By: Randy Weisz, Crop Science Extension Specialist, NCSU

Disease Identification: The first step in solving disease problems is to know what you are dealing with. Excellent small grain disease information and assistance with disease identification can be found in the 2004-2005 Small Grains Production Guide, and on-line at: http://www.smallgrains.ncsu.edu/Diseases.

 Barley Yellow Dwarf: 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.

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.

The standard preventive measure is to plant small grains after a frost has reduced the aphid population. This, however, can make timely planting impossible, especially in years when the first frost is delayed. Consequently, the best control measure is to plant varieties that are resistant to this disease (see (http://www.smallgrains.ncsu.edu/Varieties/Varieties.html) .

Powdery Mildew: One of the most yield limiting factors in North Carolina wheat production is powdery mildew. This is especially true in the coastal plains, the southern piedmont, and some tidewater areas. If the weather is warm, powdery mildew begins the infection cycle soon after planting. Under these conditions, a producer will not see the disease in the young wheat crop, but it will slowly rob the new crop of tillers and build into a full-blown infection ready to take hold in the spring. Conventional producers often do not consider powdery mildew in their planning because they can rely on foliar fungicides to control the disease if it occurs. Organic producers do not have that luxury.

The best protection against powdery mildew, is to grow wheat varieties that are resistant to it. In fact, an examination of the Small Grains Official Variety Test Results from the coastal plains, shows that the highest yielding varieties are those that have powdery mildew resistance. Varieties that have the highest levels of resistance rarely have disease levels high enough to substantially reduce yield. Organic producers who want high yielding wheat must plant powdery mildew resistant varieties.

Organic Pesticides There are a number of fungicides approved for use in certified organic production systems that can be used for control of diseases — mainly non-synthetic compounds or biocontrols. These include: neem (and its derivatives), hydrogen peroxide, potassium and sodium bicarbonate, pesticidal soaps and copper products. While these products do have potential for controlling some diseases in corn, no research has been done on them in field corn in North Carolina and, therefore, we can give no recommendations for their use in the state. The cost of these organically approved pesticides may be prohibitively expensive for field crop production. Conditions for use of a pesticide must be documented in the organic system plan (NOP 2000). See the Pest Management Web Resources for more information, or check out the OMRI site.

What varieties are resistant? Wheat varieties lose their resistance to powdery mildew very rapidly. A variety that has good resistance the first year or two after its release may deteriorate in its third year and become highly susceptible. Consequently, wheat varieties grown in North Carolina are evaluated for disease resistance every year, and new disease resistance ratings are published in the Small Grains Official Variety Test Results, and posted on the internet (http://www.smallgrains.ncsu.edu/Varieties/Varieties.html) every July in time for growers to order the best varieties for the next year’s production. Organic producers should check this web site every year (or contact their county Extension agent for the information) make a list of the most resistant varieties, and use this list as a starting point for ordering seed.

A second defense against powdery mildew is to plant late when the weather has turned cold. Powdery mildew does not grow in cold fall and winter weather. Of course wheat does not grow well in these conditions either. Consequently, while late-planted wheat may avoid powdery mildew, it also usually suffers from lower yields, and as described below, attack by cereal leaf beetle.

Thinking of planting early? Remember, powdery mildew likes warm fall weather. Research in North Carolina and Virginia has shown that planting wheat ahead of the suggested starting dates (see the 2004-05 Small Grain Production Guide or

http://www.smallgrains.ncsu.edu/Guide/Chapter7.html) puts the crop at sever risk to powdery mildew unless a fungicidal seed treatment is used. Organic producers, and especially those in the coastal plains, southern piedmont, and tidewater regions should avoid planting ahead of the recommended planting dates.

Leaf Rust: Leaf rust is a foliar disease that attacks wheat late in the growing season. While leaf rust can occur anywhere in North Carolina, it is most likely to be problematic in the tidewater region. Like powdery mildew, conventional producers rely on foliar fungicides to protect the crop from this disease. Organic producers can not. There are varieties with good resistance to leaf rust, and organic producers, and especially those in the tidewater, should try to produce varieties that have a combination of powdery mildew and leaf rust resistance. Variety resistance to leaf rust also deteriorates from year to year, so organic producers should check the most recent variety ratings (http://www.smallgrains.ncsu.edu/Varieties/Varieties.html) every year before ordering seed.

Loose Smut: 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 the 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.

Because loose smut is seedborne, control measures focus on the seed to be planted. Certified seed fields are inspected for loose smut and strict standards are enforced. Seed from fields with loose smut are rejected. Thus, using certified seed is a highly effective way to avoid loose smut. Organic producers who use farmer saved seed, should never plant seed from a crop infected with loose smut as it is a seed-borne disease and survives inside the seed.

Septoria Leaf and Glume Blotch. Septoria leaf and glume blotch may occur at any time during the growth of the plant and on any portion of the plant. Rotation away from small grains for at least three years can lessen the severity of septoria, and plowing under of 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, so certified seed can reduce introduction of the disease. Organic producers should never save seed for plant back if the wheat was infected with Septoria!

Scab or Head Blight. Scab, a fungus that is seen as prematurely bleached heads or spikelets, can occur in all small grains—wheat and barley being the most susceptible. Rye and triticale are the most resistant of the small grains to this disease. Scab occurs and is spread to small grains in the spring, and also results in toxins (vomitoxin is most common) in the harvested grain.

The first line of defense against scab is to plant wheat varieties with resistance to the fungus. However, as of 2004 there are no wheat varieties completely resistant to scab. A complete list of wheat varieties and their resistance to head scab can be found on-line at http://www.smallgrains.ncsu.edu or check with county Extension agents. Wheat producers with a history of scab should seriously consider selecting varieties based on this information. Tillage and rotation are another way to reduce the likelihood of scab, as the primary source of the disease are corn and/or wheat residues left on the soil surface from previous crops. Tillage practices that bury the residues and rotations of at least three years are an effective means of controlling scab. Since the scab fungus is spread to newly formed wheat head when rain splashes the spores from corn residues onto flowering heads in late spring, an effective way to avoid scab is to miss the weather conditions conducive to spread. Planting several wheat varieties with different “heading dates” will ensure that head emergence and flowering will be staggered through the spring, reducing the chance that environmental conditions will be suitable to scab in all wheat fields.

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. Likewise, all farmer-saved seed should be thoroughly cleaned to remove any small, light, scab-infected seed to prevent further infections in the field. Organic producers should never use farmer saved seed if head scab was present in the crop.

Other Small Grain Diseases: Several other small grain disease can be problematic in North Carolina. Growers in need of more detailed information than is contained here should check online at http://www.smallgrains.ncsu.edu.

 

Back to Pest Management Home Page

Home