Stripe rust continues to take its toll on the Kansas wheat crop this year. Spurred on by recent cool, wet weather, the disease is at moderate to severe levels in most areas of the state. The rust pressure is highly variable within many of these regions. Some fields are experiencing only minor damage and while others may experience more than 40% yield loss.
The amount of yield loss depends on the severity of the disease and the timing relative to grain development. The greatest yield losses occur when the plants suffer severe damage to the flag leaves prior to the milk stages of kernel development. Most growers are reporting major differences in disease levels between fields treated with a timely fungicide application and those left untreated. Based on observations in fields this week, it appears many growers will see significant benefits from a fungicide application this year.
The next few weeks will be a great time to do some comparisons of fields that were treated with a fungicide and those that remained untreated in your area. As you check fields, you might notice some yellow blotches and tan striping of leaves in some fields (figure 1). These symptoms might lead producers to wonder if the fungicide is working.
In most cases, a careful examination of the lesions reveals the dried remains of the stripe rust fungus and the absence of bright orange spores of the fungus. These are an indication that the fungicide may have been applied a little late but ultimately has done its job. The absence of new spores indicates that the fungus is no longer growing or initiating new infections. The remaining green leaf area should help the plant produce harvestable grain. Necrotic areas on the plant leaf will not recover and green back up after a fungicide is applied.
Figure 1. Symptoms of stripe rust that has been killed by a fungicide. Photo by Jeanne Falk Jones, K-State Research and Extension.
Erick De Wolf, Extension Plant Pathology
Jeanne Falk Jones, Sunflower Extension District Agronomist