By Ryan Flaming, County Extension Agent, Agriculture & Natural Resources
Over the next month or two, wheat stands will hopefully become established over most of the state. Ideally, the wheat will take on a solid green color, form a secondary root system, and develop one or two tillers in addition to the main tiller. But sometimes there are problems. The most common symptoms of problems are discoloration, stunting, loss of leaves, or dying of emerged seedlings.
Causes of chlorosis or poor growth
If wheat is yellow or stunted and not growing this fall, what are the possible causes? Is it something producers can correct? Will it hurt yields? Some of the most common causes of yellowing and/or stunting in the fall are:
Nitrogen deficiency. Nitrogen deficiency causes an overall yellowing of the plant with the lower leaves yellowing and dying from the leaf tips inward. Nitrogen deficiency also results in reduced tillering, top growth, and root growth in the fall. The primary causes of nitrogen deficiency are insufficient nitrogen fertilizer rates, leaching from heavy rains, early-season denitrification or volatilization, and the presence of heavy amounts of crop residue, which can immobilize nitrogen. Topdressing the field during the winter can solve the problem, provided there is enough moisture to move the fertilizer into the root zone (and the ground isn’t frozen at the time of application).
Poor root growth. Chlorosis and stunting can also be due to poor root development, which can often result in nitrogen deficiency. 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 plants are yellow or stunted because the root systems are not extensive enough to provide enough nutrients. This may be due to dry soils, waterlogging, or poor seedbed conditions at planting time. If conditions improve, plants should develop secondary roots and the color should improve. If conditions do not improve and root growth remains stunted, the plants may winterkill more easily or may not be strong enough next spring to reach their full yield potential.
Aluminum toxicity (low-pH soils). Strongly acid soils may present several problems for wheat production. Aluminum toxicity is the most common problem associated with acid soils. Typical symptoms include thin stands and lack of vigor. High concentrations of aluminum will reduce development of the roots, giving them a short stubby appearance. The roots will often have a brownish color. In general, aluminum toxicity will reduce yield potential when soil pH levels get below 5.5 and KCl-extractable (free) aluminum levels are greater than 25 parts per million. When soil pH levels are 5.0 or less, yields start dropping off rapidly in most cases. Selecting adequate varieties for low pH conditions is essential. Applying phosphorus in the seed row at planting can also help. Liming to adequate pH levels following recommendations from a soil test can fix the problem long term.
Leaf rust. If leaf rust infects young seedlings in the fall, the plants may turn yellowish. Severe fall infections of leaf rust are not common in Kansas, but can occur. Producers will be able to see the small brown pustules on the leaves. Tan spot can also cause wheat to turn yellow in the fall. These seedling infections of tan spot are often associated with wheat sown into heavy wheat residue. Viral diseases, such as soil-borne mosaic, wheat streak mosaic, and barley yellow dwarf, can infect wheat in the fall. Some yellowing can occur in the fall but in most cases the severe yellowing symptoms do not show up until early spring. It rarely, if ever, pays to treat fields with fungicides in the fall for leaf rust or tan spot, even if those diseases do cause yellowing. Cold temperatures in the winter normally reduce the risk of problems.
Cold temperatures. 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.
Greenbugs or bird cherry oat aphids. These insects most commonly infest wheat sometime after the first freeze and before Christmas. They can cause plants to turn yellow and be somewhat stunted. These symptoms can occur in the fall, but don’t usually show up until early spring. Often, greenbug and bird cherry oat aphid infestations occur in patches in a field, not uniformly distributed. Each of these patches infestations are usually initiated by one winged female landing on a susceptible wheat plant. That female starts to produce more females, which then produce more females, and so on. The resulting infestation often radiates out from the initial infested plant in a roughly circular pattern. Greenbug and aphid infestations in the fall can make the wheat more susceptible to winterkill under normal winter temperatures.
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.
Causes of seedling death or loss of leaves
If leaves are being lost, or the plants are sickly or have died, it is important to find out why before replanting. Some of the most common causes of seedling death, sickliness, or loss of leaves include:
Seedling blight. This is one of the most common causes of post-emergence seedling death or sickliness. The root system or coleoptile region may be diseased or dead in infected plants. 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).
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.
Fall armyworms and army cutworms. Where fall armyworms infest the wheat, leaves start looking ragged from the “windowpaning” effect. As the worms grow, they will chew off entire leaves, tillers, or whole plants. Fall armyworms can move across a field in a wave, starting on one side of the field. Army cutworms may also damage wheat, much like fall armyworms. Army cutworms may successfully overwinter and continue feeding during mild spells throughout the winter and spring. Fall armyworms won’t overwinter, thus they’ll only be a problem until the advent of cold weather.
Grasshoppers. Grasshoppers can be a problem along the edge of a field, where severe feeding can occur as other foliage turns brown. Three to four passes, as needed, from a sprayer with an insecticide along the edge of a field can usually minimize damage from this pest.
False wireworms. These insects typically feed on seeds or seedling roots, and can cause death.
White grubs. If young plants are dying, with no aboveground symptoms evident, white grubs may be the cause. Check to see if roots are pruned.
If you notice any problems with your wheat fields this fall, give me a call at the Extension office (316) 284-6930 and I will be happy to come out and help you with your questions.