To determine the cause of the yellowing, check the following:
What parts of the plant are affected? Is the yellowing on older lower leaves only, newer leaves only, on the tips, or on the entire plants? If the yellowing is on lower leaves, that indicates nitrogen deficiency. If it is only on newer leaves or leaf tips, that could indicate barley yellow dwarf. If entire plants are yellowing, that might indicate sulfur deficiency.
Are fields unusually dry? If soils are very dry, root growth will often be stunted and plants will gradually become chlorotic, and then turn bluish or brown.
What is the pattern in the fields? If the yellowing is in streaks in the field, that implies a fertilizer application problem, or possibly a residue distribution problem. If it is mostly on terrace tops, that might indicate a weather-related problem that would affect exposed plants first. If it is occurring in primarily in low areas, that might indicate freeze injury where cold air settled or drowning. If the yellowing is uniform throughout the field, any of the factors above could be the cause.
Are other wheat fields in the general region of yours also yellow, or just a few scattered fields? If fields in the entire region are yellowing, that would imply a weather-related problem. If it is specific to just one or two fields, that implies a management-related or field-specific soil problem.
Is there a difference between early-planted and late-planted fields? Later-planted wheat often had less root development going into winter this year, which made the plants more susceptible to nitrogen and other nutrient deficiencies
Poor root growth. This is most often due to dry soils, late planting, or poor seedbed conditions at planting time. If the root system is poorly developed, plants are more likely to have a nutrient deficiency, such as nitrogen or sulfur deficiency.
Nitrogen (N) 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 N deficiency are insufficient fertilizer rates, poor root growth, drought, a problem with application methods, leaching from heavy rains, and the presence of heavy amounts of crop residue, which immobilize N. If the yellowing occurs in streaks in the field and a surface topdress application was made, the deficiency is probably due to an application equipment problem or the presence of residue windrows. If the yellowing is more general in nature, it could be that the fertilizer has not yet moved into the soil because of a lack of rain.
Sulfur deficiency. Sulfur deficiency symptoms in wheat can be similar to nitrogen deficiency, with a general chlorosis of the leaf. However, with S deficiency the whole plant is pale, with a greater degree of chlorosis in the young leaves. Sulfur is not mobile in the plant like N, so lower leaves do not show more severe deficiency symptoms than the upper leaves unlike N. The uniform nature of the yellowing on the plants is one means of diagnosing S deficiency in wheat. Another common difference compared to N deficiency is the pattern in the fields. Sulfur deficiency often occurs first on slopes, eroded areas, on coarser soils, or wherever organic matter levels are lowest. Therefore, deficiencies are usually limited to only certain areas of the field.
Wheat streak mosaic and barley yellow dwarf. Symptoms of barley yellow dwarf and wheat streak mosaic often become more evident during a period of rapid growth than at earlier growth stages. In general, these diseases cause a yellow discoloration of plants that can be confused with nutrient deficiency or environmental injury. Viral diseases, such as barley yellow dwarf and wheat streak mosaic, tend to occur in patches within affected fields. Barley yellow dwarf is most commonly found in small patches randomly scattered within a field. Wheat streak mosaic often is more severe on the edges of a field closest to volunteer wheat. The symptoms of barley yellow dwarf and wheat streak mosaic are most intense on the upper leaves of infected plants. In contrast, nitrogen deficiency symptoms are more severe on the lower leaves and sulfur deficiency symptoms occur uniformly throughout the plants. The upper leaves of plants infected with barley yellow dwarf are normally bright yellow, but some varieties may also have a red discoloration. This discoloration is more intense near the tip of the leaf than at the base, giving the leaves a flame-like appearance. Plants infected early in their development are often slightly shorter than healthy plants. Wheat streak mosaic also causes a yellow discoloration of the upper leaves but the affected leaves have a more streaked or blotchy appearance. Plants infected with wheat streak mosaic may be dramatically shorter than healthy plants and often have a more prostrate growth pattern.
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