Wheat Disease Update

Up to this point in the season, wheat diseases have been relatively minimal. Early-season soil-borne viruses were not a significant problem, and all indications are that arthropod (insect/mite) –vectored viruses will also be minor players this year. Other foliar fungal diseases, such as those that make up the leaf blotch complex; leaf, stripe, and stem rust; and powdery mildew have also been confined to the lower canopy or can barely be found in many fields. I attribute the minimal fungal disease picture to cooler than normal conditions this spring. In general, the excellent production practices of Kentucky’s wheat producers (planting date, fertility, variety selection, etc.) have also kept disease pressure low.

I believe the recent block of rain will change everything for the worst. I already see indications of this in my plots at Princeton. As we all know, it started to rain the last couple days of April and it has been wet, more or less, ever since. A few very windy days before the rain set in and then the rain, has significantly limited the number of acres that were treated with a fungicide. Fortunately, some farmers were able to arrange for aerial applications in some fields, but ground applications were greatly reduced compared to normal. The bottom line is that many fields are completely unprotected, and many that did get sprayed were sprayed earlier or later than is desired for maximum protection from fungal diseases.

To make matters worse, disease models indicated that the Fusarium head blight (FHB; head scab) risk was high across most of the state during the wet period. Most wheat fields in the state were in some stage of flowering during the wet period. Flowering is the stage when wheat is most susceptible to infection by the fungus that causes FHB, Fusarium graminearum. So connecting the dots: moderate to high FHB risk (spore production)……limited fungicide applications….highly susceptible crop stage (flowering)…..conditions favoring spore release and infection (wet, cloudy, humid)…..and it is easy to see how FHB could be a serious problem this year, even in treated fields. I say even in treated fields, because it is well known that even the best foliar fungicides provide only suppression of FHB, not true control. When disease pressure is high, fungicides may help, but yield and grain quality can still be seriously impacted.
Over the last couple of days, I have received numerous queries asking if I thought spraying wheat that has finished flowering should be considered. My response was (and is) that no fungicides are labeled for applications this late and even if they were, they would not do much good. Fungicides targeted at FHB must be applied BEFORE infection occurs to be of any value; they do not have post-infection activity. Thus, if heads of non-sprayed wheat were infected by F. graminearum during the recent rainy weather, applications made now would not help significantly. Fungicides would most certainly afford some control of other fungal diseases, but if the crop is past flowering, I do not believe they can develop fast enough to severely impact yield. That is, they would have to reach damaging levels before the crop reaches the soft dough stage, and I do not believe this will happen.

The situation is different for the small number of fields that are still flowering. As long as the label restrictions can be met, there may still be some value in spraying crops that are at full bloom or earlier. Whatever is done, be sure to follow the pesticide label!

We will know the full extent of FHB damage in Kentucky by the end of May. Some farmers in the extreme southwest part of the state are just now beginning to see FHB symptoms.