Gary Strickland, OSU Extension Educator
December 24, 2013
Most of the county wheat fields have suffered from what appears as a browning off of the top leaf area starting around the end of the second week of December. Many have been concerned that the wheat was dying as it appears to not be recovering very quickly. To answer this question we have to address a couple of points in respect to the wheat plants growth.
First, temperatures dropped down as low as 0 - 4oF on at least four occasions, according to Jackson County mesonet data, between Dec. 4 to the 12 with highs not getting above freezing (32oF) and remaining in the low 20’s or high teens for several days. Wheat leaf material not insulated by either snow or residue suffered freeze injury during these low temperature periods. In many no-till fields the appearance of a “brown top” to the wheat was apparent where the leaf above the residue looked dead. In conventional till fields leaf that was above the snow line also looked dead. However, in nearly all fields that I have looked at when the residue is moved away from the plant the lower portion of the plant below the residue or snow line is green with the crown tissue (which is just below the soil surface) intact. This means that with warmer temperatures and adequate moisture the plant should resume growth and put out new leaf material. In fact, new leaf material that I found in the plant whorl was green and healthy in appearance. So, what levels of warmer temperatures are needed? The wheat plant has to have temperatures equal to or above 40oF to grow. The closer the temperatures are to 40oF the slower the growth but when temperatures drop below this point then growth is practically non-existent. This means that since the end of November we have had very few growing degree days for wheat, therefore recovery is slow.
Next, let’s consider the growth and developmental stage of the wheat plant heading into this low temperature time period. When the first snow occurrence came at the end of November we were experiencing dry conditions with limited moisture availability, therefore plants had not grown much. Crown root systems (the primary supporting root system of the plant) had either just started developing in areas in the county but in several cases this root system had still not developed at this point. The crown root system starts developing generally at about the 4-5 five true leaf stage, which means most wheat plants had limited leaf area developed and with loss of the top portion of this leaf area the plant looked dead. Again, when I have checked most fields, the crown area of the plant seems to be white, turgid (full of moisture) and healthy. Now we have incidences of dryland root rot that had expressed in fields prior to these storm systems arriving and I am still finding these plants as well as some plants that due to developmental stage have been killed by winter injury. But, in the majority of cases the wheat plants still seem to be healthy. The plants just need some growing temperatures!!!
In short, while we have had battles with some root rot and mites prior to the wintery storm systems, most of the brown leaf material we are seeing right now is associated with winter injury. To fully evaluate the impact from this we need 7- 10 days of good growing temperatures to allow new leaf material time to emerge. And, I expect that in most cases we will see new leaf material coming out and the wheat plant resuming growth. But the plant will now have an added benefit, moisture around the newly developing root system! This should allow for good initial recovery growth for a period of time. Hopefully, we will continue with some follow up rainfall as well!