Historically Warm February
February 2017 will go down in the record books as one of the warmest Februaries on record, not just in South Dakota, but across the United States. Some Eastern parts of the state will end up more than ten degrees above average for the month. The Western region will end February around two to six degrees above average.
As a result of these above average temperatures, the abundant snow pack that covered the state rapidly melted. As of February 27, most of the state was snow-free except for the Southern tier counties. Without snow cover to insulate the ground, soil temperatures can warm quickly, and winter wheat can break dormancy.
Winter Wheat Emergence: Threshold Indicator
A threshold indicator for winter wheat emergence is to consider average temperatures over a 14-day period. When that 14-day average temperature is equal to or above five degrees C, or 41° F, then hard red winter wheat can break dormancy.
Historically in South Dakota, this threshold is met in the first week in April. Last year, in 2016, winter wheat broke dormancy in mid-March. This year, temperatures are on track for an even earlier dormancy break date, about a month earlier than average. This is consistent with the winter wheat growing region as a whole, in which many areas have already broken dormancy a month earlier than the historical average date, and also earlier than last year. Some warmer areas of the state, and warmer areas of some fields, were seen to have already broke dormancy the week of February 20th.
This is a potentially risky situation, as historically there has been a 50% likelihood of a hard freeze (28° F) as late as May 1st-6th in most western and some central counties. In addition to the frost/freeze risk and winterkill, there are other potential concerns regarding insect pests and disease in winter wheat.
Figure 1. 14-day running mean of average daily temperature for South Dakota. Hard red winter wheat can break dormancy when the 5 deg C threshold is met. The red line is 2017 14-day average temperatures, green line is 2016, and black is average. Graph courtesy of Eric Luebehusen, USDA.
During the spring of 2016 many insect pests were observed in winter wheat fields much earlier than anticipated. This was attributed to the warmer temperatures that led to winter wheat breaking dormancy in mid-March. Insects are cold blooded organisms that rely on environmental temperatures to regulate many of their activities. In addition, many insects that can be problematic in South Dakota overwinter in the southern U.S. Warmer temperatures may lead to earlier than normal flights.
The army cutworm is one insect pest that was observed earlier than normal during 2016. Army cutworm caterpillars overwinter below the soil surface in winter wheat fields. Once the soil has sufficiently warmed up, they become active and will begin feeding on winter wheat. Due to the higher than average temperatures it is possible that the army cutworm populations may be observed earlier than normal in 2017. Once fields begin to break dormancy they should be scouted for army cutworm populations.
Mild winter weather can lead to survival of plant pathogens that normally would not survive in South Dakota. Stripe rust is one disease of concern that can survive mild winters in South Dakota. Stripe rust, which otherwise develops from spores blown from Southern states, has survived our winters in the last two years. With winter wheat likely to break dormancy early this year, this will increase the likelihood of stripe rust to develop earlier than normal. Winter wheat should be scouted as soon as wheat greens up and an early fungicide at herbicide timing planned if stripe rust is observed.