‘Yellow wheat’ not just from drought

We’ve been getting quite a few calls and diagnostic samples of yellow wheat in the last few weeks. Conditions over most of the state have been warm and dry and that is often the main problem. However, people also have some fertility, insect and disease concerns. Let’s go through the various factors that are affecting the wheat this fall.

Although most of our wheat was planted into decent moisture, a lot of it is now under drought stress. The field pattern is often diagnostic. Drought stress is going to be worse in areas that were double-drilled, on higher ground, thinner soils and near tree lines.

Symptoms are also often diagnostic. In many cases, seedlings have not been able to establish crown roots in the dry soil and are getting along solely on the seminal root system. This is usually worse where seedbeds were too loose or planting was too shallow.

Some of the drought-stressed wheat is starting to show typical symptoms of rolled up leaves and a bluish cast to the foliage. However, some is stunted and turning yellow.

Why the difference? One explanation is the stunted root systems are not able to explore much soil volume and therefore don’t have access to all the nitrogen.

In some cases, nitrogen deficiency seems to be the main problem. The typical symptoms of nitrogen deficiency are yellowing of the lower leaves from the tip back. If the deficiency is severe, upper leaves will also be pale green or yellow.

Nitrogen deficiency is often worse following sorghum, according to Ray Lamond, Kansas State agronomist. Sometimes the field pattern is a give-away because you can see fertilizer skips or overlaps. In a few cases, terrace channels look worse, presumably due to leaching or denitrification last spring or summer.

A third cause of concern is greenbug damage. Greenbugs are light green aphids with a dark green stripe down their back. The symptoms of greenbug damage are clusters of small brown spots where the greenbug colonies feed. These spots can coalesce and kill the entire leaf.

Initially, individual yellow leaves are noticed. Later, entire plants may be killed. Keep a close eye on greenbugs because they have potential to devastate large areas of fields.

Another aphid problem is the bird cherry-oat aphid. These dark green aphids have a reddish brown patch on their rear ends. Although they may be numerous, they usually do not cause significant direct damage to plants. Their main impact is that they carry barley yellow dwarf virus (BYDV).

With the long, warm fall and the high populations of bird cherry-oat aphids, conditions have been ideal for spread of BYDV. Unfortunately, there is not much you can do about it. It is impractical to spray for aphids to prevent BYDV because it only takes one aphid to transmit the disease.

By the time you see a big population of aphids it is too late.

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