Source: Kansas State University Extension
By late March, wheat should be uniformly green. If there is yellowing in the field, determining the cause of the problem can be important. Some causes require a solution, such as applying more nitrogen (N), some are temporary and do not require any corrective actions, and some causes are beyond the control of growers.
To determine the cause of the yellowing, check the following:
1. 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 N deficiency. If it is only on newer leaves or leaf tips, that could indicate cold temperature leaf burn, sulfur deficiency or barley yellow dwarf. If entire plants are yellowing, that might indicate atrazine carryover, liquid fertilizer burn, sulfur deficiency or drowning.
2. What have the temperature and growing conditions been over the past 30 days? If there has been a sudden drop in temperatures while leaves were green, it might be cold injury or leaf tip burn.
3. Are fields unusually wet or dry? If soils are very dry, root growth will often be stunted and plants will gradually become chlorotic, then turn bluish or brown. If soils are excessively wet, roots can drown and nutrient uptake can be greatly reduced, resulting in yellowing of lower leaves first, then entire plants.
4. Can the plants be pulled easily from the soil? If so, the root system is stunted and could be at least one cause of the yellowing.
5. What is the pattern in the fields? If the yellowing is in streaks in the field, that implies a fertilizer application problem, or possibly atrazine carryover. If it is mostly on terrace tops, that might indicate a weather-related problem that would affect exposed plants first or an issue related to eroded or low organic-matter soils, such as sulfur deficiency.
If the yellowing is occurring primarily in low areas, that might indicate freeze injury where cold air settled, drowning, soilborne mosaic, or spindle streak mosaic. If only the edge of the field is yellow, look for symptoms of wheat streak mosaic. If the yellowing is in roundish spots scattered throughout the field, suspect barley yellow dwarf. If the yellowing is uniform throughout the field, nitrogen deficiency, poor root growth due to drought or poor seed-to soil contact, cold weather leaf burn or topdress fertilizer leaf burn are likely causes.
6. Are other wheat fields in the general region 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.
7. What herbicides had been applied to the previous crop? If atrazine had been applied to the previous crop, check on the rate used and the environmental conditions since the application. If soils have been drier than normal after the atrazine was applied, this would increase the chances for atrazine carryover into the wheat crop.
8. What did the most recent soil test show? Are there nutrient deficiencies or lime requirements that haven’t yet been corrected?
9. Is there a difference between early-planted and late-planted fields? Earlier planting usually results in bigger plants when going into winter, which can sometimes result in more drought stress or cold temperature injury to the leaves over the winter. Later-planted wheat often has less root development going into winter, which can make the plants more susceptible to N and other nutrient deficiencies. Plants will grow out of the yellowing from either of these causes if growing conditions are good in spring.
Will yellow wheat at this point in the season lead to reduced yields? Can it be corrected?
In general, growers can correct a general N or sulfur deficiency, but not without some yield loss if the wheat is already jointed. If there are streaks in the field due to a misapplication problem, that may reduce yields a bit but it is probably not worthwhile to try to correct it unless the streaks are very wide and persistent throughout the field.
Poor root growth may cause yield loss if it persists through the stem elongation stage. Virus diseases and atrazine carryover will very likely cause some yield loss, and nothing can be done about it at this point.
Wheat will generally grow out of any temporary leaf burn due to cold weather or fertilizer applications with little or no yield loss. Yellowing due to late spring freeze injury after jointing, however, indicates death of the growing point and can result in yield loss.
For more information, see the following K-State Research and Extension publications:
Diagnosing Wheat Production Problems in Kansas
http://www.ksre.ksu.edu/bookstore/pubs/S84.pdf
Diagnosing Nutrient Deficiencies in the Field
http://www.ksre.ksu.edu/bookstore/pubs/MF3028.pdf
