There may be large areas, small patches, or streaks of yellowish wheat in some fields this spring. What are some of the main causes of yellow wheat in the spring?
The most common causes of yellow wheat in the spring are:
Poor root growth. This may be due to dry soils, waterlogging, or elevated crown height caused by shallow planting depth or excessive residue in the root zone. If the plants have a poor root system, then the plants are yellow because the root systems are not extensive enough to provide enough nutrients.
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. The primary causes of nitrogen deficiency are insufficient fertilizer rates, application problems, applying the nitrogen too late, leaching from heavy rains, denitrification from saturated soils, and the presence of heavy amounts of crop residue, which immobilize nitrogen.
Figure 1. Nitrogen deficiency on wheat. The lower leaves are the first to become chlorotic. Photo by Dorivar Ruiz Diaz, K-State Research and Extension.
Sulfur deficiency. Sulfur deficiency is not as common as nitrogen deficiency, but there has been an increase in the numbers of fields with sulfur deficiency in recent years. Deficiency can be more common in areas where organic matter levels are low -- especially on sandier soils or eroded areas of a field. It can also occur where soils are cold and dry in the spring. Under these conditions, the rate of release of sulfur from organic matter is greatly reduced. The symptoms of sulfur deficiency are very similar to nitrogen deficiency. However, sulfur deficiency does differ from nitrogen deficiency in that the whole plant is pale with a greater degree of chlorosis in the young leaves. The pattern of chlorosis may show gradation in intensity with the younger leaves at the tip yellowing first because sulfur is not easily translocated within the plant. But the entire plants quickly become totally chlorotic and take on a light yellow color. Symptoms often become more pronounced when plants begin growing rapidly while soil conditions are such that organic matter mineralization and sulfur release rates are low. Symptoms may disappear as the temperature warms up and moisture conditions improve, which increases the rate of mineralization of sulfur from organic matter and the rate of root growth in the plants.
Figure 2. Sulfur deficiency in wheat, with symptoms appearing first on the younger leaves. Photo by K-State Research and Extension.
Iron chlorosis. Iron chlorosis is not common on wheat in Kansas, but does occur on certain high-pH, calcareous soils in western Kansas. Newly emerging leaves will have green veins, with yellow striping between the veins. Eventually, the entire leaf may turn yellow or white.
Soilborne mosaic or spindle streak mosaic. Soilborne mosaic and spindle streak mosaic are viral diseases that occur primarily in eastern and central Kansas, but can also occur in western Kansas. These diseases are most common in years with a wet fall, followed by a cool, wet spring. Lower areas of the field are most commonly affected. Symptoms are usually most pronounced in early spring, then fade. Leaves will have a mosaic of green spots on yellowish background; and plants will be stunted.
Wheat streak mosaic complex. This viral disease is vectored by the wheat curl mite. Yellow areas in field will appear in spring; usually on field edges adjacent to volunteer wheat. Leaves will have a mosaic of yellow streaks, stripes, or mottling. Plants will normally be stunted. Unlike soilborne/spindle streak mosaic, wheat streak mosaic is not associated with any particular type of weather pattern or soil condition.
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 boot stage. Leaf tip turns yellow or purple, but midrib remains green. The yellow color is more intense, and in an even distribution pattern on the leaf surface compared to the yellowing caused by the mosaic diseases. Plants are usually, but not always, stunted.
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 topgrowth, often giving the field a yellowish cast but not reducing yield potential.
Figure 3. Yellowing wheat from cold weather injury at the tillering stage. Photo by K-State Research and Extension.
Freeze injury at the jointing stage. Jointing wheat can usually tolerate temperatures in the mid to upper 20’s with no significant injury. But, if temperatures fall into the low 20’s or even lower for several hours, the lower stems, leaves, or developing head can sustain injury. If the leaves of tillers are yellowish when they emerge from the whorl, this indicates those tillers have been damaged.
Jim Shroyer, Crop Production Specialist Emeritus
Dorivar Ruiz Diaz, Nutrient Management Specialist
Erick DeWolf, Extension Plant Pathologist