Powdery mildew starts as small white spots on leaves. It usually occurs on squash, cucumber, and beans. As it spreads it covers leaves and stems with white powdery coating.

Powdery mildew is a true mildew and appears as a powdery or felty white growth on the surface of affected plants. The problem most often occurs during periods of high humidity. This mildew feeds on the plant by means of tiny suckers, draining the plant of important nutrients.

Powdery mildew, caused by Blumeria graminis f. sp. Tritici, is widely distributed throughout the world, particularly in humid regions. Over the past 20 years, powdery mildew has been the most common disease of wheat in Ohio. It is most damaging in years with relatively mild weather during April and May. Mild temperatures, high relative humidity and dense stands of wheat favor mildew development. It is most prevalent on the lower leaves of susceptible varieties in late April or early May when wheat is in the joint to flag-leaf stage of development. This disease results in reduced kernel size and test weight, and ultimately lower yield. The earlier in the spring mildew begins to develop on the plant and the higher on the plant it develops by flowering the greater the yield loss. Greatest yield losses occur when the flag leaf becomes severely diseased by heading. Losses up to 45 percent have been documented in Ohio on susceptible varieties when plants are infected in April and weather conditions are favorable for spread of the fungus throughout the growing season.

Symptoms

Powdery mildew is characterized by a powdery white to gray fungal growth on leaves, stems and heads. The fluffy white pustules are first detected on the lowest leaves of plants in early to mid April. As the plant matures, the white powdery growth changes to a grey-brown color. The leaf tissue on the opposite side of the leaf from the white mold growth becomes yellow, later turning tan or brown. Small, black fruiting bodies (cleistothecia) develop on leaves as plants mature in June. Cleistothecia are recognized as distinct round, black dots within older, grey colonies of powdery mildew. Cleistothecia contain spores (ascospores) that serve to infect wheat.

Disease Cycle

Figure 1. Powdery mildew can be recognized as fluffy white mold growth on leaf surfaces during cool, humid weather.

Most years, wheat becomes infected with powdery mildew in the fall soon after planting. Autumn infections on newly planted wheat result from spores produced on volunteer wheat plants or spores developing within cleistothecia. The mildew fungus survives over winter as cleistothecia on wheat straw or as mycelium on infected wheat. Conidia produced on wheat plants are wind dispersed. Conidia germinate and infect plants under cool, moist conditions. Infection does not require free water on the plant surfaces, but high relative humidity (near 100 percent) favors infection. Optimum development of powdery mildew occurs between 59 and 71 degrees F and is retarded above 77 degrees F. Under optimum conditions, a new crop of conidia are produced every 7 to 10 days. Mildew is more severe in dense stands of heavily fertilized wheat. Plants are most susceptible during periods of rapid growth, especially from stem elongation through heading growth stages.

Controls

1. Growing mildew resistant varieties is the most economical way to control powdery mildew. Wheat varieties vary in their resistance to powdery mildew and new races of the fungus develop that attack previously resistant varieties. Therefore, it is important to get current information on the varieties with effective resistance to powdery mildew.

2. Powdery mildew thrives where high rates of nitrogen have been used. Nitrogen not only promotes tiller formation, causing dense stands, but also increases the susceptibility of the crop. Use a correct and balanced fertilization program with proper levels of N, P and K. Watch for mildew in fields receiving greater than 70 lb N/A.

3. In fields with persistent disease problems, the wheat stubble and other residues should be tilled into the soil to permit disease causing fungi to die out before another wheat crop is planted. Incorporating wheat residues into the soil, destroying volunteer wheat and crop rotation will lessen the amount of overwintering inoculum in the field.

4. Fungicides are available that provide excellent control of powdery mildew. Their application is based on scouting fields for symptoms and assessing disease severity from tiller elongation through flowering stages of growth. It is important to keep the top two leaves of the plant as disease free as possible so that the plant can use its full potential to fill the grain. Fungicides can be applied based on the level of disease in the field, the known susceptibility of the variety, and the selling price of the grain.

Source: The Ohio State University Extension, Plant Pathology
Powdery Mildew of Wheat,AC-10-96
Patrick E. Lipps

On Blackberries Sphaerotheca macularis (Wallr.:Fr.) Lind

Disease Cycle: The fungus overwinters within infected buds near the tips of heavily infected canes. Shoots that emerge from these buds the following spring are infected, and spores produced are distributed by air currents to spread the disease. Repeat cycles of infection can continue throughout the summer. Unlike most fungal diseases, powdery mildew infections do not require periods of wetness in which to develop. However, they are more likely to become severe during humid weather conditions.

Symptoms: Infected leaves are covered with a white powdery growth, usually on their undersides, and may curl upwards. Some cultivars simply develop light green blotches on the leaf surfaces. Infected shoots may be long and spindly and have dwarfed leaves.

Monitoring: No thresholds are currently in use within the commercial industry.

Control: Maintain good air circulation around the planting and remove late-developing primocanes that may be infected. Fungicidal control is generally not effective or practical. Nova is registered for control of powdery mildew.

Biological Control: No effective biological control agents are commercially available.

Powdery Mildew

Powdery mildew is an important fungal disease of grapes. If not controlled on susceptible cultivars, the disease can reduce vine growth, yield, quality, and winter hardiness. Cultivars of Vitis vinifera and its hybrids (French hybrids) are generally much more susceptible to powdery mildew than are native American varieties such as "Concord" (Table 4.2). On susceptible cultivars, the use of fungicides to control powdery mildew is an important part of the disease management program.

The fungus can infect all green tissues of the grapevine. It was previously thought that the fungus overwintered inside dormant buds of the grapevine. Research in New York has shown that almost all overwintering inoculum comes from cleistothecia, which are fungal fruiting bodies that overwinter primarily in bark crevices on the grapevine. In the spring, airborne spores (ascospores) released from the cleistothecia are the primary inoculum for powdery mildew infections.

NOTE: Ascospore discharge from cleistothecia is initiated if 0.10 inch of rain occurs at an average temperature of 50 degrees F. Most mature ascospores are discharged within 4 to 8 hours.

Ascospores are carried by wind. They germinate on any green surface on the developing vine, and enter the plant resulting in primary infections. The fungus grows and another type of spore (conidia) is formed over the infected area after 6 to 8 days. The conidia and fungus mycelia give a powdery or dusty appearance to infected plant parts. The conidia serve as "secondary inoculum" for new infections throughout the remainder of the growing season. It is important to note that a primary infection caused by one ascospore will result in the production of hundreds of thousands of conidia, each of which is capable of causing secondary infections. Therefore, as with black rot, it must be emphasized that early season control of primary infections caused by ascospores is necessary. If primary infections are controlled until all the ascospores have been discharged, the amount of inoculum available for causing late season (secondary) infections is greatly reduced.

Conditions that favor disease development

Although infection can occur at temperatures from 59 degrees to 90 degrees F, temperatures of 68 degrees to 77 degrees F are optimal for infection and disease development. Temperatures above 95 degrees F inhibit germination of conidia and above 104 degrees F they are killed. High relative humidity is conducive to production of conidia. Atmospheric moisture in the 40 to 100% relative humidity range is sufficient for germination of conidia and infection. Free moisture, especially rainfall, is detrimental to the survival of conidia. This is in contrast to most other grape pathogens, such as black rot and downy mildew, that require free water on the plant surface before the spores can germinate and infect. Low, diffuse light seems to favor powdery mildew development. Under optimal conditions, the time from infection to production of conidia is only about 7 days.

It is important to remember that powdery mildew can be a serious problem during growing seasons when it is too dry for most other diseases, such as black rot or downy mildew, to develop.

Cleistothecia are formed on the surface of infected plant parts in late fall. Many of them are washed into bark crevices on the vine trunk where they overwinter to initiate primary infections during the next growing season.

Fungicides for Powdery Mildew

Protectants: Sulfur is highly effective against powdery mildew if used in a protectant program with a minimum of 7 to 14 days between applications (Table 4.3). There are many formulations of sulfur (wettable powders, dusts, dry flowables and flowables). The flowable formulations appear to be most effective and result in much less applicator exposure when preparing sprays.

NOTE: On sulfur-tolerant varieties that are susceptible to powdery mildew (Table 4.2), sulfur will be a major component of the fungicide program. On highly-susceptible varieties, spray intervals shorter than 14 days (7-10 days) will probably be required with sulfur. Although sulfur is highly effective for powdery mildew control, it has little or no effect on the other grape diseases (Table 4.3) it is important to remember that sulfur will cause severe injury on some grape varieties. Sulfur should only be used on varieties known to be sulfur tolerant (Table 4.2).

NOTE: Chancellor, Concord, DeChaunac, Foch and Rougeon grapes are highly sensitive to sulfur. Sulfur injury may occur even on sulfur tolerant cultivars when temperatures of 80 to 85 degrees F or higher are experienced during or immediately after application.

Copper fungicides (fixed coppers or Bordeaux mixture) have been rated moderately effective against powdery mildew, however, care must be taken when using copper due to the danger of foliage injury (phytotoxicity). Under heavy disease pressure, copper fungicides may not provide adequate control. Copper is not the preferred fungicide for powdery mildew control. However, if copper is applied for downy mildew control, it will provide some protection against powdery mildew. On less susceptible cultivars, such as Concord, copper fungicides may provide satisfactory control.

Source: Midwest Small Fruit Pest Management Handbook,Bulletin 861

Preventative measures of controlling mildew includes providing adequate ventilation and drainage. Applications of sulfur can help to control the problem on beans and peas.

Prevent mildew by planting in a sunny, well-drained area. Plenty of air flow and ventilation will also discourage the growth of mildew. Wet media and spray with elder leaves.