Many fields of wheat around Kansas have begun to take on a yellowish cast over the past couple weeks. There are several factors that can cause yellowing at this time of year:
- Cold temperature leaf injury
- Nitrogen deficiency
- Poor root development
- Leaf burn from liquid fertilizer application
- Atrazine carryover
- Leaf rust or tan spot
Determining the cause of the yellowing can be important. Some causes require a solution, such as applying more nitrogen; some causes are temporary and do not require any corrective actions; and some causes are beyond the control of producers.
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 cold temperature leaf burn or barley yellow dwarf. If entire plants are yellowing, that might indicate atrazine carryover, liquid fertilizer burn, or drowning.
* 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, you might suspect cold injury or leaf tip burn.
* Are fields unusually wet or dry? 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. If soils are very dry, root growth will often be stunted and plants will gradually become chlorotic, 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 atrazine carryover. 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.
* 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.
* 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.
* What did the most recent soil test show? Are there nutrient deficiencies or lime requirements that haven’t yet been corrected?
* 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 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 nitrogen and other nutrient deficiencies. Plants will grow out of the yellowing from either of these causes if growing conditions are good in late winter and early spring.
In the current situation, the likely cause of yellowing in most cases is cold temperature injury. I say that because: (1) the plants in most cases are showing yellowing or burning on upper leaves and leaf tips; (2) the symptoms are occurring over a wide region, especially in southern Kansas; (3) the symptoms are mostly on earlier-planted wheat with more leaf area exposed; and (4) the weather had been mild for much of December and January, and plants were green, followed by a period of cold temperatures recently.
Yellowing due to cold weather injury at this time of year is temporary, and should not cause any yield loss. Expect the wheat to return to a healthy green color when stem elongation begins if growing conditions are good -- unless another problem crops up.