Powdery Mildews

Symptoms

Even though there are several types of powdery mildew fungi, they all produce similar symptoms on plant parts. Powdery mildews are characterized by spots or patches of white to grayish, talcum-powder like growth. Tiny, pinhead-sized, spherical fruiting structures that are first white, later yellow-brown and finally black, may be present singly or in a group. These are the cleistothecia or overwintering bodies of the fungus.

The disease is most commonly observed on the upper sides of the leaves. It also affects the bottom sides of leaves, young stems, buds, flowers, and young fruit. Infected leaves may become distorted, turn yellow with small patches of green, and fall prematurely. Infected buds may fail to open.

Conditions That Favor the Disease

The severity of the disease depends on many factors: variety of the host plant, age and condition of the plant, and weather conditions during the growing season.

Powdery mildews are severe in spring and fall due to favorable warm weather (70-80°F). The fungus needs both high and low humidity to complete its life cycle. High humidity, often present in evenings and early mornings in the spring and fall, allows for spore formation, while low humidity during the day favors spore dispersal. Additionally, the fungus does not need the presence of water on the leaf surface for infection to occur. The disease is common in crowded plantings where air circulation is poor and in damp, shaded areas. Incidence of infection increases as relative humidity rises to 90 percent, but it does not occur when leaf surfaces are wet (e.g., in a rain shower). Young, succulent growth usually is more susceptible than older plant tissues.

About the Fungi

Powdery mildews are host specific – they cannot survive without the proper host plant. For example, the species Uncinula necator, which causes powdery mildew on grape and linden, does not attack lilac. Similarly, Microsphaea alni affects elm, catalpa, lilac and oak but not turfgrass.

Powdery mildews produce mycelium (fungal threads) that grow only on the surface of the plant. They never invade the tissues themselves. The fungi feed by sending haustoria, or root-like structures, into the epidermal (top) cells of the plant. The fungi overwinter on plant debris as cleistothecia or mycelium. In the spring, the cleistothecia produce spores that are dispersed to susceptible host tissue by splashing raindrops, wind or insects.

Control

Several practices will reduce or prevent powdery mildews. Many ornamental plants, such as roses, vegetables and Kentucky bluegrass, have cultivars, which have been developed to be resistant or tolerant to powdery mildew. Inquire about resistant varieties before a purchase. If resistant varieties are unavailable, do not plant in low, shady locations. 

Once the disease becomes a problem: 

• Avoid late-summer applications of nitrogen fertilizer to limit the production of succulent tissue, which is more susceptible to infection. 
• Avoid overhead watering to help reduce the relative humidity. 
• Remove and destroy all infected plant parts (leaves, etc.). For infected vegetables and other annuals, remove as much of the plant and its debris in the fall as possible. This decreases the ability of the fungus to survive the winter. Do not compost infected plant debris. Temperatures often are not hot enough to kill the fungus. 
• Selectively prune overcrowded plant material to help increase air circulation. This helps reduce relative humidity and infection. 

Chemical 

If cultural controls fail to prevent disease buildup or if the disease pressure is too great, an application of a fungicide may be necessary. These include: 

These include: 

• sulfur 
• neem oil (Rose Defense, Shield-All, Triact) 
• triforine (Ortho Funginex), ornamental use only 
• potassium bicarbonate (Kaligreen, First Step) 

Chemicals are most effective when combined with cultural controls. Apply fungicides at seven to 14-day intervals to provide continuous protection throughout the growing season. Follow the instructions on the fungicide label for use on specific plant species, varieties, rates to be used, timing of applications, and waiting periods before harvest. 

An alternative nontoxic control for mildew is baking soda (similar to the potassium bicarbonate listed above) combined with a lightweight horticultural oil such as Sunspray. Researchers at Cornell University have discovered the fungicidal properties of this combination against powdery mildew on roses. Applications of one tablespoon baking soda plus 2.5 tablespoons of Sunspray oil in 1 gallon of water are still experimental. Use it at your own risk.

Reference: CSU Fact Sheet 2.902