|It’s been said that once you get the wheat crop seeded, 90% of the work is done. That doesn’t keep farmers from worrying about stand establishment over the next several weeks – particularly as drought conditions continue to plague the young wheat crop.
Jim Shroyer, Extension agronomist at Kansas State University, says that ideally, over the next few months 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, including discoloration, stunting, leaf loss or dying of emerged seedlings.
Let’s look at some of the causes of yellowing or stunting of the crop this fall. They could include:
* Nitrogen deficiency causes an overall yellowing of the plant with the lower leaves yellowing and dying from the leaf tips inward. It can also result 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. 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) 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 terms 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. In addition, liming to adequate pH levels following recommendations from a soil test can fix the problem long term.
* Leaf rust: while not common in Kansas in the fall, it can occur. It could cause the plants to turn yellowish. 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 cure this problem.
* 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. Both 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.
* 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.
This is not a complete list of possible problems on early-season wheat by any means, just some of the most commonly found problems.
For a complete discussion, see K-State’s publication S-84, Diagnosing Wheat Production Problems.
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