. Make sure soil has proper drainage. Also, try a mulch that allows for aeration, but prevents soil from splashing on the plants in heavy rains.
How to Treat Downy Mildew
Fungicides can eliminate downy mildew if used early in the process. Prevention is the best way to keep this blight from forming. Try something like GreenCure Organic Fungicide. This organic fungicide is a good all around gardening product and can be used on a wide range of plants and gardening diseases. This way if you aren’t 100% that you’re dealing with downy mildew, chances are a wide range product like this fungicide will catch the plant disease.