Powdery Mildew in Cereal Crops

Powdery mildew, caused by the fungus Blumeria graminis, is one of the most common and damaging foliar diseases in wheat worldwide. Powdery mildew exists almost everywhere wheat is grown. As an obligate parasite, the fungus grows only on living tissue. Different strains of the powdery mildew fungus also cause barley mildew.

Powdery mildew is particularly a problem in regions with high nitrogen fertilization and high stand densities. Cool, moist weather conditions with high relative humidity – 97 to 100 percent – enhance germination of the fungal infection. Optimum temperatures for development of powdery mildew are 59 to 70 degrees F, typically making it the first leaf disease of the season. When temperatures climb above 77 degrees F, powdery mildew begins to deteriorate. In much of the Midwest growing regions, powdery mildew in wheat occurs from April to July, but it can infect newly emerged wheat in the fall.

Powdery Mildew

Powdery mildew is characterized by white, cottony patches (colonies) of mycelium and conidia (asexual spores) on the surface of the plant. They can occur on all aerial parts of the plant, including stems and heads, but are most conspicuous on the upper surfaces of lower leaves. The white colonies later turn dull gray-brown.

Identification and Lifecycle

Symptoms of powdery mildew are white, cottony patches (colonies of mycelium) and asexual spores (conidia) on the surface of the wheat plant. These patches of white mold are typically seen in early to mid-April through July in most of the Midwest. Although the white patches can occur on all aerial parts of the plant, including stems and heads, they are most easily visible on the upper surfaces of lower leaves.

Later, the white colonies or patches turn dull gray to gray-brown in color. Severe infections can result in wheat plant stunting. Heads on the later tillers may show more heavy infection because they reside lower in the wheat canopy, where humidity remains high. The leaf tissue on the opposite side of the white mold growth turns yellow and then changes to tan or brown.

As plants mature in June, small, distinct black dots containing spores develop on plant leaves. In this phase of the disease, spores reside in older, gray-colored powdery mildew colonies that infect the wheat field.

Typically, the lifecycle of powdery mildew begins in the fall, soon after winter wheat planting. The fungus overwinters on straw residue as white, cottony patches of asexual spores. Spring infections occur when spores are dispersed by wind and rain. As the major cause of powdery mildew, these spores are produced in large numbers from initial infections.

Windborne spores land on plant surfaces and germinate, resulting in new infections on healthy plants or secondary infections on plants already infected. New spores can reproduce in seven to 10 days. The fungus can spread throughout the growing season, especially on susceptible varieties infected in early spring and when weather conditions are favorable.

Crop Damage

The greatest yield loss occurs when the flag leaf of a wheat plant becomes severely diseased by powdery mildew by the time of heading. Damage caused by the powdery mildew fungus increases the number of non-productive tillers, resulting in yield loss even at low levels of infection.

Disease infection robs yield by reducing kernel size, number of seeds per unit area and test weight. A rule of thumb is the earlier a spring mildew infection begins and the higher up the plant it spreads by flowering, the greater the resulting yield loss. According to research by The Ohio State University Extension, wheat yield losses from powdery mildew may be as high as 45 percent.

Managing Powdery Mildew

Farmers have several options to manage powdery mildew in wheat, ranging from seed treatments, variety selection, scouting, cultural practices and fungicide applications.

Seed treatments

Using the correct fungicide seed treatment and rate is one way to provide effective and economical disease control in wheat production. Seed treatments protect seed and young plants from disease and other threats to plant health and yield. Areas commonly infested with powdery mildew also will need a foliar fungicide treatment to provide protection beyond the seedling growth stage.

Variety selection

Every commercially available wheat variety has a unique disease package, and excellent disease resistance is not available to manage all disease threats in high-yielding varieties. It’s best to select two or three high-yielding varieties that offer the best resistance to common diseases found on your individual farm.

Scouting

When scouting for weeds and insects, check for the presence of powdery mildew and other leaf diseases. Scouting helps you make the best decision about whether or not a fungicide application is needed, and it also provides information to help you select appropriate disease management practices for future crops. If you suspect powdery mildew, take samples and work with your county Extension agent to confirm a diagnosis. Crop identification guides are also helpful.

Cultural practices

Crop rotation can help deter diseases. In areas where continuous wheat is grown, cultivation can break down crop residue that harbors certain diseases. Plant disease-free seed. Good weed control from preplant, at planting and throughout the growing season also helps protect against disease and other pests.

Fungicides

Excellent powdery mildew control can be achieved with commercially available fungicides. The decision to use fungicides should be based on scouting for symptoms. It’s important to assess disease severity from the growth stages of tiller elongation through flowering. Experts recommend application timing to keep the upper leaves of the wheat plant disease-free, allowing full potential for grain fill. Other factors affecting fungicide application are infection levels in the field, the susceptibility of the variety and the market price for wheat grain.



Crop Science Solutions to Prevent Powdery Mildew

A well-thought-out disease-management program, including best management practices, proper seed protection and selection and fungicide applications using multiple modes of action, should be implemented to sustainably manage diseases. The following Crop Science solutions are valuable tools to consider for your program.      

Because cereal fungal diseases can overwinter and survive in crop residue, crop rotations that include dicot crops can help reduce the inoculum of fungal leaf spot pathogens. Fungicide seed treatments can provide a healthy start for seedlings, especially in cool and damp spring conditions. EverGol® Energy from Crop Science is a seed treatment fungicide that promotes more root growth for faster crop establishment and controls seed and soilborne disease, such as Rhizoctonia. It features a new combination of fungicides incorporating a complementary mode of action that supports resistance management.

A number of fungicides are available for both early-season and late-season control of these common leaf diseases. Chemistries from two of the most commonly used classes of fungicides, triazoles and strobilurins, provide good-to-excellent activity against wheat leaf diseases. When used in conjunction with best management practices, they can also help manage disease resistance. Wheat growers should consider fungicides with systemic movement and curative properties for the broadest protection from cereal foliar diseases.

With a combination of two chemistries, Prosaro® fungicide provides preventive and curative action against key cereal leaf diseases such as various types of rust, Septoria leaf blotch, tan spot and powdery mildew. Additionally, Prosaro provides unsurpassed activity against head diseases such as scab (Fusarium head blight) and glume blotch. It’s a good choice to ensure grain quality and enhance yield potential.

For more information on wheat disease control options from Bayer, please contact your local Crop Science US representative or visit the cereals section.

Before selecting a seed treatment or applying any fungicide, please read the entire label for the best possible results and to confirm that the product is effective on the disease you need to control. Not every product is suitable for every situation, and correct application techniques will ensure the best results.

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