13. Small Grain Disease Management: Cultural Practices, Varieties, Seed Treatments, and Foliar Fungicides

By Randy Weisz and Tom Melton

Small grain diseases are a serious threat to high yields and grain quality in many parts of North Carolina. While some small grain diseases can be treated after they appear, others are difficult or impossible to treat after they are established. Additionally, when commodity prices are low, the costs associated with controlling diseases once they have started can cut deeply into already thin profits. The best way to minimize disease management costs and to maximize yield potential is to include disease control in every stage of small grain management, including variety selection, choice of seed source, seed treatment, planting date, seeding rate, fertility, and all aspects of spring pest control. A comprehensive disease management plan starts with an understanding of the major diseases common to small grains in North Carolina.

COMMON DISEASES OF SMALL GRAINS IN NORTH CAROLINA

Barley Yellow Dwarf

Description. Barley yellow dwarf virus (BYDV) is the most important viral disease of wheat, oats, barley, and rye in this state. Symptoms of BYDV are often overlooked and can easily be confused with nutritional problems. Infected plants are normally found in small areas usually only a few feet in diameter. Leaves are discolored in shades of yellow, red, or purple, especially from the tip to the base and from the margin to the midrib. Plants may also be stunted. When infected early in the fall, discoloration and stunting can be severe. Infections that occur in the spring generally result in less severe symptoms. (For a color photo of BYDV, see http://greengenes.cit.cornell.edu/gifs/wheat_diseases/054.gif.)

The virus is transmitted by aphids that over-summered on nearby corn crops or host grasses, such as orchard grass, tall fescue, or ryegrass. The aphids that carry BYDV can only survive on living plants and do not live in plant debris or in the soil. The bird cherry-oat aphid and English grain aphid are the most important transmitters of BYDV to small grains in the fall and spring, respectively.

Weather. BYDV is most likely to occur after a warm fall and mild winter, which foster the growth and development of both the host grasses and aphid populations.

Control Measures

Cultural Practices. The standard preventive measure is to plant small grains after a frost has reduced the aphid population. This, however, can make early planting impossible, especially in years when the first frost is delayed.

Varieties. Information on wheat varieties resistant to BYDV is limited. Varieties known to be resistant as of 2003 are listed in Table 13-1. The table will be updated as new information is received, and it will be posted on the Web at http://www.smallgrains.ncsu.edu/Varieties/VarietyCharacter.pdf.

There is no available information on variety resistance to BYDV for other small grains.

Insecticides. Insecticides sprayed in the fall, either on the previous crop residue or on the small grain foliage, have not been consistently effective in controlling the spread of BYDV. The one possible exception to this generalization are long residual foliar pyrethroid insecticides, such as Warrior T. On-farm tests have suggested that when applied shortly after wheat emergence (at the 2- to 3-leaf stage) Warrior T can provide aphid protection and limit BYDV infestation.

Seed Treatments. Gaucho and Cruiser are insecticidal seed treatments for wheat. They prevent the development of aphid populations in small grains, thereby helping to prevent the spread of BYDV. Since they are applied as a seed treatment, the decision to use them must be made before planting, which is before a grower knows if BYDV is going to be a problem. Consequently, these seed treatments may be most cost effective when used in high-risk situations (early planting, warm fall weather, a field history of BYDV, and selection of varieties that do not have “good” BYDV resistance).

Leaf Rust

Description. Leaf rust attacks wheat, rye, triticale, and barley. Lesions are small, circular, and vivid orange in color. They may occur on stems, but are most common on the upper surface of the leaves. When heavily infected, the whole leaf will die. Once leaf rust is seen in wheat, it should be treated as soon as possible because it can develop very rapidly. (For color photos of leaf rust, see http://oak.ppws.vt.edu/stromberg/smallgrain/biology/wlrust.html.)

Weather. Winds can carry rust spores great distances. Rapid development occurs between 60 and 80°F when moisture is not limiting.

Control Measures

Varieties. Many resistant varieties are available and are the most economical control measure. Because resistance breaks down over a period of several years, growers should check with the county Extension agent, refer to the latest Small Grain Official Variety Test Report, or check the latest variety characteristics data at http://www.smallgrains.ncsu.edu/Varieties/VarietyCharacter.pdf.

Foliar Fungicides. If an infected wheat crop has reached threshold disease levels (Table 13-2), and has a yield potential of at least 55 bushels per acre, a foliar fungicide may be economical. This assumes a wheat price of at least $3.00 per bushel. A yield potential of 65 bushels per acre is required if the price is $2.50. Fungicides are not recommended for wheat if the price and yield thresholds are not met. Threshold levels are not available for other small grains.

Loose Smut

Description. Loose smut symptoms occur between heading and maturity. At first, diseased heads are blackened and clearly visible among newly emerged green, healthy heads. Infected heads emerge slightly earlier than normal and have their spikelets, except for a delicate membrane, entirely transformed into a dry olive-black spore mass. The membrane tears easily as heads emerge, and once the spores are dispersed by wind, all that remains is the stem or rachis (stem of the head). Infected seed appears normal. The fungus, which is found inside the embryo of the seed, will grow within the plant from germination until a�er seed heads emerge and smutted grains appear. Therefore, symptoms from infection that takes place in one year are not seen until plants from the infected seed mature in another year. (For color photos of loose smut, see http://oak.ppws.vt.edu/stromberg/smallgrain/biology/wlsmut.html.)

Weather. Infections occur only during flowering and are favored by wet weather and cool-to-moderate temperatures (61 to 72°F).

Control Measures

Since loose smut is seedborne, control measures focus on the seed to be planted.

Using Certified Seed. Certified seed fields are inspected for loose smut and strict standards are enforced. Seed fields with loose smut are rejected. Thus, using certified seed is a highly effective way to avoid loose smut.

Seed Treatments. Many seed treatments, such as Baytan-Thiram, Dividend Extreme, Raxil-Thiram, and RaxilXT, are effective against loose smut. See the Chapter 5, Do You Need a Seed Treatment, which is at http://www.smallgrains.ncsu.edu/Guide/Chapter5.html.

Powdery Mildew

Description. Powdery mildew can be a serious disease of wheat, barley, or oats. Lesions are first noticeable as white, powdery spots on the lower leaves and stems. As the lesions mature, they become darker, and sometimes salmon-colored with black spots (perithecia). If there is a heavy infestation, clouds of white spores can be seen when walking through the infected grain. (For color photos of powdery mildew, see http://oak.ppws.vt.edu/stromberg/smallgrain/biology/wpmildew.html.)

Weather. Spores are dispersed by wind. High humidity (with or without rain) and cool temperatures (59 to 72°F) favor disease development. The disease is markedly slowed when temperatures are above 77°F. Since late spring temperatures typically are above 77°F, waiting for warmer weather can be an effective control if threshold levels have not been reached.

Control Measures

Cultural Practices. Volunteer wheat plants often carry this disease, so plow them under where feasible. Rotating out of small grains, reducing the frequency of small grains in a crop rotation, and planting late also can reduce the severity of powdery mildew.

Varieties. Many resistant varieties of wheat and barley are available, and resistance is the most economical control measure. Because resistance breaks down over a period of several years, growers should check with the county Extension agent, refer to the latest Small Grain Official Variety Test Report, or check the latest variety characteristics data at http://www.smallgrains.ncsu.edu/Varieties/VarietyCharacter.pdf.

Seed Treatments. Baytan seed treatment can reduce the amount of powdery mildew during the fall and winter and result in lower disease incidence the following spring. Baytan must be applied by a certified seed conditioner and is not currently labeled for on-farm or hopperbox use. Raxil-Thiram, and Dividend are also labeled for powdery mildew, but research at NC State University and at Virginia Polytechnic Institute has not shown them to be effective for powdery mildew control.

Foliar Fungicides. If an infected wheat crop has reached threshold disease levels (see Table 13-1) and has a yield potential of at least 55 bushels of wheat per acre, application of a foliar fungicide may be economical. This assumes a wheat price of at least $3.00 per bushel. A yield potential of 65 bushels per acre is required if the price is down to $2.50. Fungicides are not recommended for wheat if either of these two conditions is not met. Thresholds for other small grains are not available.

Septoria Leaf and Glume Blotch

Description. Septoria leaf and glume blotch can be a serious disease of wheat. Symptoms may occur at any time during the growth of the plant and on any portion of the plant. Lesions are round to lens-shaped and are found on the oldest leaves first. Lesions begin with a water-soaked appearance, later drying to a yellow or reddish-brown color. Tissue death eventually extends beyond the lesion, and sometimes the entire leaf is killed. A diagnostic feature in older lesions are the small, dark, “pimple-like” spots known as pycnidia. (For color photos of septoria, see http://oak.ppws.vt.edu/stromberg/smallgrain/biology/wgblotch.html.)

Weather. The fungus is dispersed by windblown rain. Wet, windy weather favors spore dispersal and increases the severity of this disease. Dry periods prevent infections and stop disease development.

Control Measures

Cultural Practice. Rotation away from small grains for at least three years can lessen the severity of septoria, and plowing under wheat stubble will prevent infection from the previous wheat crop. Potash, copper, and magnesium should be kept at recommended levels. In some cases, septoria can be seedborne; therefore, certified seed can reduce introduction of the disease.

Seed Treatments. Seed treatments alone are not good preventive measures for septoria.

Foliar Fungicide. If the infected wheat crop has reached threshold disease levels (Table 13-2), and has a yield potential of at least 55 bushels of wheat per acre, applying a foliar fungicide may be economical. This assumes a wheat price of at least $3.00 per bushel. A yield potential of 65 bushels per acre is required if the price is down to $2.50. Fungicides are not recommended for wheat if both conditions are not met. Additionally, when selecting a foliar fungicide for powdery mildew or leaf rust, select one that is also effective against septoria.

Scab or Head Blight

Description. Scab, a fungus that is seen as prematurely bleached heads or spikelets, can occur in all small grains. Wheat and barley are the most susceptible to the disease, oats are a little less susceptible, and rye and triticale are the most resistant. If the rachis is infected, everything above that point will be faded. Small dark spots (perithecia) and superficial pink or orange fungal growth (mycelium and spores) can be seen at the base of the spikelets. Only partly filled seed will be found in the infected spikelets. Scab also results in toxins in the harvested grain. The most common scab toxin is “DON” or vomitoxin. When DON reaches 2 parts per million, the grain is no longer fit for human consumption and cannot be sold to a flour mill. When DON reaches 5 parts per million, it is no longer fit even for swine feed. (For color photos of scab, see http://www.smallgrains.ncsu.edu/Diseases/ScabPictures.html.)

Weather. The fungus is spread by air currents. When small grains flower, moist warm weather (77 to 86°F) favors scab.

Control Measures

Sadly, there are no single management practices that will guarantee that scab will not develop and there are no effective foliar fungicides that will control scab. However, there are important steps wheat producers can take, that when added together can greatly reduce the likelihood of a major scab outbreak. These steps are outlined below.

Plant Wheat Varieties With Resistance to Scab: As of this writing (2004) there are no wheat varieties that are completely resistant to scab. However, research at Virginia Polytechnic Institute and across the Southeast has demonstrated that wheat varieties McCormick, NC-Neuse, Roane, and Vigoro-Tribute are moderately resistant to scab. North Carolina producers who have had a history of scab should give these varieties serious consideration. Updated wheat variety scab resistance will be published every year on the Web at: http://www.smallgrains.ncsu.edu/.

Tillage and Rotation: The primary sources of the disease are corn and/or wheat residues left on the soil surface from previous crops. Thus, conventional-tillage practices that bury these residues are the most effective means of controlling scab. Conversely, no-tillage wheat planted into corn stubble, or no-tillage wheat planted after double-cropped soybeans when wheat straw is still present from the previous crop are at the highest risk to scab.

For producers who wish to plant no-tillage wheat, the best rotation might be to follow full-season soybeans, or to follow cotton (as long as it was not planted minimal-tillage into standing wheat straw that is still present). If producers must plant no-tillage wheat after corn, anything they can do to mow, chop, grind, and/or shred the corn stalks may help reduce the risk of scab at least a little.

Whether planting conventional- or no-tillage wheat, rotations that have wheat following crops other than corn or double-cropped soybeans will be the most effective in reducing scab infestations.

Force Heading to Occur Over a Series of Spring Dates: The scab fungus is present on corn residues left on the ground prior to planting wheat. The fungus is spread to newly formed wheat heads when rain splashes the spores off the corn and onto flowering heads in late spring. If the weather is warm and moist, the disease infests the heads and spreads. So an effective way to avoid scab is to miss these weather conditions by planting several wheat varieties with different “heading dates”. That will ensure that head emergence and flowering will be staggered through the spring, reducing the chance that environmental conditions will be conducive to scab in all wheat fields. A second way to force wheat to flower over a period of dates in the spring is to stagger planting dates.

At Harvest: Scab infested grain heads have a pinkish hue. If scab is present the combine may be adjusted so that the lightweight diseased grain is removed along with the chaff. This will not remove all the infected grain, but can help reduce mycotoxin levels in grain heading to market.

Build a Plan of Attack

Step 1 : Pick at least three wheat varieties for production. These should include varieties with moderate scab resistance and be from at least two different heading date classes.

Step 2 : Stagger planting dates for the chosen varieties across several weeks.

Step 3 : Plant conventional-till. If planting no-till avoid following corn or planting into wheat residues. If no-till production following corn is unavoidable, then shred, chop, or grind corn stalks as early as possible.

Step 4 : Scout the field before harvest. If scab is present, adjust the combine to remove as many of the diseased kernels as possible.

Seed Treatments. Planting scab-infected seed is not recommended. All farmer-saved seed should be thoroughly cleaned to remove any small, light, scab-infected seed. Germination should be tested, and if it is low, a seed treatment may be used to improve stand establishment. Since scab is spread to small grains in the spring, seed treatments will not protect the crop from new infections.

Take-All

Description. Take-all symptoms are most obvious near heading when plants appear uneven in height, begin to die prematurely, and have white, bleached-out heads. Earlier in the season, plants with take-all are stunted, mildly chlorotic, and have fewer tillers. Heads on tillers killed by take-all are distinctly bleached, sterile, have shriveled grain, and are subject to darkening by “sooty” molds. Diseased plants typically pull up easily, and on close examination their roots appear sparse, blackened, and brittle. Blackened roots and sometimes-blackened stems just above the root crown are diagnostic. (For color photos of take-all, see http://oak.ppws.vt.edu/stromberg/smallgrain/biology/wtall.html.)

Weather. High winds can cause diseased plants to lodge.

Control Measures

Short rotations like continuous wheat double-cropped soybeans are a common cause of root rots, such as take-all.

Cultural Practices. Rotating out of small grains even for one year can reduce the severity of take-all.

Seed Treatments. Dividend may reduce take-all, but it is not an effective control measure and should not be used in place of rotation away from small grains.

Tan Spot

Description. Tan spot (also called yellow leaf spot) primarily infects wheat and rye. It develops on both upper and lower leaves. The lesions initially appear as tan-brown flecks that expand into lens-shaped lesions similar to the lesions produced by the septoria leaf blotch fungus. Tan spot lesions range in size from 1/4 to 1/2 inch in length and are surrounded by a chlorotic (yellow) border. As the lesion matures, the center becomes darker due to the production of airborne olive-brown spores that can infect other leaves, stems, and plants. (For color photos of tan spot, see http://oak.ppws.vt.edu/stromberg/smallgrain/biology/wtspot.html.)

Weather. Temperatures ranging between 60 to 82°F and periods of heavy dew favor infection.

Control Measures

As the wheat matures, the tan spot lesions produce oversummering reproductive structures called pseudothecia. The infested straw with pseudothecia remaining after harvest serves as the source of inoculum for the next season’s wheat crop. Therefore, no-till wheat planted into fields where wheat residue remains is at increased risk to tan spot.

Cultural Practices. Since infected wheat straw is a primary source of infection, plowing under the straw and rotating away from small grains are the best preventive measures.

Foliar Fungicide. Several foliar fungicides are effective against tan spot. However, thresholds have not been developed for this disease. Apply fungicides for tan spot in wheat only if the infected crop has a yield potential of at least 55 bushels of wheat per acre. This assumes a wheat price of at least $3.00 per bushel. A yield potential of 65 bushels per acre is required if the price is down to $2.50. Fungicides are not recommended for wheat if these conditions are not met.

USE RISK MANAGEMENT TO DEVELOP A MANAGEMENT PLAN

Determining the risk of small grain diseases is important creating a sound management plan. The three most common and economically damaging diseases of small grains in North Carolina are leaf rust, powdery mildew, and septoria leaf and glume blotch. These diseases rarely occur in the mountain or northern and central piedmont counties. Consequently, growers in these counties are in a “low-risk zone” for small grain diseases (Figure 13-1) and rarely have to use seed treatments or foliar fungicides to control these three small grain diseases. In eastern and southern North Carolina, disease pressure increases and small grains grown in the coastal plain and tidewater counties are at “high risk” to infection. Growers in a high-risk zone need to make disease control part of their small grain management plan.

Low-Risk-Zone Management Plan

Though small grains in this region rarely need to be treated for powdery mildew, rust, or septoria, growers should scout in the spring to be sure that disease levels are below the threshold (Table 13-2). In years when weather conditions are unusually warm and moist, scouting will catch these diseases in time to apply fungicides. If a grower in the low-risk zone has a field with a history of exceeding disease thresholds, the disease management plans for the high-risk zone should be followed.

High-Risk-Zone Management Plan

By far the most common small grain grown in the eastern North Carolina is wheat. Consequently, the following recommendations are designed for wheat producers in the high-disease-risk zone. Some producers in the high-risk zone grow wheat primarily as a cover crop and are not “pushing” their wheat for high yields. Seed treatments or fungicides for disease control will rarely be economical for these producers since their yields generally will not be high enough to cover the input costs of these controls. These producers should use high-quality disease-free seed of varieties that have good or very good resistance to powdery mildew, and if possible, good resistance to leaf rust. As long as the yield potential of these crops is low, scouting and application of foliar fungicides will not be economical.

Growers in the high-risk zone who are attempting to achieve maximum profits from their wheat yields will need a comprehensive disease management program. The goal is to achieve good disease control without the expense of a foliar fungicide whenever possible. Disease control planning begins with powdery mildew. Powdery mildew is the easiest disease to plan against. Plant a resistant variety. Because resistance breaks down over a period of several years, growers should check with the county Extension agent, refer to the latest Small Grain Official Variety Test Report, or check the latest variety characteristics data at http://www.smallgrains.ncsu.edu/Varieties/VarietyCharacter.pdf. Growers in the high-risk zone who plant varieties with only fair or poor powdery mildew resistance can still hold powdery mildew at bay by using a specialty seed treatment, such as Baytan. Otherwise, growers will frequently have the added expense of applying foliar fungicides. No matter which varieties or seed treatments are used, follow good cultural practices: do not plant too early, destroy volunteer wheat plants, and regularly scout in the spring for powdery mildew. (see Table 13-2).

After planning for powdery mildew control by using resistant varieties or a seed treatment, consider leaf rust. Leaf rust can develop very rapidly late in the season and cause serious yield loss. Seed treatments or fungicide applications made when nitrogen is topdressed are not effective against leaf rust. Making control even more difficult is that most foliar fungicides that could be used to stop leaf rust infections are off label by the time the disease develops. Consequently, the best defense against this disease is to plant resistant varieties. Check with the county Extension agent, refer to the latest Small Grain Official Variety Test Report, or check the latest variety characteristics data on the Internet at: http://www.smallgrains.ncsu.edu/Varieties/VarietyCharacter.pdf.

When possible, choose a variety resistant to powdery mildew and leaf rust to minimize the chances of needing to apply a late-season fungicide. Since leaf rust can develop rapidly and cause serious damage, scout all high-yield-potential wheat fields regularly in the spring for leaf rust (see Table 13-2).

No wheat variety is resistant to septoria leaf and glume blotch, and seed treatments are not effective against this disease. Since septoria can be seedborne, it is important to use high-quality disease-free seed. Additionally, septoria can be carried over the summer on wheat stubble, so destroying wheat residues can be beneficial. Scout high-yield-potential wheat fields in the spring, and apply fungicides if the threshold has been exceeded (Table 13-2). Additionally, if a fungicide is applied for either powdery mildew or leaf rust, give the crop additional protection by using a material that is effective against septoria.

Scouting, Fungicide Timing, and Disease Thresholds

All small grain growers, and especially growers in the high-risk zone (Figure 13-1, above) who are pushing their crop for high yield, should scout their small grains for powdery mildew, leaf rust, and septoria weekly in the spring. Ideally, begin scouting when the flag leaf emerges and continue weekly through heading. If a crop consultant is available, weekly scouting during this time frame is ideal, and fungicides can be applied only if and when thresholds are exceeded (Table 13-2). A timesaving, but less than ideal, procedure is to check only areas of the field that have the thickest growth, as they are most at risk to disease. Another timesaving, but less than ideal, approach is to look for diseases when checking for cereal leaf beetle (Figure 13-2, below). Current fungicide labels allow fungal diseases and cereal leaf beetle to be treated with a single tank-mix application from flag leaf emergence until the grain heads have emerged.

Figure 13-2. Foliar disease and cereal leaf beetle scouting and spray timing in wheat

Research has provided good thresholds for leaf rust, powdery mildew, and septoria (Table 13-2). If an infected wheat crop has reached threshold disease levels (Table 13-2) and has a yield potential of at least 55 bushels of wheat per acre, application of a foliar fungicide may be economical. This assumes a wheat price of at least $3.00 per bushel. A yield potential of 65 bushels per acre is required if the price is down to $2.50. Fungicides are not recommended for wheat if both conditions are not met. Thresholds for other small grains are not available.

SPECIAL CONSIDERATIONS FOR NO-TILL

Head scab is the major disease risk associated with no-till small grain production. Small grains infected with head scab can have reduced yield and lower test weight. They may also contain mycotoxins harmful to humans and livestock. Additionally, the disease is seedborne and will consequently infest the subsequent small grain crop if the seed is untreated. The fungus overwinters on infested corn stalks and wheat residues, and it produces spores that infect wheat at flowering. Spores are transmitted to the small grain at flowering by wind or rain splash during hot, humid weather. Once the disease is present, no control measures are effective. No-till small grains following corn or double-cropped soybeans, especially in the warm high-risk zone (Figure 13-1, above) are at increased risk to head scab. All no-till small grain producers should examine their crops after heading for head scab.

Septoria leaf and glume blotch and tan spot may also be more problematic in no-till small grains. Tan spot and septoria are transmitted to small grains from wheat residues left on the soil surface. Consequently, when no-till small grains are planted into mixtures of residue that contain wheat straw from previous crops, both these diseases may increase in severity. Tan spot is most frequently seen in North Carolina in no-till fields.

SUMMARY

An appropriate disease management program is an important part of high yielding small grain production in North Carolina. The state can be roughly divided into two disease-risk zones. The counties to the west are mostly in the low-disease-risk area (Figure 13-1, above), whereas counties in the east and south generally fall into the high-disease-risk zone. Small grain producers in the low-risk counties generally do not have as difficult a time managing foliar diseases, such as powdery mildew, leaf rust, or septoria leaf and glume blotch. For these producers, scouting their fields in the spring may be the primary disease control practice required. Producers in the high-risk counties need a much more proactive disease management program. These producers need to integrate variety selection, choice of seed treatment, scouting, and application of foliar fungicides into a comprehensive program.

Thanks to Dr. Eric Stromberg for his thresholds and figures.

This file is a chapter from Small Grains Production Guide, 2004-05. Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the NC Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension Center.

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Last Revised Sept. 2004