Many homeowners have noticed the appearance of a strange white, powdery substance on their garden plants. Questions arise such as, what is it, will it harm my plants and how can it be controlled?

What they are observing is powdery mildew, a fungal disease, which affects many species of plants. The primary symptom is the appearance of grayish-white, powdery material visible on the surface of leaves, stems and flower petals.

As the disease progresses, these parts will begin to curl and become distorted, and in severe cases, they will turn yellow or brown. It can lead to a decline in vigor and growth of the host. Unlike many fungi, it can be troublesome in dry weather and does not require free water to grow. It thrives in high humidity and cool nights. Hot temperatures discourage the fungus. Several different species exist, and each has a limited host range. The mildew that gets on crape myrtles will not infect other plants or vice-versa.

One of the most noticeable plants that powdery mildew is troubling is crape myrtles. The disease can prevent the flower buds from opening and reduce their ability to bloom. Several varieties of crape myrtles are resistant to the fungus, such as “Natchez,” “Arapaho,” “Muscogee” and “Catawba.”

The disease attacks dogwoods, causing the leaves to curl and turn yellow. The powdery growth may be difficult to see. It will not harm the trees. The flowers are seldom infected since the tree blooms before conditions are conducive for the fungus to become problematic.

Dogwoods are understory trees, meaning they grow under larger trees in the forest, preferring partial shade. If grown in full sun, they are more susceptible to the fungus. Make sure they have well-drained soil, and cover their root zones with a 2- to 4-inch layer of organic mulch, such as pine straw or pine bark, to ensure they stay in optimal condition.

Some vegetables, such as squash, cucumbers, melons and other cucurbits, are susceptible to powdery mildew fungus, and severe infestations can cause the plants to die. To reduce the likelihood of being infected, the plants should receive at least six hours of full sun a day.

Some varieties of vegetables are resistant to the disease. If we do not receive rain, apply water twice a week and ensure it penetrates deeply into the soil. Water in the early morning before noon and direct it to the base of the plants to avoid getting the leaves wet.

The golden euonymus is quite susceptible to powdery mildew in addition to suffering from heavy infestations of the scale insect. They should never be planted in the landscape, and existing ones should be removed. Use plants that are not troubled by the fungus, such as hollies.

Some herbaceous annuals and perennials suffer from the disease. Zinnias, phlox and dahlias are examples. Select varieties that have resistance to the disease. They should be planted in open areas that receive full sun. Avoid excessive fertilization.

Fungicides have minimal effect and should be used in conjunction with other control tactics. Daconil and the copper based ones, such as those sold by Bonide and Hi-Yield, have some efficacy on the disease. Please observe all label directions and safety precautions when using pesticides.

Powdery mildew is problematic and troubles many plants in the home landscape. However, it causes minimal harm, and most plants will survive as it runs its course. Employing preventative tactics may help reduce the disease and its impact.

Timothy Daly is an Agricultural and Natural Resource Extension Agent with UGA Extension Gwinnett. He can be contacted at 678-377-4011 or tdaly@uga.edu.