BERTAUSKI COLUMN: Mildew a pesky problem for plants

Powdery mildew on crape myrtle.

Everyone has seen fungus outside somewhere, but it's not necessarily small. The largest organism in the world is a fungus that spreads over 2,000 acres. Unlike photosynthetic plants, fungi cannot produce their own food and find it in other places.

Quite often, fungi are harmlessly decomposing logs, leaves and other organic matter. In some cases, they team up with algae to form lichens that attach to tree trunks and branches. The photosynthesizing algae provide the fungi with food, and the fungi provide water. When a tree declines, it allows more light into the canopy that stimulates lichen growth. Lichens are sometimes falsely accused of killing the host when they're just hanging on to it.

Parasitic fungi, however, infect plants and steal food. Sometimes the host just looks bad, having blemishes such as spotty leaves. Sometimes they die.

Fungal diseases generally prefer wet conditions so that spores can penetrate the plant. Overhead irrigation often encourages disease because it extends leaf wetness. Irrigating turf in the morning is preferable to the evening to reduce the length of time leaves are wet.

Mildews are unique from other fungal pathogens because they inhabit the outer surface of the leaf and show up as a white fuzzy coating. The powdery or dusty appearance is the actual fungus and only penetrates the epidermal layers.

Powdery mildew
Powdery mildew commonly occurs on plants as a white powder on the top and bottom of foliage as well as stems. While it isn't considered harmful, it can distort new growth and cause minor discoloration. Generally, it is not fatal.

The disease is caused by several fungi that tend to be very host specific. One powdery mildew infection isn't likely to cause an outbreak on other plant species.

Unlike most fungal diseases, powdery mildew does not require free water to be infectious. It prefers warm temperatures and high humidity, which is exactly the type of weather the Lowcountry has been experiencing. You may be noticing more of the disease on crape myrtles, dogwoods, roses and cucurbits. Hydrangeas tend to get powdery mildew in late summer, which causes the leaves to prematurely drop.

Localized powdery mildew infections on single branches can be pruned away. The disease survives on fallen debris, so cleaning up infected foliage can reduce its spread. Avoid succulent growth by reducing nitrogen fertilization. The disease is likely to be more prevalent on succulents growing at the base of crape myrtles, particularly older varieties.

Certain roses can become covered with the disease and require fungicide applications. Since powdery mildew is primarily on the outer surface of the foliage, applications of horticulture or neem oil often can be effective in addition to being safe to use around people and pets.

Downy mildew
Downy mildew may not be as innocuous as powdery mildew. Technically, it's a pathogen that infects the lower portion of the leaf and frequently appears grayish and, sometimes, angular.

It's not as common in the landscape as powdery mildew but often can be controlled with fungicides when it becomes a problem. However, a new downy mildew disease recently has been discovered infecting impatiens. The results are quick and deadly.

Impatiens' downy mildew appeared in the greenhouse industry in 2004 and didn't have much of a presence in the landscape until this year. It is specific to common impatiens (Impatiens wallerana). Initially, the growth is stunted and the leaves turn light green before suddenly dropping off, leaving behind bare stems.

The pathogen will not infect other plants. New Guinea impatiens are resistant to the disease, too. Fungicide treatments are not recommended for treating infected common impatiens because the disease is too aggressive. If impatiens show symptoms, remove all infected plants and destroy them.

The disease can overwinter in the soil for years, so avoid replanting common impatiens in same bed. Consider other shade-tolerant flowers such as coleus, begonia, torenia, alternanthera or New Guinea impatiens.

Tony Bertauski is a horticulture instructor at Trident Technical College. To give feedback, email him at gardening@postandcourier.com.

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