Guide to diagnosis of chlorotic and poor vigor wheat in fall
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.
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