Alyssa Koehler, Extension Field Crops Pathologist; akoehler@udel.edu
Fusarium species that cause FHB can infect both corn and small grains. Walking through fields with corn stubble, you may see orange growth on old debris (Figure 1). Since much of our small grain acreage is planted into corn fodder, FHB inoculum is maintained over winter. Wet spring conditions favor fungal sporulation that can lead to infected wheat or barley heads. As the fungus grows on corn fodder or other debris, spores are released that are then rain-dispersed or moved through air currents. As the grain crop is flowering, spores land on the head or anthers, colonize these tissues, and move into the grain. Once inside the grain, water and nutrient movement is disrupted which results in the bleached florets we associate with FHB (Figure 2). Shriveled and wilted “tombstone” or “scabby” kernels can reduce yield and result in grain contaminated with mycotoxins (Figure 3). Deoxynivalenol (DON), also referred to as vomitoxin, is a health hazard to humans and animals. Wheat heads colonized later in development may not show dramatic symptoms, but can still have elevated DON.
Figure 1. Corn stubble with Fusarium that can contribute to FHB in wheat.
Figure 2. Wheat heads showing bleached florets from FHB.
Figure 3. Healthy kernels (left) White, shriveled, scabby kernels from FHB (right).
Optimal wheat fungicide application is at early flowering (10.5.1) to about 5 days after. A well-timed fungicide application can help to reduce disease severity and DON levels. It is important to remember that fungicides help to reduce disease levels and DON (traditionally around 50% reduction on a susceptible variety), but they do not eliminate all FHB or DON. To try to maximize the efficacy of fungicides, it is important to apply at the correct timing. Fungicides for FHB are most effective when applied during flowering in wheat and at head emergence in barley. Although new products like Miravis Ace can be applied earlier, it is still best to wait for main tillers to be at 10.5.1 or a few days beyond, so that secondary tillers have a greater chance of being at 10.3-10.5.1. If you spray too early, heads that have not emerged will not be protected by the fungicide application. When wheat heads begin to flower, look for yellow anthers in the middle of the wheat head. When at least 50% of main stems are flowering, you will want to initiate fungicide applications. As the flowering period continues, anthers will emerge from the top and then the bottom of the wheat heads. Anthers can stay attached after flowering, but usually become pale white (Figure 4). Triazole (FRAC group 3) fungicides that are effective on FHB include Caramba (metconazole), Proline (prothioconazole), and Prosaro (prothioconazole + tebuconazole). Miravis Ace (propiconazole + pydiflumetofen) offers a triazole + SDHI, FRAC group 7. Other newer products to the market include, Spharex (metconazole [3] + prothioconazole [3]) and Prosaro Pro (prothioconazole [3] + tebuconazole [3] + fluopyram [7]). As a reminder, fungicides containing strobilurins (QoI’s, FRAC 11) should not be used past heading because these fungicides can result in elevated levels of DON. Flat fan nozzles pointed 90° down are great at covering foliage, but they do not provide as good of coverage on heads, which is the target for FHB management. Nozzles that are angled forward 30-45° down from horizontal (30 degrees is better than 45) or dual nozzles angled both forward and backward give better contact with the head and increase fungicide efficacy. For ground sprays, fungicides should be applied in at least 10 gallons of water per acre.
Thinking beyond this season, an integrated approach can improve management of FHB and help to keep DON levels low. In your field rotation plan, if you have soybean fields that can be harvested early enough for a timely wheat or malting barley planting, this rotation helps to reduce the amount of pathogen overwintering and starting the season. In addition to rotation considerations, seed selection is another important piece of FHB management in wheat. There is no complete host resistance against FHB, but you can select wheat varieties with partial resistance. The University of Maryland sets up a misted nursery to compare FHB index and DON levels across local wheat varieties under very high disease pressure to aid in variety selection decisions (https://psla.umd.edu/sites/psla.umd.edu/files/files/documents/Extension/Small%20Grains%20in%20MD/Wheat%20and%20barley_trial_data_2022-combined_withCF.pdf)
Figure 4. From left to right Feekes 10.3, Anthesis, Feekes 10.5.1 (yellow anthers beginning flowering), 4 days after anthesis (white anthers post flowering).