Time to scout wheat crop for freeze injury, rust diseases

By Kirstin Vohs, Kansas Wheat Communications Intern

Over the past few weeks farmers have taken major hits, not only financially, but also when it comes to the weather. The week of April 13-17 saw another round of cold temperatures that have potential to cause freeze injury to the 2020 wheat crop. With Kansas reaching devastatingly cold temperatures, it’s important for farmers to scout their wheat.

Factors to watch out for include growth stage of the crop, air and soil temperature, duration of cold temperatures and snow cover. Based on simple wheat development models and observations from K-State Research and Extension personnel, the wheat growth stage around Kansas ranges from tillering to Feekes 5 in the northwest part of the state, to flag leaf emergence or boot in the southeast region. Most of the crop in south central Kansas is at the first or second node, and the crop is less developed as we move to the northwest. For fields that have not jointed yet, the crop generally withstands temperatures of 15-20°F fairly well, especially if the growing point is still below ground. This is the condition for most of northern Kansas.

If the growing point is already above ground (first joint visible), wheat can sustain temperatures down to about 24°F for a few hours. Minimum temperatures below 24°F for extended periods of time increase the risk of crop injury.

While soil temperatures can help buffer freezing air temperatures if the growing point is below ground or near the soil surface, the buffering capacity decreases as the crop develops and the growing point moves away from the soil surface. Thus, expect a positive effect of the soil temperatures in north central and northwest Kansas where soil temperatures were sustained above 38°F during the entire week, and the crop is still at tillering through Feekes 5 stages of development. 

Along with watching your wheat during the changing weather, it is also important to scout wheat for stripe and leaf rust.

According to Dr. Erick De Wolf, Plant Pathologist at Kansas State University, “It is prime time for growers to be out looking for diseases while they are checking for potential freeze injury as well.”

There were a few reports of stripe rust from Oklahoma and a Twitter report of the disease in southeast Kansas bordering Missouri. Several of De Wolf’s colleagues made visits to research sites in south central Texas and reported severe leaf rust on varieties known to be susceptible to that disease. To date, extension agents, crop consultants and growers all indicate that rust is not widely active in Kansas.

“Look for the characteristic yellow rectangular lesions of the stripe rust pathogen,” said De Wolf, adding that sometimes on the young plants around the jointing stages of growth, “we don’t often get the rectangular look to some of the stripe rust lesions.” When people think of stripe rust, they often visualize the characteristic bright yellowish-orange lesions on adult plants. Symptoms of stripe rust on younger leaves are often less rectangular because the fungal growth within the plant is not limited by the veins of younger leaves.

With the wheat crop in south central and southeast Kansas approaching or already at the flag leaf emergence stages of growth, farmers are encouraged to be on the lookout for diseases. More in-field observations will be happening over the next few weeks.

More information about freeze injury and how to identify wheat rust diseases is available from K-State Research and Extension’s April 17th Agronomy eUpdate at https://webapp.agron.ksu.edu/agr_social/issue/k-state-agronomy-eupdate-issue-795-fri-apr-17-2020.