Possible causes for white heads in wheat
White heads have been appearing in many wheat fields around the state. Sometimes the white heads are just single tillers scattered throughout part or all of a field, and sometimes they occur in small to large patches. Heads might be completely white starting from the stem, or may just have a few spikelets showing the discoloration.
There are many causes of white heads. Here are some of the most common causes and their diagnosis.
Premature dying (hot weather). Fields in parts of the state are showing off-white color similar to take-all but in widespread areas, despite grain development still many times at the milk stage of development. This is premature dying is likely due to heat and drought stress, although many years it could be due to drowning, hot dry winds, or some other stress. The pattern of off-colored heads will often follow soil types or topography, and may occur in large patches. The grain will be shriveled and have low test weight. The area that seem to be most affected by premature dying spans the region between Marion, Dickinson, Harvey, Sedgwick, Reno, Saline, Clay, Cloud, and Riley counties (Figure 1).
Figure 1. Large patches of wheat fields in central Kansas prematurely turning color due to heat stress. Photos taken May 24, 2018 in Cloud County by Romulo Lollato, K-State Research and Extension.
Freeze injury to stem or crown. Depending on the stage of growth at the time of a late spring freeze, parts or all of the heads may die and turn white.
In years when the freeze occurs about the boot stage or a little earlier, such as this growing season, there can be injury to the lower stem, which then cuts off water and nutrients to the developing head and that stem simply does not develop. This year, we estimated as much as 20-40% tiller loss in the region spanning McPherson to Barber Counties, depending on field. Typical symptoms include a much denser lower canopy as compared to the number of actual heads emerged, as many tillers died back and never made to the heading stage of development. In years when the wheat is in the early heading stage at the time of the freeze, the freeze can damage the heads directly.
Often, wheat on north-facing slopes, on ridge tops, or in low-lying areas will be most affected by freeze injury. But freeze injury can also be so severe that it occurs throughout the fields, in no particular pattern. Crown rot is another potential problem that can be traced back to freeze injury.
When the crown is damaged by cold temperatures or a freeze, part or all of the tillers can die. If the tiller from a damaged crown forms a head, this head will almost always be white. The crown will have internal browning, and stands will usually be thinner than normal.
Hail. Hail can occasionally damage just a portion of a head, and cause that damaged portion to turn white. The hail impact to the heads may also remove spikelets and expose the rachis (Figure 2).
Figure 2. The heads in this photo have had a few spikelets removed due to hail impact and have their rachis exposed. Photo by Romulo Lollato, K-State Research and Extension.
Dryland root rot (also known as dryland foot rot). This disease, caused by the Fusarium fungus, causes white heads during the grain filling stages of growth. Plants infected by dryland root rot often have lower stems that are brown. In many cases, splitting the diseased stems will reveal a pinkish-white mold growth near the base of the plant (Figure 3). This disease is usually most common under drought stress conditions, and is often mistaken for either drought stress or take-all.
Figure 3. White wheat head caused by Fusarium root rot. Detail on the right shows pink discoloration inside the stem typical of the Fusarium pathogen. Photo by Romulo Lollato, K-State wheat extension specialist.
Head scab. When there are periods of rainy weather while the wheat is flowering, some heads may become infected with Fusarium head blight (head scab) and turn white. The heads of some red-chaffed varieties turn a darker red when infected with scab, but the heads of most varieties turn white. Symptoms can be restricted to one or few spikelets in the head, but often times the upper half or the entire head might be affected (Figure 4). Head scab is most common where wheat is grown after corn, or after a wheat crop that had head scab the previous year. Head scab can be identified by looking for orange or pink spores of the Fusarium fungi at the base of diseased spikelets, as well as by a darker discoloration to the rachis of the wheat head. During the current growing season, head scab has been observed at low levels in southeast Kansas, but it is probably still early to see symptoms in central and north-central Kansas as it takes approximately three weeks from flowering for the first symptoms to appear.
Figure 4. Wheat heads affected by head scab or Fusarium head blight. Symptoms range from one or few spikelets that turned white, to the upper half or entirety of the head. Photo by Romulo Lollato, K-State Research and Extension.
Take-all. This disease often causes patches of white heads scattered throughout the field. It occurs most frequently in continuous wheat, and where there is a moderate to high level of surface residue. Take-all is also favored by high pH soils, so a recently limed field might also show symptoms. To diagnose take-all, pull up a plant and scrape back the leaf sheaths at the base of a tiller. If the base of the tiller is shiny and either black or dark brown, it is very likely to be take-all. All tillers on a plant infected with take-all will have white heads. Plants will pull up easily because the roots are damaged by the disease.
Sharp eyespot. This disease is common in Kansas, but rarely causes significant yield loss. Sharp eyespot causes lesions with light tan centers and dark brown margins on the lower stems. The ends of the lesions are typically pointed. If the stems are girdled by the fungus, the tiller may be stunted with a white head. Each tiller on a plant may be affected differently.
Wheat stem maggot. Wheat stem maggot damage is common every year in Kansas, but rarely results in significant yield loss. It usually causes a single white head on a tiller, scattered more or less randomly through part or all of a field. One typical symptom of white heads caused by wheat stem maggot is that the flag leaf and lower stem are often green, and only the last internode (peduncle) and head are white. If you can grab the head and pull the stem up easily just above the uppermost node, the tiller has probably been infested with wheat stem maggot. Scout for symptoms of chewing close to the base of the plants, which could indicate that the head has died as function of wheat stem maggot (Figure 5).
Figure 5. White wheat head due to wheat stem maggot, characterized by a white head and peduncle but healthy and green lower stem. Detail on the right shows chewing of the base of the peduncle by the maggot. Photo by Romulo Lollato, K-State Research and Extension.
Romulo Lollato, Wheat and Forages Specialist
Erick DeWolf, Extension Wheat Pathologist