Wheat progress and early season diseases

Wheat in the southernmost part of Illinois is near Feekes growth stage (FGS) 7, while areas in the mid south are nearing FGS 5.  Wheat in the central part of Illinois is greening up.  Warm temperatures this week will move our wheat a stage or two, and we should be out in fields scouting.  Now is the time you may see some powdery mildew building in some fields.  Powdery mildew develops best under cool temperatures and high humidity.  Interestingly, this disease does not like it wet- water can actually inhibit spore germination.  Powdery mildew typically starts up in low lying pockets in fields and spreads over time.  Often the disease slows down once temperatures increase into the 80’s and is not frequently observed in the upper canopy as a result.  Resistance is very effective for managing powdery mildew.  However, in parts of the U.S., some populations of the powdery mildew pathogen have overcome resistance genes commonly used in wheat varieties.  If we start to see this disease more frequently or often, this may be part of the issue.  Remember that the powdery mildew that affects wheat is different from powdery mildew that you see on other crops, such as barley, soybeans, or vegetables.

Powdery mildew on wheat is easily identified by white to grey tufts of fungal hyphae and spores on the leaf surface. Stems and heads can also be affected.




Powdery mildew often is found in the lower canopy due to its need for persistent humidity


Another fungal disease that you will encounter early in the season is Septoria or speckled leaf blotch.  This is a residue-borne fungal disease that produces irregular shaped lesions on foliage.  The center of these lesions will have very obvious large black dots, giving it a “speckled” appearance.   Like powdery mildew, Septoria leaf blotch does best in cool, humid conditions.  Thus, we see it early in the season, and once things warm up, the disease slows down.  Most often you will see it in the lower to mid canopy.  Unlike Stagonospora leaf blotch, this disease does not move to the head.


Speckled (Septoria) leaf blotch does best under cool, humid conditions and does not affect heads.
As far as fungicides go, early season applications may not be overly beneficial as the flag leaves and upper tissues, which contribute the majority of carbohydrates for grain fill, will not be protected.  In addition, these diseases often are not impacting these important tissues due to temperature and humidity constraints.  However, if susceptible varieties are planted and conditions favor disease (cool and wet) they can help reduce disease onset.  Most often a cheap triazole (DMI-group 3) fungicide will work, and provide about 2 weeks of protection, but products with QoI (group 11) and SDHI (group 7) active ingredients also work well and provide a little more residual disease control.  Be aware that these fungi can develop insensitivity to fungicides.

Typically we see symptoms of the soil borne viruses at this point in time, when weather is cool.  Examples include wheat spindle streak and wheat soilborne mosaic viruses.  These are spread by soilborne protists and do best under cool temperatures, and symptoms often look like nitrogen deficiency.  Consequently, when temperatures warm up and nitrogen goes on, symptoms disappear, but the virus may still be present and active.  As a result you may see reduced yields but no visual symptoms.

Symptoms of wheat soilborne mosaic virus. Note stunting and chlorosis that can be misdiagnosed at nitrogen deficiency.



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