Wheat disease update: Stripe rust, leaf rust, and powdery mildew
This week has brought more reports of stripe rust in Kansas. Stripe rust can be found in the lower and middle canopy of many fields in central Kansas, but the severity remains low (Figure 1). Stripe rust is more severe in the southeast region of the state and has moved to the upper leaves in some fields. The weather conditions the past 14 days have not favored the rapid spread of stripe rust. Stripe rust is favored by cool, wet weather and temperatures in recent weeks were too warm for the stripe rust fungus to function efficiently. For example, most areas of the state had more than 30 hours of temperatures above 75 F in the last two weeks (Figure 2). Some areas of southwest and south central Kansas had more than 50 hours of unfavorable temperatures. The threat of stripe rust has not passed, however. We know stripe rust is present at low levels in many fields in the state. The disease could increase rapidly if we get into another period of favorable weather with frequent rainfall and temperatures in the 40-50F range at night. I still think there is a moderate risk of Kansas having a serious problem with stripe rust this season.
Figure 1. Distribution of wheat stripe rust in Kansas as of April 21, 2017.
Figure 2. Duration of time that temperature was unfavorable for the development of stripe rust between April 7 and April 21, 2017.
Leaf rust was reported previously in south central and southeastern Kansas. This week brought a few new reports of leaf rust and indications that leaf rust has moved to the upper leaves in few areas (Figure 3). This movement of rust to the upper leaves is important because these leaves provide most of the resources the plants will use produce grain. Any damage done to the upper leaves increases the risk of yield loss.
Figure 3. Distribution of wheat leaf rust in Kansas as of April 21, 2017.
Powdery mildew (Figure 4) is becoming severe in fields planted to moderately susceptible and susceptible varieties. 1863, Gallagher, KanMark, LCS Pistol, SY Flint, WB4458, WB-Grainfield, and WB-Redhawk are vulnerable to powdery mildew. In some fields, the powdery mildew has moved to the leaf just below the flag leaf prior to heading. This early establishment of the disease is cause for concern and growers should consider both rust and powdery mildew into their fungicide decisions. Fields with multiple diseases in the middle canopy and those where disease has moved to the upper leaves prior to heading have a more than 80% chance of experiencing a yield loss of >4.0 bu/a.
Figure 4. Wheat with symptoms of powdery mildew. Photo by Erick DeWolf, K-State Research and Extension.
Erick De Wolf, Extension Plant Pathology
Romulo Lollato, Wheat and Forages Specialist
Mary Knapp, Weather Data Library
Chip Redmond, Kansas Mesonet