Wheat | Texas Plant Disease Handbook

Triticum aestivum

Common (dry land) Root Rot (fungus – Bipolaris sorokiniana): The fungus infects the sub-crown internode of young wheat seedlings. Root rotting can continue throughout the season and is most severe when new root formation is slowed. Later in the season, drought and warm temperatures favor root rot. Seed treatment and balanced fertility programs can reduce disease severity. Crop rotation may also reduce the disease and Common root rot is widely distributed in Texas.

Leaf Blotch (fungus – Septoria tritici): Lesions appear as pale green to yellow spots on the leaf. As cells in the spot are killed it turns brown. Lesions also appear on the head (see illustration). Later, gray to black fruiting bodies are formed in the dead spot. Control with sanitation (early plowing) and crop rotation.

Stem Rust (fungus – Puccinia graminis f. sp. tritici): Stem rust is recognized by the elongated, ragged pustules it produces on stem, leaf, sheath, blade, chaff, beard and occasionally on young kernels. Fragments of epidermis adhere to the sides and ends of the pustules, giving a ragged appearance. The brick-red color and large elongated pustules distinguish it from leaf rust which has small round pustules and orange-red spores. Stem rust is far more devastating than leaf fust on susceptible varieties. Soft wheat varieties are generally more susceptible than hard red winter wheat varieties. Barley, rye, wild barley, and goatgrass are also susceptible.

Take All (fungus – Gaeumannomyces graminis var. tritici): Take-all is most obvious near heading on plants growing in moist soil. Diseased plants have basal internodes that are shiny black and have few tillers. Heads ripen prematurely and are bleached white and sterile. Plants may break free at the crown when pulled from soil. The fungus causing take-all persists in infected wheat stubble. Take-all is favored by alkaline, compacted, infertile (esp. nitrogen and phosphorus-deficient) and poorly drained soils. The disease has only been observed in Texas under irrigated conditions on the High Plains and Rolling Plains. Rotation may reduce disease incidence in fields where take-all has been a problem.

Soil-borne Mosaic (virus): Symptoms of soil-borne mosaic appear early in the spring, usually when weather is cool and damp, but are rarely seen in fall or winter. Fields observed from a distance have irregular patches of light green or yellow plants. Symptoms on plant leaves range from mild green to yellow mottling and striping, giving the mosaic appearance. Stunting varies from moderate to severe and may be accomplished by rosetting. Symptom expression favors temperatures below 68^o F. Symptoms gradually disappear before harvest time if normal temperature persists. A soil-borne fungus, Polymyxa graminis, which is a parasite of roots of many grass plants, is the vector of this disease. Virus particles are either inside spores of the fungus or are attached to the spores. The fungus invades the roots in the fall, when soils are cool and wet, carrying the virus particles with it. There is no practical way to rid the soil of the soil-borne mosaic virus. Rotation out of wheat to other crops will reduce losses.

Foot Rot (fungi – Fusarium avenaccum, Fusarium spp.): Foot rot generally begins as an invasion of seedling roots. Foot rots are favored by cool temperatures. The fungus is more a secondary invader of roots initially infected by the common root rot fungus. The fungus surviving on crop residues and rotation may reduce disease severity.

Leaf Rust (fungus – Puccinia recondita): Leaf rust occurs on either side of the leaf and on the leaf sheath as small, reddish-orange pustules. In most years, leaf rust causes more damage in Texas than any other wheat disease. Growth is rapid between 59 – 72^o F. It causes a reduction in the number and size of kernels. The disease reduces forage production in fields where it is utilized for grazing. New races of the rust fungus originate naturally and challenge wheat varieties. The leaf rust fungus of wheat does not attack oats or barley.

Powdery Mildew (fungus – Erysiphe graminis f. sp. tritici): Powdery mildew is usuall found only on the leaves, but the fungus may attack all aboveground parts of the plant. It is noticeable first as small, irregular or circular, light gray spots on the upper surface of a leaf. The spots enlarge as the fungus grows and often may involve large parts of the leaf. As the spots age, the fungus on them takes on a flowery appearance which is due to the production of an enormous number of spores. Often the lower surface of the leaf beneath the diseased spots turns yellow and older parts of the spots turn brown. Affected leaves become deformed and crinkled and in severe cases they become brittle or they may die prematurely. As affected wheat approaches maturity, small, black fruiting structures (seen as black specks) may be scattered throughout the fungus growth on infected spots. This disease is not seed-borne but can be soil-borne. Close grazing to keep top growth reduced to a minimum will allow sunlight and air to keep the crown of the plant dried out, thus reducing the occurrence of powdery mildew. Rotations with non-host plants will also help reduce the soil-borne phase of this disease.

Stinking Smut or Bunt (fungus – Tilletia foetida): Heads affected by stinking smut fungus have a distinct blue cast. At bloom time infected heads are more slender than healthy heads and do not put out pollen sacks. At maturity they appear plumper but lighter in weight than normal heads. The smut ball consists of a mass of foul-smelling, dark-brown powder (spores of the stinking smut fungus). In the field, smutted heads usually stand more nearly erect than healthy heads because of their lighter weight. In some varieties, it is necessary to crush the kernels to determine if heads are diseased. The offensive odor indicates the presence of heavy infections either in fields or in shipped grain. Infested wheat is usually condemned before shipment. While smut balls may be removed by cleaning and recleaning seed, spores will still be carried on kernels.

Tan Spot (fungus – Pyrenophora trichostoma): Tan spots can occur on both leaf surfaces. Initial symptoms will be tan-brown flecks and the spots will enlarge into lens-shaped lesions. The fungus grows as a saprohpyte on crop residue. Because of this it has been observed to be more severe in no-till wheat. Cultural practices such as deep burial of straw and rotation will help reduce tan spot.

Wheat Streak Mosaic Virus: The vector of this virus is a microscopic eight-legged, cigar-shaped, wheat curl mite, Aceria tulipae. This disease is more severe in the High Plains area of Texas. Although most severe infection occurs in the fall, the characteristic yellowish streaking and mottling of leaves usually are first observed after warmer spring weather. As plants approach maturity, the leaves turn brown and die. WSMV causes red streak on corn kernels. Control for this disease is possible by clean tillage around fields, early destruction of volunteer wheat, late planting after frost

Glum Blotch (fungus – Septoria nordorum): Glume blotch occurs on the nodes, spikes, and glumes causing blackened areas. Stems are weakened and may bend or break just above the nodes. Seed may be shriveled, reducing yields and quality of grain. Seed treatment, rotation, deep burial of crop residue, foliar fungicides, and resistant varieties will reduce disease losses.

Loose Smut (fungus – Ustilago tritici): Loose smut destroys the grain and all glume structures of the spike leaving only the central rachis which head early. Infected plants, produce smut spores that are wind-borne to healthy plants at flowering time. Spores germinate and germ tubes penetrate the young wheat ovaries where the fungus remain dormant until those seed germinate. Use of seed treatment fungicides and disease-free seed will prevent this disease.

Rhizoctonia Root Rot and Sharp Eyespot (fungus – Rhizoctonia solani): Many Rhizoctonia strains infect wheat roots and culms. Culm infections are sometimes observed in North Central and East Texas. These are referred to as sharp eyespot. Sharp eyespot begins as a gray colored lesion on the lower leaf sheath. Lesions may later turn straw colored. Acid, sandy and dry soils increase disease risk as do cool spring temperatures. Bare Patch of wheat is also caused by a particular strain of Rhizoctonia solani. This disease has not yet been identified in Texas.

Barley Yellow Dwarf Virus: New growth of wheat plants infected in the seedling stage is chlorotic or yellowish in color. The entire plant will be severely dwarfed, tiller sparsely and produce few heads with little or no seed. Plants that become infected after the tillering stage are not dwarfed. The yellow dwarf virus can be carried from plant to plant by several species of grain aphids that overwinter on perennial grasses. No effective control measures are known other than utilizing tolerant varieties.