Wheat Disease Update: Stripe Rust and Fusarium Head Blight
Carl A. Bradley, Extension Plant Pathologist
Fig. 1. Symptoms of Fusarium head blight (scab) of wheat (note the “bleached” heads) (Photo by C. Bradley).
Several wheat fields in Kentucky are now at the anthesis stage (Feekes 10.51, “flowering”) or fast approaching it. Anthesis is a critical time, as wheat becomes susceptible to infection by Fusarium graminearum, the primary causal agent of Fusarium head blight (FHB; also known as scab) in Kentucky (Fig. 1). This disease can cause reduced grain yield, test weight, and quality. In addition, the fungus can produce toxins that will contaminate grain such as deoxynivalenol (DON; also known as vomitoxin). Harvested grain with high levels of DON may be discounted or outright rejected at the elevator. To achieve the best management of FHB, different management practices must be implemented, such as planting wheat into fields that were previously cropped to soybean (rather than corn), planting wheat varieties with moderate to high levels of resistance to FHB, and applying foliar fungicides at the proper timing. Of these different management practices, the application of foliar fungicides is the only one that can be done during the growing season.
Multiple fungicides are registered for use on wheat, but only a few have efficacy in managing FHB. Over the last couple of years, two new fungicides became available, which are Prosaro Pro (Bayer CropScience) and Sphaerex (BASF Corp.). These two relatively new fungicides join a lineup of other fungicides available that have efficacy against FHB, which are Caramba (BASF Corp.), Miravis Ace (Syngenta Crop Protection), Prosaro (Bayer CropScience), Proline (Bayer CropScience), and multiple tradenames of products that contain tebuconazole as their solo active ingredient. Multi-year, multi-university research has shown that the best efficacy against FHB and the associated mycotoxin DON has been achieved with Caramba, Miravis Ace, Prosaro, and Proline compared to products that contain only tebuconazole. Results from multi-year, multi-site research trials evaluating Prosaro Pro and Sphaerex are more limited but have shown that these products have efficacy in managing FHB and DON similar to Caramba, Miravis Ace, Prosaro, and Proline. The most recent edition of the “Fungicide Efficacy for Control of Wheat Diseases” publication is available on the Crop Protection Network (https://cropprotectionnetwork.org/publications/fungicide-efficacy-for-control-of-wheat-diseases). This publication provides additional information about the efficacy of fungicides for management of several wheat diseases, including FHB.
Currently, the primary fungicide group that is being used to manage FHB and DON is the demethylation inhibitor group (DMI; also known as triazoles); however, Miravis Ace and Prosaro Pro also each contain an active ingredient from the succinate dehydrogenase inhibitor (SDHI) group of fungicides (pydiflumetofen in Miravis Ace and fluopyram in Prosaro Pro). Table 1 provides a breakdown of active ingredient rates for each product available for FHB and DON control in Kentucky.
Proper fungicide application timing is critical in achieving the best efficacy. The best application timing is when plants are beginning to flower (early anthesis – Feekes growth stage 10.51), but some efficacy may still be achieved within a few days after Feekes 10.51. Regarding fungicide application timing, it is important to always follow the label recommendations and consider the preharvest interval (PHI) requirements, which can vary from product to product. Fungicide products that contain quinone outside inhibitor (QoI; strobilurin) active ingredients should not be applied for control of FHB, and most do not list FHB control or suppression on their label. In multiple university research trials, strobilurin fungicides have been shown to increase DON levels in grain compared to non-treated checks. Therefore, it is extremely important that only effective fungicides be applied for management of FHB.
When making a decision on if a fungicide application is needed, FHB risk should be assessed. A FHB Prediction Tool is available on-line at www.wheatscab.psu.edu. This risk is based on weather conducive for FHB, and should be assessed for each field as they begin to develop heads in anticipation of flowering. It is important to continually monitoring the FHB Risk Prediction Tool as more and more wheat fields get closer to the anthesis stage.
Fig. 2. Stripe rust of wheat (Photo by C. Bradley).
On another wheat disease note, stripe rust (Fig. 2) got an early start this year in southern states and has recently made its way into Tennessee and Illinois. Although I have not yet observed stripe rust in any Kentucky wheat fields, it is likely already here, based on its presence in neighboring states. Fungicides that are registered for control of FHB also have efficacy against stripe rust, so a Feekes growth stage 10.51 application of an effective fungicide for protection against FHB will also provide protection against stripe rust.
Acknowledgement:
This article references the results of research supported by the U.S. Department of Agriculture under agreement no. 59-0206-0-183, which is a cooperative project with the U.S. Wheat and Barley Scab Initiative.