By Connie Strunk and Emmanuel Byamukama
The weather conditions we have had in the past few days are conducive for powdery mildew to develop in winter wheat. Several fields scouted in Turner and Hutchison counties had low to moderate levels powdery mildew (Figure 1).
What causes powdery mildew?
Powdery mildew is caused by a fungal pathogen Blumeria graminis f. sp. tritici. This pathogen survives on infected crop residue. Infection of wheat can take place any time from when seedlings emerge in fall and throughout spring, but wheat is most susceptible during the rapid growth period in spring. Powdery mildew develops under humid conditions and at temperatures between 60 and 72° F. Dense stands or heavy tillering also encourage infection. Moisture trapped in the lower canopy provides the perfect microclimate for powdery mildew development. Early infection of wheat by powdery mildew may lead to reduced tillering, and infected young tillers may fail to produce heads. The pathogen damages wheat by utilizing nutrients and increasing transpiration leading to poor plant growth.
Figure 1. Powdery mildew developing in winter wheat near Tukey Ridge, SD.
How do you scout for it?
When scouting, separate plants and examine the lower leaves and stems. A field may have plants whose upper canopy looks symptomless but the lower canopy may have heavy symptoms (Figure 2). Look for cottony mycelium on the stems and the upper surface of leaves (Figure 3). Yellow spots on the leaves may indicate a new lesion developing.
Figure 2. A wheat field with upper canopy looking healthy. Inset picture shows plants in the same field with heavy powdery mildew developing in the lower canopy.
How can you manage it?
For fields with potential for high yield and with moderate powdery mildew and other fungal diseases developing, a fungicide application at herbicide timing (tank mix with an herbicide) or at flag leaf may be profitable. Several fungicides are effective in controlling powdery mildew (see the NCERA working group fungicide efficacy table).
Cultural practices such as crop rotation and tillage may help reduce inoculum but this pathogen is capable of producing millions of spores in a short time with favorable weather, and the spores can also be blown from a long distance. Most wheat cultivars are susceptible to powdery mildew but some are more tolerant than others.
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