Powdery Mildew: Late Summer Landscape Problem

Published: September 3, 2019

Various species of powdery mildew can be found on garden plants throughout the growing season. However, it is in the late summer and fall of the year that they become more common. Plants frequently afflicted include lilac, zinnia, phlox, dahlia, rose, monarda and crabapple. Additionally, there are numerous other flowers, trees, shrubs and even grasses that may be attacked by the disease when conditions are conducive for infection.

Powdery mildew on plantain

Powdery mildew is a somewhat generic term used to describe numerous species of fungi in the phylum Ascomycota whose members commonly are called "sac fungi". Widespread in occurrence, powdery mildew fungi derive their common name from their production of whitish, "powdery" spots on the surface of leaves of plants they infect. Most tend to be "host specific," meaning a single species of fungus infects only a fairly narrow range of host plants.

The life cycle of powdery mildew begins when spores are produced and dispersed into the air. Infection occurs when these spores contact a susceptible host growing in an environment suitable for infection to occur. After germinating, the spores produce the whitish mycelia responsible for the disease's name. Later, structures called haustoria grow from branching segments of the mycelia called hyphae and penetrate host cells to absorb nutrients.

Unlike many other plant diseases caused by fungi, powdery mildew does not require standing water on the surface of leaves to become infective. Most species of powdery mildew only require high humidity. The mild days and cool nights associated with late summer and fall present ideal conditions for powdery mildew infection. Additionally, powdery mildew can become a problem when plants are grown in crowded, low-light or shady conditions with poor air circulation. The latter tends to promote the high relative humidity needed for infection.

Powdery mildew on beans

Powdery mildew damage varies between species. On zinnia, phlox or rose, the disease can seriously deform foliage and ruin the appearance of the plant. On trees such as crabapple, or shrubs such as lilac, the fungus normally grows late enough in the season that it does little damage. However, plant appearance in the landscape still suffers.

Even plants within the same species vary in their susceptibility to powdery mildew. Rose is a good example. Many hybrid tea and floribunda rose cultivars have been developed with excellent resistance to powdery mildew. In contrast, other roses are quite susceptible. Fortunately, powdery mildew is less of a serious problem on roses in our climate than in many others.

Left unchecked, powdery mildew can become increasingly problematic. The first line of defense in controlling powdery mildew as well as many plant diseases is sanitation. Seriously diseased annuals and perennials plants (or their affected foliage) should be removed promptly from the garden.

Diseased tree and shrub leaves should be removed from the landscape as soon as they drop in autumn. Avoid using them as mulch. Although the fungi responsible for powdery mildew do not grow on dead plant tissue, their overwintering structures can be found on both living and dead tissue.

When chemical control is warranted, there are many fungicides from which to choose. Sulfur dust and lime/sulfur sprays have been used for years as a control method for powdery mildew. Some copper-containing fungicides also have been used. Sulfur dust or sprays should not be applied when temperatures are very high, since leaf damage in some plants can result. These materials are preventative in nature and are best applied before the fungus becomes established.

Newer fungicides available which have demonstrated good control of powdery mildew include thiophanate-methyl (e.g. Clearys 3336F®)*, chlorothalonil (e.g. Daconil®), propiconazole (e.g. Banner Maxx®) and myclobutanil (e.g. Spectracide Immunox®). Before using any of the preceding compounds for mildew control, always check their label to insure safe use on the plants to be treated and follow label directions carefully.

Home remedies that reportedly have been used with success to control powdery mildew include milk (10% solution), baking soda (most effective when combined with an oil), garlic, compost tea and various mint oil extracts.

Where fungicides cannot be used, preventative measures such as locating susceptible species where air movement is good is helpful. Trimming or pruning established plants to open their canopies will help to improve air circulation. The better the air circulation, the less chance of high humidity developing in stagnant air. Perhaps the best alternative is to use genetically resistant cultivars when establishing a landscape, or replacing susceptible plants in existing planting.

*Mention of brand names is for informational purposes only, and does not imply an endorsement by University of Missouri Extension.