Wheat Growing Conditions: April 21, 2017

 

There is a southeast-to-northwest gradient in wheat development across Kansas. Wheat in far southeast is now around flowering, with some more fields even further along in the early stages of grain development (Fig. 1). The majority of wheat in the south central region is already at boot or heading. Wheat in parts of southwest Kansas and the majority of the central portion of the state is now at flag leaf emergence or at boot. Northern Kansas and northwest Kansas have the majority of the fields now past the second node, and approaching the flag leaf emergence.

 

 

 

This week, K-State Research and Extension agronomists have visited several different fields across the state. The route is summarized in Figure 2. Many fields visited had good yield potential, especially in the central portion of the state. Fields in far northwest Kansas (Rawlings and Thomas counties) and in parts of southwest Kansas (Wichita, Finney, and Meade counties) also had good yield potential when not infected by wheat streak mosaic virus (Figure 3).

 

The major issues being faced across Kansas in the current wheat crop involve viral diseases (mostly wheat streak mosaic in western Kansas), stripe rust (please see accompanying eUpdate on stripe rust), some scattered poor emergence in parts of northwest Kansas, and some nitrogen and sulfur deficiencies.'

 

 

 

 

 

 

 

The most prevalent viral disease across the route shown above was wheat streak mosaic virus (WSMV), especially in the western portion of the state. While some fields were completely infected by the disease (Fig. 4, upper panel) and showing the typical spread pattern across the field (wide near the infection point, narrowing towards deeper portions of the field), many fields were actually only showing scattered plants or patches of infected plants across the field (Fig. 4). This patchy distribution is more typical of barley yellow dwarf, but is re-occurring in WSMV-infected fields this growing season. 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. The level of infection varied from field to field, and symptoms ranged from a pale-yellow, light green streaking (typical infection) to a bright yellow response, depending on variety susceptibility.

 

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.

 

 

 

 

 

Some fields in the region encompassing Wakeeney, Hill City, and Oberlin had very scattered stands composed of a mixture of large fall-emerged plants and small spring-emerged plants (Figure 5). It was clear that those fields had limited fall precipitation, resulting in poor fall emergence. Yield potential in these fields will depend on what percentage of the plants emerged in the fall, and what the distribution uniformity of those fall-emerged plants. Previous K-State research has shown that a fall stand of about 50-60% can still result in good yield potential, provided the distribution of these plants is relatively uniform so that the individual plants can tiller out and compensate. However, if final stands are less than those percentages or plants are irregularly distributed, yield can be severely compromised. Plants which emerged during the spring will have about half of the potential of the fall-emerged plants, depending on weather conditions in the growing season.

 

 

 

Fields with symptoms of nitrogen or sulfur deficiencies are also present in the 2017 Kansas wheat crop. Nitrogen deficiency is characterized by a pale green color in the lower leaves, while sulfur deficiency results in light green upper leaves. Severe N deficiency will turn all leaves pale green and reduce plant size. Some fields were showing signs of N deficiency, which could be differentiated by the darker green portions of the field where there was some extra N available (Figure 6). In the specific field shown in Figure 6, the lighter green plants were also showing more streaky symptoms in the leaves, indicating possible infection by wheat streak mosaic. Plants are currently being tested for both N concentration and WSMV.

 

 

Nitrogen applied late season has resulted in yield gains as late as Feekes 8-9 in K-State research. However, the yield gain from late-applied nitrogen is not as large as from early applications because it does not increase number of grains per head. 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 number of heads per area