Powdery Mildew vs Downy Mildew
When dealing with disease in the garden, it is always best to know what you’re up against before you start treating. Keeping symptoms straight can be confusing, and keeping names straight adds another layer of deduction. Downy mildew or powdery mildew—while both are called “mildew,” there are differences in how they look on your plants and how you treat the disease.
So how do you tell the difference at first inspection?
|Powdery Mildew||Downy Mildew|
|Fungal spots have a circular white appearance||Fungal spots have an angular and gray appearance|
|Fungus can be anywhere on the leaf surface||Fungus is limited by leaf veins|
|Leaves yellow after fungus has been present for a while||Leaves may yellow before the presence of fungus is even evident|
Powdery mildew is a serious fungal disease that attacks a wide range of plants. While nonresistant crapemyrtle trees are especially susceptible, this disease can also attack flowers like zinnias, roses, and gerbera daisy, as well as edibles like squashes, strawberries, and tomatoes.
One of the most noticeable symptoms of powdery mildew is the spots or patches of white powder that can be found usually on the leaves. These spots enlarge and become a dusty white or gray coating. Symptoms of powdery mildew usually appear late in the growing season, especially when nights are cooler and there is little rain but humidity is high.
Powdery mildew thrives in moist conditions—including humid weather—shady areas, and areas where plants are crowded. This fungus robs your plant of nutrients and will result in stunting of leaves, buds, and fruits.
The symptoms of downy mildew can be a bit misleading, because at first glance, they're similar to the symptoms of nutritional deficiencies or even virus diseases. Young plants and new leaf growth are most vulnerable to this fungal disease. In cases of downy mildew infection, leaves become yellowish or speckled, leaf edges may curl downwards, and faint gray fuzz may appear on the undersides of leaves.
As the disease progresses, white-ish to gray, downy-looking growth will be visible along the undersides of leaves, sometimes flecked with tiny black spots. On the upper surface, yellow leaf spots will appear angular in shape and bound by leaf veins, sometimes creating a “quilted” look. Eventually leaves and flowers drop, leaving you with a plant of mostly stems.
Basils, impatiens, melons, and viburnum are particularly susceptible to downy mildew. There are certain conditions that are more ideal for downy mildew to flourish: high humidity, cool temperatures, and plant crowding.
You can treat powdery or downy mildew by spraying the infected plants with a fungicide labeled specifically for the type of mildew you are dealing with, be it powdery or downy. As always, it is important to follow all label instructions.
A Pound of Prevention
While treatments exist, prevention is always the best way to deal with plant diseases. Preventing powdery or downy mildew can be accomplished by following common recommendations for managing fungal diseases. Susceptible plants should be planted in sunny locations when possible, and regardless of whether they're planted in the sun or shade, air should be able to move freely around plants (i.e., no crowding). Additionally, if you are providing your plants with supplemental water you should water only the roots—not the foliage—in the morning. Watering only the roots is important to avoid getting the foliage wet, and doing so in the morning allows any foliage that does get splashed an opportunity to dry out.
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