Cereal Diseases Cause Significant Concerns

Wheat and other cereals are a staple in most diets, making foliar (leaf) disease and kernel fungal disease management critical to world food supplies. Demand for wheat is constantly increasing.

According to projections by the Food and Agriculture Organization (FAO) of the United Nations, global cereal production would need to increase by 40 percent to meet the future food demand of the projected world population in 2050.

University and other industry experts research as many as a dozen foliar diseases in cereals. Their findings can help growers recognize the signs of foliar diseases, and help cereal growers determine the best managements practices to protect winter and spring wheat yields.

Today’s cereal growers can realize an opportunity to enjoy significant profits by understanding common diseases and taking steps to protect yields, from variety selection to fungicide application. Common cereal diseases to manage include the following:

Fusarium Head Blight

Commonly known as head blight (scab), this fungal disease, caused mainly by the fungus Fusarium graminearum, is present wherever cereal crops are produced. According to Dr. Don Hershman, Extension plant pathologist, University of Kentucky, head scab is the disease most feared by growers of wheat and other cereals, including barley.

“You can go from having a beautiful-looking crop to a terrible-looking crop almost overnight and, with head blight infection, other diseases begin to creep in,” Hershman says.

The telltale symptom of Fusarium head blight is heads that are partially to completely bleached. This occurs 14-21 days after flowering (anthesis). If flowers in grain spikes are infected just after emergence, kernels that develop will be lightweight and highly shriveled.

Portions of the head that remain unaffected appear green and healthy. Over time, the fungus grows and infects nearby spikelet tissue in the same head; eventually all or most of the head may become diseased. In warm and humid weather conditions, pink-to-orange clusters of spores can be seen at the base of infected spikelets. Eventually, the fungus infects developing kernels, causing them to shrink and wrinkle. Dead, infected kernels, called “tombstones,” are lightweight, discolored and will not germinate if planted.

The Fusarium fungus can produce mycotoxins, which could be harmful to humans and livestock. These mycotoxins are subject to Food and Drug Administration (FDA) regulatory guidance. Growers attempting to sell grain containing mycotoxins, such as deoxynivalenol (DON), may have to settle for a “docked” or reduced price, or suffer outright rejection of the grain at delivery to the elevator. It’s economically important for growers to diligently watch and identify disease symptoms; otherwise, they may not be able to sell their grain.

“Crop protection for head blight begins with variety selection,” Hershman says. “There are no immune varieties, but some varieties are fairly resistant, and growers need to be aware of the disease resistance in the seed they choose.” Crop rotation, residue management, planting multiple varieties with different flowering dates to encourage disease escape, and timely fungicide applications will also help manage head blight and DON.

Leaf Rust

This fungal disease, caused by the fungus Puccinia triticina, occurs wherever wheat and other cereal crops are grown. It is most prevalent during mild temperatures and when dew is frequent during the jointing through flowering growth stages. Wind enhances the spread of spores during the day, while calm nights enhance dew formation.

Individual fields can be destroyed when leaf rust is severe prior to heading. Grain shrivels, and any nutrients produced – primarily in the flag leaf – are used by the fungal infection instead of being transported to the grain. Early infection can result in weak plants and poor root and tiller development.

Leaf rust, also called brown rust, attacks only foliage and can be identified by the dusty, reddish-orange to reddish-brown marks produced on the upper leaf surfaces.

In recent years, fungicide trials at Texas A&M, College Station, showed that leaf rust can reduce yields up to 50 percent on untreated check plots.

Planting resistant seed varieties and timely fungicide applications offer the best protection.

Stripe Rust

Wheat stripe rust, also known as yellow rust, appears early in the season because it prefers cool, moist weather, while leaf rust becomes prevalent later in the spring, when temperatures warm. Growers need to be alert to identify and manage stripe rusts for a longer time period and realize that leaf rust pressure increases as stripe rust pressure begins to fade.

Caused by the fungus Puccinia striiformis, stripe rust typically produces yellow or orange blister-like lesions, or pustules, that are arranged in stripes. The disease primarily occurs on leaves, although glumes and awns may also be affected. The pustules produce massive amounts of spores that are easily dislodged and dispersed through the wind. These spores may appear as orange dust on the clothing of individuals walking through a heavily infested field.

If wheat stripe rust spores land on another living wheat leaf, they can germinate and infect the leaf. The rust grows inside the leaf, then produces new pustules containing new spores. During high humidity in winter, most spores remain in small clumps, which are relatively heavy and fall out of the air quickly, confining their spread to short distances. This part of the life cycle leads to “hot spots” of infection seen in crops in later winter and early spring.

In lower humidity, stripe rust spores disperse more freely into the air and can travel for much greater distances. This may result in a uniform pattern of disease development from mid-spring onward.

Kansas State University research shows that stripe rust can cause losses of 40 percent or more when the disease becomes established on susceptible varieties before heading.

Where available, choose resistant wheat varieties and use timely fungicide applications for the best protection.

Fungal Leaf Spot Diseases

Most fungal leaf spot diseases are common infections in wheat and other cereals. Symptoms are similar and can be observed as lesions varying in color from brown to purple to gray. Common fungal leaf spot diseases include:

Septoria Leaf Blotch

Septoria leaf blotch, caused by the fungus Septoria tritici, typically develops on wheat on the lower leaves in early spring. It moves to upper leaves under cool, wet weather conditions when temperatures range from 59 to 77 degrees F. Irregular spots will be reddish-brown in color and are spread randomly over leaf blades.           

The greatest yield losses happen when infection attacks the flag leaf and two leaves below in late spring or early summer before flowering. Yield reduction can be as high as 20 to 30 percent, according to The Ohio State University.

Tan Spot

This fungus, caused by the Pyrenophora tritici-repentis fungus, produces lesions measuring 1/5 to 3/5 inches with a dark brown center. Early in the season, tan spot lesions often have a distinctive yellow border. A reddish discoloration, called “red smudge,” can form on the seed coats of infected kernels. These infected kernels are usually plump, not shrunken, but the discoloration in the grain may prompt market discounts.

When wet weather lasts more than 24 hours, tan spot spores germinate and infect surrounding plants. Each wet weather episode releases new spores, and symptoms become apparent within five to seven days.

Tan spot is considered to be the most destructive leaf spot disease found in all wheat classes throughout the growing season, according to North Dakota State University research. Tan spot is among the foliar leaf diseases that can cause reduced wheat test weights and yield losses of up to 50 percent.

Net Blotch

The net blotch fungus, caused by Pyrenophora teres, is restricted to barley crops and is abundant during cool, damp periods. The disease first appears on seedling leaves as oblong brown spots. These blotches are often confined between the veins and marked internally with a hallmark netting appearance. Infections occur on leaves throughout the growth cycle, from seedling until near maturity. The lesions may cover a large portion of the leaf, reducing or destroying its photosynthetic ability.

Stagonospora

Stagonospora is caused by Parastagonospora nodorum and Parastagonospora avenae f. sp. triticea. This fungus initially causes small, water-soaked lesions on the lower leaves of the plant that become yellow and, eventually, red-brown. As the disease advances, the lesions form a gray to gray-brown center encompassing brown specks. After flowering occurs, wet weather can contribute to the development of lesions on the glumes, frequently beginning at the tip. Whole areas of the glumes may eventually become covered with dark purple to dark brown lesions with ash-gray areas. Severe leaf blotch infections damage crops by resulting in lightweight, shriveled kernels.

Powdery Mildew

The spores of this fungus, caused by the fungus Blumera graminis, germinate on the leaf surface. They then invade the wheat or other cereal plant by extracting nutrients from the plant cells, but without killing the plant. The disease becomes economically important under humid, rainy conditions and in dryland areas, where irrigation is used for high yield production, because of reductions in seed size and number of seeds per unit area.

Powdery mildew increases in some regions because of increased nitrogen fertilizer applications, which are used to boost soil fertility and enhance yield.

Rapid growth of the disease typically begins on the lower leaves and sheaths when plants begin to joint and temperatures are cool. After the flowering stage and when temperatures rise, infection and disease development decline.

Early-season powdery mildew stimulates the production of tillers that become nonproductive later in the growing season. According to a study produced by the Food and Agriculture Organization of the United Nations, yield losses may be as high as 40 percent. Fungicide application can reduce losses.

Managing Cereal Diseases

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 Bayer 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 seed treatment is a powerful combination of fungicides that promotes fast crop emergence while protecting seedlings from stand-damaging diseases such as Rhizoctonia solani, Fusarium spp. and Pythium spp., as well as smuts.

Raxil® PRO Shield is a convenient, all-in-one formulation, offering the same complete disease package as Raxil® PRO MD with the added power of Gaucho 600FS. It is a broad-spectrum seed-applied fungicide containing two modes of action that provides protection for seeds and seedlings against infection from Fusarium, Rhizoctonia, Pythium, smuts, bunt and seed decay fungi, such as Penicillium. It offers the added benefit of imidacloprid to protect seedlings against damage from wireworms, aphids and hessian fly.

A number of fungicides are available for both early-season and late-season control of these common leaf spot diseases. Chemistries from two of the most commonly used classes of fungicides, triazoles and strobilurins, provide good-to-excellent activity against wheat leaf spot diseases, and when used in conjunction with best management practices, can help manage disease resistance. Wheat growers should consider fungicides with systemic movement and curative properties for the broadest protection from cereal foliar diseases. However, strobilurin-containing fungicides should not be applied to headed wheat if Fusarium head blight is a concern. Using strobilurin chemistries after flag leaf emergence may result in increased mycotoxin levels should Fusarium head blight infect the field later.

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 and tan spot. 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. Many plant pathologists call it the “top-of-the-line” fungicide.

Absolute® Maxx contains two chemistries, a triazole and a strobilurin, that work together through different MOAs. The two most common factors limiting yields of wheat in the High Plains are moisture stress and rust. The strobilurin component in Absolute Maxx promotes higher productivity when precipitation is limited. The triazole provides fast activity against the various rust pathogens. Both components come together to give residual control of the important cereal foliar diseases.


For more information on wheat disease control options from Bayer, contact your local US representative or visit www.cropscience.bayer.us/contact.


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