Powdery Mildews Fact Sheet No

Powdery Mildews Fact Sheet No. 2.902 Gardening Series|Diseases by S. Newman and L.P. Pottorff* Powdery mildews are one of the most Powdery mildews are severe in warm, Quick Facts widespread and easily recognized plant dry climates. This is because the fungus does diseases. They affect virtually all kinds not need the presence of water on the leaf • Powdery mildew is one of the of plants: cereals and grasses, vegetables, surface for infection to occur. However, the most widespread and easily ornamentals, weeds, shrubs, fruit trees, relative humidity of the air does need to be recognized plant diseases. and broad-leaved shade and forest trees. In high for spore germination. Therefore, the Colorado, powdery mildews are common on disease is common in crowded plantings • Powdery mildews are ash, lilac, grape, roses, turfgrass, vegetables where air circulation is poor and in damp, characterized by spots or (such as cucurbits and peas), euonymus, shaded areas. Incidence of infection increases patches of white to grayish, cherry, apple, crabapple, pear and Virginia as relative humidity rises to 90 percent, but talcum-powder-like growth. creeper, among others. it does not occur when leaf surfaces are wet • Powdery mildews are severe (e.g., in a rain shower). Young, succulent in warm, dry climates growth usually is more susceptible than older Symptoms plant tissues. • Many plants have been Even though there are several types of developed to be resistant or powdery mildew fungi, they all produce tolerant to powdery mildew. similar symptoms on plant parts. Powdery About the Fungi mildews are characterized by spots or Powdery mildews are host specific – they • Succulent tissue is more patches of white to grayish, talcum-powder- cannot survive without the proper host plant. susceptible to infection. like growth. Tiny, pinhead-sized, spherical For example, the species Uncinula necator, Once the disease is a fruiting structures that are first white, later which causes powdery mildew on grape problem, avoid late summer yellow-brown and finally black, may be and linden, does not attack lilac. Similarly, applications of nitrogen present singly or in a group. These are the Microsphaea alni affects elm, catalpa, lilac and fertilizer. cleistothecia or overwintering bodies of the oak but not turfgrass. fungus. Powdery mildews produce mycelium • Plant resistant varieties if The disease is most commonly observed (fungal threads) that grow only on the available. on the upper sides of the leaves. It also affects surface of the plant. They never invade the the bottom sides of leaves, young stems, tissues themselves. The fungi feed by sending • Chemicals are most effective buds, flowers and young fruit. Infected leaves haustoria, or root-like structures, into the when combined with may become distorted, turn yellow with epidermal (top) cells of the plant. The fungi cultural controls. small patches of green, and fall prematurely. overwinter on plant debris as cleistothecia Infected buds may fail to open. or mycelium. In the spring, the cleistothecia produce spores that are moved to susceptible host tissue by splashing raindrops, wind Conditions That Favor or insects. the Disease The severity of the disease depends on Control many factors: variety of the host plant, age and condition of the plant, and weather Cultural conditions during the growing season. Several practices will reduce or prevent powdery mildews. Many plants, such as roses, vegetables and Kentucky bluegrass, ©Colorado State University have cultivars, which have been developed Extension. 10/99. Revised 5/13. * S. Newman, Colorado State University Extension to be resistant or tolerant to powdery www.ext.colostate.edu specialist, greenhouse crops, and professor. Originally written by Laura Pottorff, former plant pathologist and mildew. Inquire about resistant varieties horticulturist, Integrated Pest Management Program, Jefferson County. 5/2013 before a purchase. If resistant varieties Chemicals are most effective when are unavailable, do not plant in low, combined with cultural controls. Apply shady locations. fungicides at seven to 14-day intervals to Once the disease becomes a problem: provide continuous protection throughout • Avoid late-summer applications the growing season. Follow the instructions of nitrogen fertilizer to limit the on the fungicide label for use on specific production of succulent tissue, which is plant species, varieties, rates to be used, more susceptible to infection. timing of applications, and waiting periods • Avoid overhead watering to help reduce before harvest. the relative humidity. An alternative nontoxic control for • Remove and destroy all infected mildew is baking soda (similar to the plant parts (leaves, etc.). For infected potassium bicarbonate listed above) vegetables and other annuals, remove as combined with a lightweight horticultural much of the plant and its debris in the oil (Sunspray). Researchers at Cornell fall as possible. This decreases the ability University have discovered the fungicidal of the fungus to survive the winter. Do properties of this combination against not compost infected plant debris. powdery mildew on roses. Applications Temperatures often are not hot enough of one tablespoon baking soda plus 2.5 to kill the fungus. tablespoons of Sunspray oil in 1 gallon of • Selectively prune overcrowded plant water are still experimental. Use it at your material to help increase air circulation. own risk. 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: • sulfur • neem oil (Rose Defense, Shield-All, Triact) • triforine (Ortho Funginex), ornamental use only • potassium bicarbonate (Kaligreen, First Step) Colorado State University, U.S. Department of Agriculture and Colorado counties cooperating. CSU Extension programs are available to all without discrimination. No endorsement of products mentioned is intended nor is criticism implied of products not mentioned..

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