Wheat producers may start seeing some wheat fields turn yellow during this time of the year. The pattern may vary from field to field, sometimes as large areas, small patches, or streaks of yellowish wheat in some fields this spring. What are some of the main weather and disease-related factors that can cause yellow wheat in the spring?
Poor root growth. Many potential causes exist for reduced root growth: dry soils, later sowing, waterlogging, or elevated crown height caused by shallow planting depth or excessive residue in the root zone (Figure 1). If the plants have a poor root system, then the root systems are not extensive enough to access enough water and nutrients, causing the plant to turn yellow. More information on poor root growth associated with soil fertility factors can be viewed in a companion article in this eUpdate issue.
Figure 1. The left panel shows the lack of development of the crown rooting system of a wheat field due to drought conditions in the topsoil. Photo by Romulo Lollato, K-State Research and Extension. The right panel shows a slightly more developed but also extremely shallow rooting system, likely due to a restrictive dry topsoil layer. Photo by Tyler Ediger, wheat producer in Meade County, KS.
Cold weather injury at the tillering stage. A sudden drop in temperatures after the wheat has greened up but before it reaches the jointing stage will burn back the top-growth, often giving the field a yellowish cast but not necessarily reducing yield potential (Figure 2). This injury is likely cosmetic, provided the growing point is still healthy. Variety release from winter dormancy can also affect the extent of the symptoms, as early varieties would have been less cold-hardy and thus likely sustain more injury.
Figure 2. Yellowing wheat from cold weather injury at the tillering stage. Wheat variety on the left (WB-Grainfield) has a later release from winter dormancy as compared to WB-Cedar (variety depicted on the right). Thus, WB-Cedar sustained more leaf injury. Photo by Romulo Lollato, K-State Research and Extension.
Freeze injury at the jointing stage. Jointing wheat can usually tolerate temperatures in the mid-to-upper 20s with no significant injury. But, if temperatures fall into the low 20’s or below for several hours, the lower stems, leaves, or developing head can sustain injury (Figure 3). This has not been a problem this year yet, but in some cases, there are severe temperature drops in the near future as the crop greens up and joints, producers should scout their fields to assess the yield potential. If the leaves of tillers are yellowish when they emerge from the whorl, this indicates those tillers have been damaged.
Figure 3. Comparison between a healthy developing wheat head (left-hand side, typically light green and firm) versus a developing wheat head that sustained freeze injury (right-hand side, whitish/brown and mushy). Photo by Romulo Lollato, K-State Research and Extension.
While the extent of potential freeze damage depends on minimum temperatures achieved, duration of cold temperatures, and stage of wheat development; other factors such as crop residue, position on the landscape, wind speed, snow cover, and soil temperatures also play a role. Figure 4 shows an example of the effect of heavy residue on potential wheat damage. In this photo, parts of the field with a heavier layer of residue show greater cold damage than lighter residue. This can be partially explained because, under a thicker layer of residue, the wheat crown tends to form closer to the surface and therefore is more exposed to freezing temperatures.
Figure 4. Effect of soil residue on wheat freeze damage. Wheat is showing more damage from freezing temperatures in thicker residue layers. Photo by Tyler Ediger, wheat producer in Meade County, KS.
Leaf senescence and opportunistic leaf spotting diseases. After the winter, it is normal for some of the leaves in the lower canopy to go through senescence and perish, sometimes translocating nutrients to the new growth and sometimes just due to different natural reasons. This causes a yellowing of the lower wheat canopy. Some opportunistic saprophytic fungi or fungal diseases such as leaf spots (Septoria tritici blotch, Stagonospora nodorum leaf blotch, and tan spot), may colonize these dying tissues as shown in Figure 5. For the most part in Kansas, these diseases do not cause economic damage as long as they remain on the lower leaves, especially if they occur in tissue that is dying already. They might become a problem and warrant a fungicide application in specific situations, such as when a susceptible variety is planted into heavy wheat residue – especially under no-tillage practices, and when symptoms appear in the upper canopy after the flag leaf has emerged (see Stagonospora nodorum leaf blotch, in Figure 6).
Figure 5. Septoria tritici blotch (leaf spot) colonizing tissue from the lower wheat canopy that was already senescing. Photo by Romulo Lollato, K-State Research and Extension.
Figure 6. Stagonospora nodorum leaf blotch symptoms in the upper wheat canopy. Photo by Romulo Lollato, K-State Research and Extension.
Soilborne mosaic or spindle streak mosaic. Soilborne mosaic and spindle streak mosaic (Figure 7) are viral diseases that occur primarily in eastern and central Kansas but are rare in western Kansas. These diseases are most common in years with a wet fall, followed by a cool, wet spring. These diseases are often most severe in low areas of a field where soil conditions favor infection. Symptoms are usually most pronounced in early spring, then fade as temperatures warm. Leaves will have a mosaic of green spots on a yellowish background. Infected plants are often stunted in growth. Many varieties in the eastern part of the state have high levels of resistance to these viral diseases.
Figure 7. Wheat with symptoms of wheat spindle streak mosaic. Notice the yellow, linear lesions that are tapered at both ends. Photo by Erick DeWolf, K-State Research and Extension.
Wheat streak mosaic complex. This viral disease is vectored by the wheat curl mite. Yellow areas in the field will appear in spring around the jointing stages of growth; usually on field edges adjacent to volunteer wheat. Leaves will have a mosaic of yellow streaks, stripes, or mottling (Figure 8). Plants infected with wheat streak mosaic are often smaller than healthy plants. There are two additional viruses, Triticum mosaic virus and high plains mosaic virus, that also result in similar symptoms.
Figure 8. Typical symptoms of wheat streak mosaic virus. Photo by Kelsey Andersen Onofre, K-State Research and Extension.
Barley yellow dwarf. This viral disease is vectored by bird cherry oat aphids and greenbugs. Small or large patches of yellow plants will occur, typically around the boot stage (Figure 9). The leaf tip turns yellow or purple, but the midrib remains green. The yellowing caused by barley yellow dwarf is less botchy than the yellowing caused by other viral diseases. Plants infected by barley yellow dwarf are often stunted.
Figure 9. A typical patch of plants showing symptoms of barley yellow dwarf virus infection. (Photo by Romulo Lollato, K-State Research and Extension) as well as up-close symptoms of barley yellow dwarf (Photo by Kelsey Andersen Onofre, K-State Research and Extension)
Romulo Lollato, Wheat and Forages Specialist
Kelsey Andersen Onofre, Extension Wheat Pathologist
Erick DeWolf, Plant Pathologist