Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
The wet weather continues to keep Delaware and Eastern Maryland at moderate to high risk for Fusarium head blight (FHB). Elevated FHB risk may persist through next week. There is still a large amount of wheat that just started to flower or will flower next week. If we are lucky the late-flowering wheat could somewhat escape the current elevated period of risk.
Remember that applications of Caramba, Prosaro, or Proline should be made from the start of flowering and can be made up to 5-6 days after this point without significant drop-off in efficacy. The start of flowering is when 50% of the main tillers have anthers emerging from the center of the heads. We target this timing because the main tillers constitute the majority of yield. Infections of anthers can result in the fungus colonizing the head and rachis, resulting in reduced nutrient and water flow to the grain. This action results in blank or lightweight kernels and reduced yield. If the fungicide is applied too early or too late expect reduced efficacy (Figure 1).
Figure 1. The effects of Prosaro and Caramba, applied at different timings, on DON. Lower levels are better in this case. Note that the FGS 10.5 application was only 3 days from flowering, which may explain the efficacy in application. There is no way to tell if you are 3, 5, or more days from flowering. For optimal impact on FHB and DON, try and hit the FGS 10.5.1 to 10.5.1+5 window. Chart based on data from A. Grybauskas.
The application of the right fungicide, applied perfectly, to a susceptible variety may not result in satisfactory suppression of FHB and DON in a high risk year, as these products only reduce DON by 45% and visual symptoms by 50% on average, compared to untreated controls. This reduction can be increased to over 75% compared to untreated, susceptible controls when a good moderately resistant variety is combined with proper fungicide application. This is why FHB management starts with the selection of a good moderately resistant variety.
I suggest growers assess their wheat fields for FHB 18-20 days after the start of flowering. Assessing fields earlier than that will not give the disease time to develop, and later than that may result in masking of symptoms as the wheat starts to mature. Run 10-20 transects through different parts of the field. Pick 10-20 per location without looking at heads. Bring the heads back to your vehicle and determine the percent of heads with FHB-derived bleaching. If you have more than 10% bleached heads, you may have an FHB issue. Although bleaching does not guarantee elevated DON, it may indicate that there is a potential for this to occur. Look for areas of the field where coverage may have been poor. If possible, harvest and store highly symptomatic fields or areas separately from good fields to minimize contamination. FHB symptoms are very pronounced, making the disease often appear worse from afar. The same can be said for other diseases such as BYDV-a little disease looks like a lot.
Unmanaged wheat may have reduced yields if you planted varieties susceptible to stripe rust or powdery mildew (Figure 2). The cool weather and high humidity has allowed both of these diseases to continue to develop in susceptible varieties and in some cases, powdery mildew has moved onto the flag leaf around heading, which will likely result in yield losses if favorable conditions persist. This of course, will be coupled with potential FHB associated issues.
Figure 2. An area of a research plot in Middletown, DE infected with stripe rust. A note about rusts- they start as isolated patches, or foci, and develop and spread over time. Early detection can save you from potential yield losses later on if favorable environmental conditions persist.