The wheat crop continues to develop at a fast pace across the state. Our estimates of crop development for different portions of the state are provided in Figure 1. The most advanced fields in far southeast corner of the state are between boot and flowering, and the majority of the region is already at or past flag leaf emergence. Parts of south central Kansas and the northern-most counties in southeast Kansas are mostly now at flag leaf emergence or at boot. Wheat in central Kansas and in fields that emerged last fall in southwest Kansas are past the second node and approaching flag leaf emergence. Northern Kansas and northwest Kansas have the majority of the fields now at jointing growth, or just past it.
Figure 1. Estimated wheat growth stage as of April 14, 2017. Growth stage is estimated for each county based on temperatures accumulated in the season and adjusted by observations of crop stage by K-State personnel. Local growth stage may vary with planting date and variety.
Wheat conditions update
The major issues being faced across Kansas in the current wheat crop involve viral diseases (mostly wheat streak mosaic in western Kansas and barley yellow dwarf in central Kansas), stripe rust (please see accompanying eUpdate in this April 14, 2017 issue of the Agronomy eUpdate), and nitrogen and sulfur deficiencies which are still showing up in many Kansas wheat fields.
Viral diseases were most likely favored by the warm conditions experienced in the fall, winter, and early spring, and by volunteer wheat not controlled before the growing season. Temperatures above 70°F were observed in February and March, which would favor both the aphids that transmit barley yellow dwarf virus, and the wheat curl mite that transmits wheat streak mosaic virus. Warm temperatures might also shut down the genetic resistance to wheat streak mosaic virus of some varieties, such as Oakley CL. We are getting reports of entire fields being severely damaged by wheat streak mosaic in western Kansas.
Early reports of wheat streak mosaic centered around west central and southwest Kansas; however, we are now getting reports of above-normal levels of disease in northwest Kansas as well. The yield loss led by an early infection of these diseases can be significant in fields where the disease is widespread. There are also reports of this viral disease occurring on scattered plants in many fields in the western portions of Kansas. This patchy distribution in the field is likely the result of low levels of wheat curl mites blowing in from distant areas of higher mite populations. The mites began to feed and transmitted the virus but then died out locally. These scattered infections should be a minor issue in terms of crop yield if the disease remains isolated to the plants (often <1% of the plants showing symptoms) currently showing symptoms, because the remaining plants will compensate for the damaged plants.
Nitrogen or sulfur deficiencies
Fields showing symptoms of nitrogen or sulfur deficiencies are also occurring in the 2017 Kansas wheat crop, both in central and south central Kansas, and into the northwest portion of the state. Nitrogen deficiency is characterized by a pale green color in the lower leaves, while sulfur deficiency results in light green upper leaves.
Producers who have not yet applied their fertilizer are now debating whether to do so, due to the above-average moisture forecast and the good chances of getting the fertilizer in the root zone. The question now is whether there a yield gain from late-applied nitrogen. K-State research has shown yield gains at times from nitrogen applied as late as Feekes 8-9. However, the yield gain from late-applied nitrogen is not as large as from early applications. Here is the reason:
Grain yield is a function of number of grains per head, numbers of head per area, and individual grain weight. The number of grains per head is determined early in the spring, in between spring greenup and Feekes 5. Thus, the number of grains per head (which is among the most important yield components for wheat) will not be affected by late-applied N. The main effect of late-season N on wheat yield will come from reducing tiller abortion due to nitrogen deficiency or, in other words, maintenance of the number of heads per area.
Thus, producers wanting to make the decision of applying N now need to first define what growth stage their wheat is. Greater yield gains will occur from applications at earlier stages of development. Very limited grain yield increase can be expected from fields that are past Feekes 8-9. For fields still at Feekes 6-8, the nitrogen might help maintain the tillers already present.
Romulo Lollato, Wheat and Forages Specialist
Erick DeWolf, Extension Wheat Pathologist