If wheat does not emerge, the problem is usually due to dry soils, a hard crust on the soil surface, or seed with poor germination. If the wheat emerges, but is chlorotic or has poor seedling vigor, several factors may be the cause. Often, a combination of factors is involved.
The most common causes of yellowish, chlorotic wheat in the fall are:
* Poor root growth. This may be due to soils that are too dry, soils compacted by a heavy rain, seedlings buried by soil moving into the seed furrow, or poor seedbed conditions at planting time. If the plants have been emerged for several weeks or more, can be pulled up easily, and have only a couple primary roots visible, then the root systems are not extensive enough to provide enough nutrients. This will often cause yellowing.
* Nitrogen deficiency. In the seedling stage, nitrogen deficiency causes the plants to look pale or slightly chlorotic. Nitrogen deficiency also results in reduced tillering, top growth, and root growth in the fall. The primary causes of nitrogen deficiency in the fall are poor root growth, insufficient fertilizer rates, leaching from heavy rains, and the presence of heavy amounts of crop residue, which immobilize nitrogen.
* Leaf rust or tan spot. If leaf rust infects young seedlings in the fall, the plants may turn yellowish. Fall infections of leaf rust are not common in Kansas, but can occur. Producers will be able to see pustules on the leaves. Tan spot can also cause wheat to turn yellow in the fall. Viral diseases, such as soil-borne mosaic, wheat streak mosaic, and barley yellow dwarf, can infect wheat in the fall, but yellowing symptoms do not normally show up until early spring. It rarely, if ever, pays to treat fields in the fall for leaf rust or tan spot, even if those diseases do cause yellowing. Cold temperatures in the winter normally cure this problem.
* Cold temperatures. This has not been a factor yet this year, but if recently planted wheat has not yet emerged, it may not come up until the weather is cold. When temperatures are quite cold at the time wheat emerges, it can result in yellow banding on the leaves. If this is the cause of the yellowing, symptoms should eventually fade away.
* Mites. (See items in next section.)
Other poor vigor problems
Where wheat stands are thin and lack vigor but are not yellowish and chlorotic, the most common causes are drought stress and aluminum toxicity. Other possible causes include severe phosphorus (P) deficiency, insect or mite feeding injury, root rot and seedling diseases, and herbicide carryover problems.
Drought stress is an obvious cause of poor vigor, and needs no explanation or special diagnostic skills. If the wheat is not under drought stress and is not chlorotic or N-deficient, but still has poor vigor, it will take some effort to determine the problem. In this case, the producer should dig up some plants and look at the roots; take a routine soil test for pH, P, and potassium (K); determine the recent history of herbicide applications; examine the field for insects and mites; and examine the crowns and base of tillers for lesions or other signs of disease.
* Low pH. (See Article No. 2 below for a full discussion.)
* Winter grain mite. During daylight, winter grain mites can be found around the base of plants or hiding just under the soil surface. They thrive in cool, moist weather and retreat deeper into the soil under hot, dry conditions. Fields with loose, sandy, or loamy soils are more at risk than those with hard, clay soils. Significant infestations are ordinarily confined to central Kansas.
Mites feed on plants mostly at night, puncturing individual cells and causing leaves to turn silvery-gray. Leaf tips may turn brown. Young plants are most susceptible and may become stunted, producing little grain. Control may be necessary if large portions of a field show symptoms and mites are abundant in relation to wheat plant growth. Because fall populations develop from eggs laid the previous spring, problems are worse in continuous wheat. Crop rotation is preventive to some degree, but field borders may be affected when mites migrate from wild grasses.
* Brown wheat mite. The brown wheat mite is a common pest of dryland wheat in western Kansas that can be a problem as far east as U.S. Highway 77 (running from near Wichita to Manhattan) in dry years. The dark brown body is rounded or slightly oval, with the first pair of forelegs notably longer than the others. Affected plants have finely mottled leaves that appear yellowed or bronzed at a distance but lack the webbing produced by the Banks grass mite. Activity is highest in late fall and early spring, with populations usually peaking around mid-April. Outbreak potential is high because all adults are female, and each can produce 70 to 90 winter eggs in a three-week period. Damaging populations are usually limited to continuous wheat fields or those where volunteer wheat was present during the previous spring. Mite populations can be quickly reduced by heavy rains, but if feeding damage is apparent and there is no rain in the forecast, a treatment may be needed.
NOTE: For treatment options and a description of the appearance of both winter grain mite and brown wheat mite, see Wheat Insect Management 2011 at your local Extension office, or at: http://www.ksre.ksu.edu/library/ENTML2/MF745.PDF
* Hessian fly. Seedlings infested by Hessian fly in the fall are typically not yellow, but are often stunted. Affected plants usually have an unusually large, broad greenish leaf for about a month in the fall. Stem elongation is typically much shorter than normal.
* Flea beetles. These tiny insects cause whitish streaks on the upper surfaces of leaves. If streaking is severe, plants may die.
* Poor quality seed. Seed with unusually low test weights may emerge, but have poor seedling vigor. Certified seed in Kansas has a minimum test weight of 56 pounds per bushel. If test weights are less than that, seedling vigor may be less than desired. Seed size does not in itself have an impact on seedling vigor. Some varieties tend to have small seed, but this does not have any impact on seedling vigor. If the seed is smaller than normal for a particular variety and has low test weight (in the low 50s or less), this can result in poor seedling vigor, however.
* Seedling blight. This is one of the most common causes of post-emergence seedling death or sickliness. If the plants may emerge just fine, then seem to “go backwards” in the fall, the root system or coleoptile region may be diseased or dead. Several fungi cause seedling blight, and these diseases are often worse on early-planted wheat. Seedling blight may not kill the seedlings outright, but can lead to later problems with common root rot, crown rot, sharp eyespot, and dryland root rot (also known as dryland foot rot). Suspected plants can be sent to the K-State Plant Diagnostic Lab in the Department of Plant Pathology for diagnosis. Contact your county Extension agent for more information on submitting samples.
* Atrazine carryover. Wheat planted into soils with atrazine residue emerges then dies back from the tips of the oldest leaves first. Atrazine carryover is most likely to occur where there were high application rates, high soil pH, coarse-textured soils, and under dry conditions.
* Low soil P levels. Phosphorus deficiency symptoms are not common on seedling wheat, but can occur under extreme conditions. Symptoms will include stunting and purpling of the leaves.