Though horticultural mildew and general mold have many similar characteristics, the two are very different—particularly in color and texture. While both are indeed fungi, mildew is usually white or gray, while mold can be black, green, red or even blue.

Mold often looks fuzzy or slimy and looks like a colorful plant growth covering the surface of a fruit. It is also often found on organic material like clothing, leather and paper, as well as inorganic materials and places like shower curtains. Though mold spores can survive in a dry environment, mold itself generally does not grow without moisture. Some types of horticultural mildew, on the other hand, can thrive in a relatively dry climate.

If you search the web on the issue of mildew, you will likely find many websites that confuse varieties of mold with mildew. Some sites even provide incorrect information, so be very careful when you do your own research on this. To minimize the chance for confusion here, the only horticultural mildews that will be referenced are powdery and downy mildew. The others that might better fit into the mold category will not be mentioned.

Horticultural mildew

Mildew thrives in a moist environment, but does not necessarily require high moisture. It is started by spores, which generally start off airborne. The two main types of horticultural mildew are powdery and downy.

Powdery mildew is mostly found on flowering plants and is propagated by an ascomycota-type of fungus. It typically gives leaves the appearance of having been dusted by cooking flour.

Downy is found more on agricultural plants and is from an oomycota fungus. Its appearance is often like leaf spots, distortions, fuzzy patches and crystalline patches that look like sugar. Downy mildew is a primary concern for potatoes, grapes, tobacco and cucurbits (pumpkins, squash, cucumber, watermelons and the like).

Recognizing and understanding the difference between powdery and downy mildew is essential if you want to be able to obtain control of these two extremely damaging pests in your garden.

Powdery mildew

Characteristics: Light, powdery spore growth covering shoots, leaf surfaces and sometimes flowers.

Controls: Garden sanitation, resistant varieties, spacing plants for good air movement, pruning infected tissue, providing adequate bright direct sunlight and appropriate fungicide sprays.

Powdery mildew is one of the oldest plant diseases known and almost every gardener has seen or experienced this pest on their plants (both ornamental and agricultural). The main symptom is thick webbing or a felt-like covering on the either side of the leaf or stem surfaces of the plant. This can make the plant look like it has been dusted with some sort of powder—hence the name powdery mildew. The leaves then can turn yellow, curl and die or fall off.

This type of mildew can infect many different plants. It can be especially serious on crops like cucurbits (squash, melons, cucumbers and more), as well as a frequent threat to ornamental plants like roses, phlox, lilacs and crape myrtle. Though a bit less likely to be infected, beans, peas, lettuces, grapes and small fruit trees can also be affected by powdery mildew. Ornamentally, various types of grass and euonymus are also quite susceptible.

Powdery mildew can, of course, seriously affect the health and vigor of a plant, as well as crop yield. Since different species and genera of fungi attack different varieties of plants, the actual identification and control might be tricky. The most common genera found around and in gardens are erysiphe, sphaerotheca and podosphaera.

All powdery mildew fungi, however, infect the plant tissue in the same way. They send hollow tubes from a spore on the leaf surface into the plant and then suck out nutrients. The mildew then grows in a radius from the initial location of inoculation and after about four days, additional spores start forming in chains on upright stalks.

The spores are what actually give the visual powdery effect. On perennial host plants like roses, fruit trees and soft fruits (like berries); mildew spores can even survive for more than a single season in those buds that have become infected. In some types of plants, certain special spores that enable the fungi to survive over the off-season are also produced.

Powdery mildew fungus spreads rapidly when spores become airborne. If plants are grown too close together, the likelihood for the disease spreading is high. Even a small or singular infection can rapidly damage your whole crop.

Low and shady locations lacking sufficient air circulation also provide the conditions in which mildew prefers to take hold and multiply, and so is sufficient to initiate an epidemic. Unlike most fungi, many powdery mildew spores do not need water in order to germinate.

Some species do require considerable humidity, but the moisture provided on the leaf surface after a cold night followed by a warm day is sufficient. Though moist conditions increase the chances that your plants could get powdery mildew, preventing moisture is not enough of a preventative measure in itself.

The most favorable outdoor condition for powdery mildew is when nighttime temperatures are around 60ºF and there is high relative humidity. The higher the nighttime temps, the lower the humidity required. Still, not all species require this much humidity and warmth, so keep an eye out on your crop or you might lose a lot of yield and effort. Since the spores are sensitive to high heat and strong direct sunlight, you might be able to use these conditions to your advantage.

Downy mildew

Characteristics: Tops of leaves have yellow blotches, while the undersides develop a faint frost of gray, white, blue or violet fuzz.

Controls: Garden sanitation, resistant varieties, spacing plants for good air movement, pruning infected tissue and appropriate fungicide sprays.

Downy mildew differs from its powdery cousin in a number of ways. Unlike powdery mildew spores (which are spread by the wind), downy mildew is spread by splashing water. Also unlike powdery mildew, which appears on both sides of the leaf surface, downy mildew produces spores primarily on the undersides of leaves.

Growth of these spores is often in a tree-like formation on branched fruiting structures (unlike the powdery mildew spores, which are produced in chains). The tops of leaves will have yellow blotches, while the undersides develop a faint frost of fuzz. Fruits on infected plants are often small and bitter—or, worse yet, they won’t even make it to harvest.

There are various species and genera of downy mildew fungi, just as with powdery mildew, and of course these will attack different plants. However, all downy mildew fungi are in the peronosporaceae family.

Downy mildew fungi prefer cool and moist conditions, which makes it less of a problem in arid climates. As such, when growing indoors, you’ll want to have good ventilation to avoid too much humidity in order to keep this pest away.

When conditions are very humid, such as following a rainstorm or in a poorly ventilated indoor tent, spores will germinate quickly—often within several hours. The population of these spores on leaf surfaces might develop in several days under ideal conditions of 65 º to 75ºF. Below 40ºF, the spores won’t germinate, and exposure to 80ºF or above for 24 hours will exterminate the spores.

Dry winds and warm, clear days also inhibit spore production. Roses, for example, are unaffected by downy mildew when humidity is less than 85%.

Horticultural mildew is a real nuisance to gardeners, both indoors and out. Gardening is a practice where frequent vigilance and maintenance have big pay-offs. Observing and controlling the mildew pest will help you maximize your yield every time.