Our recent cold night time temperatures have raised questions about possible injury to the winter wheat crop. As I write this the forecast for Saturday night April 9 is calling for a low of 18 degrees F. Laura Lindsey OSU Extension soybean and wheat specialist has been doing some trials to evaluate the freeze tolerance of winter wheat in Ohio. Evaluations are done by going out to wheat fields, digging up plant samples and placing those samples into a freeze chamber and exposing plants to various low temperatures for several hours and then evaluating plants 24 hours later.
Plants dug from a Pickaway County field on March 30 were at Feekes 5 growth stage. This growth stage is defined as: strongly erect leaf sheaths. Plants will have an upright appearance but the growing point is still below the soil surface. These plants were subjected to temperatures of 27, 21, 14 and 5 degrees F. Very little injury occurred between 14 to 27 degrees.
The effect of cold weather depends on the wheat growth stage. Maximum resistance to cold weather occurs in December-February. As wheat greens-up, the plant becomes less tolerant of freezing temperatures. Feekes 6 growth stage, is the beginning of the jointing stage in wheat and is a critical stage. It is defined as: first node visible. On reproductive tillers a visible knot, bump, or swollen tissue called the node is noticeable above the soil surface. The growing point, which includes the developing head or spike on reproductive tillers is above this node. Once the growing point is above the soil surface plants are more susceptible to freeze injury. According to a Kansas State publication (http://www.ksre.k-state.edu/bookstore/pubs/C646.PDF), at Feekes 6 growth stage when wheat is exposed to temperatures of less than or equal to 24°F for at least two hours the growing point can be killed.
As if the cold weather and potential freeze injury were not enough, there is also some concern about early season disease issues, probably because of the warmer than average winter we had. Rust is being reported in some wheat stands and Pierce Paul, OSU Extension wheat and corn disease specialist, has the following advice for wheat growers concerning disease development and management:
Although Septoria and Stagonospora leaf blotches have not yet been reported, it is highly likely that the fungi causing these diseases also survived the winter in higher-than-usual numbers and will likely infect the crop as conditions begin to warm up, especially is it stays wet over the next few weeks. For powdery mildew, a disease that thrives in dense wheat stands with high nitrogen, severity will likely increase after the wheat is top-dressed.
Rust, powdery mildew, Septoria and Stagonospora are all polycyclic diseases, meaning that once they get an early start, several batched of spores will be produced between now and grain-fill, if not managed and conditions are favorable. Spores produced early in the season will spread up the plant and to neighboring plants, causing new lesions or pustules to develop, and these new lesions will themselves produce new spores within 7-14 days that will again infect new leaves and plants, causing the diseased to spread rapidly, particularly if the variety is susceptible. The more cycles we have, the more disease we will see and the greater the grain yield and quality reduction. Keep monitoring the progress of these diseases and be prepared to apply a fungicide. We typically do not recommend fungicide application at green-up for foliar disease control, since we rarely see high levels of disease this early, however, this may be the year for a green-up application if it stays cool and your variety is susceptible. Remember to always use label-recommended rates; half-rates do not provide adequate protection and may increase the risk of fungicide resistance. Also, early applications will not protect the flag leaves and spikes against infections later in the season.