Source: Jim Shroyer, Extension Agronomy State Leader, Kansas State University
Many fields of wheat this spring have had areas of yellowish wheat. What are some of the main causes of yellow wheat in the spring? Will it hurt yields?
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, 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.
- Sulfur deficiency. Sulfur deficiency is not as common as nitrogen deficiency, but it can occur where organic matter levels are low — especially on sandier soils or eroded areas of a field. The symptoms of sulfur deficiency are very similar to nitrogen deficiency. However, sulfur deficiency does differ from sulfur deficiency in that the whole plant is pale with a greater degree of chlorosis in the young leaves. The pattern of chlorosis in the new leaves may show gradation in intensity from tip to base, but they quickly become totally chlorotic and take on a light yellow color.
- 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.
- 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.