Alyssa K. Betts, Extension Field Crops Pathologist; akoehler@udel.edu
Last winter was very wet, while this winter we have been trying to get out of drought conditions. The rain that happens over the next few weeks will play an important role in the overall risk for Fusarium Head Blight (FHB) in small grains this season. FHB is caused by species of the fungus Fusarium. 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.
Shriveled and wilted “tombstone” or “scabby” kernels can reduce yield and result in grain contaminated with mycotoxins (Figure 2). 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. While we are not able to eliminate FHB, we can take steps to try to reduce disease and mycotoxin problems.
1.Choose a variety with partial resistance Ok, we can’t change what is already in the ground, but for future seasons, resistance is one of the best tools to try to manage this disease. There is no complete host resistance against FHB, but there are now many 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, we will share the link with this year’s trials results later this summer. If you did plant a variety with resistance last fall, in the Fusarium Risk Tool, you can select the level of resistance you are expecting and see the adjusted risk level across susceptibilities (Figure 3).
2.Watch the weather and use a well-timed fungicide
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. 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, this gives a greater chance that secondary tillers are 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 a pale white (Figure 4). Triazole (FRAC group 3) fungicides that are effective on FHB include 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.
3.Manage Irrigation – limit water during flowering
Each spring we walk a fine line with water. The crop needs water to grow, but so do the pathogens. When we are in a drought year where supplemental irrigation is being used it is important to have an understanding of your fields water holding capacity and try to get water out in the weeks prior to flowering. Ideally, you want to be at field capacity and pause irrigation for the 8-10 days around flowering. With our sandy soils it can be challenging to wait this long under very dry conditions. Once flowering has concluded you can then bring soil moisture back to near field capacity.
4.Timely Harvest, adjust combine settings, dry quickly if storing
A few weeks after flowering, scout fields to get an idea about FHB incidence across fields. This is much easier to do before the wheat or barley dries down. In fields where you know you have a problem, taking a few moments to adjust combine settings and increase the fan speed can help to try to blow out lightweight, diseased grain with chaff. Ideally you do not want to store grain from fields with high levels of FHB but try to get moisture below 15% for short-term storage and below 12% for long-term storage. If you have some fields or field areas with low disease, consider keeping the high disease grain separate.
5. Avoid planting behind corn
In your field rotation plan, if you have soybean fields or vegetables that can be harvested early enough for a timely wheat or barley planting, this rotation helps to reduce the amount of pathogen overwintering and starting the season. 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
Since much of our small grain acreage is planted into corn fodder, FHB inoculum is maintained over winter. Last year I happened to have a great example of this were we had trials that were planted on the same day, with the same susceptible variety, just a few hundred yards apart. One area was behind corn, the other behind a vegetable rotation. Both trials were treated the same with mist systems set up in the spring to try to increase disease for fungicide efficacy screening. While the fungicides did reduce DON in the corn rotation, all treatments remained above 1 ppm. Meanwhile in the vegetable rotation the control was 0.5 ppm and the fungicide treatments even lower (Figure 6). The yield average in the control plots of the veg rotation were ~19 bu/a higher than the control plots following corn. While there are more logistics involved with not planting behind corn, for those growing malting barley or high value wheat with strict DON requirements to meet, rotation is something to consider.
