Alyssa Koehler, Extension Field Crops Pathologist; akoehler@udel.edu
The majority of wheat anthesis has wrapped up. This season started dry, which kept us at low FHB risk. Hopefully this led to reduced early season spore production. The rains in the past week have increased us to high risk for very susceptible varieties, and medium to high in Kent and Sussex counties for susceptible (Figure 1) and moderately susceptible (Figure 2). Once wheat has flowered, symptoms of FHB are usually visible in 18-24 days, but cool weather can slow symptom development and warm weather can speed things up. With the amount of frost damage we have seen this year, it will be important to take note of head health in the week to 10 days after anthesis to get a sense of frost damage versus damage from disease. Depending on what growth stage the wheat was in, you may see bent heads or grain that did not fill out (Figure 3). Heads with FHB will have bleached florets or bleached sections of the head, which can look similar to frost damage. If you have not scouted the field earlier in the season, it may be hard to distinguish the cause of head bleaching. A unique feature of FHB is that you may be able to see pink/orange growth on spikelets (Figure 4). You can follow these steps to assess the level of frost damage and later the level of FHB present in your field.
- For every 10 acres of field, randomly select one spot to survey.
- Keeping your line of sight above the wheat heads, walk 40-50 yards and randomly pick 10-20 heads to look at on the plant or detach and place into a bag. (You don’t want to be looking down and biasing the heads you select).
- Once you have randomly selected the heads, rate the percent of each head with frost damage or symptoms of FHB (bleaching or pink growth on spikelets).
- After you have recorded values for each head, you can determine the average percent FHB severity by dividing the sum of disease severities by the total number of heads collected. (Ex. You rate 10 heads with severity values: 0, 10, 30, 0, 0, 20, 10, 0, 0, 0. These add up to 70. 70/10 heads = 7% FHB severity). Higher levels of FHB are typically associated with elevated levels of DON and possible issues with yield and test weight. It is possible to have delayed or lower levels of symptoms and still have DON.
- Repeat this assessment as needed to get an overall rating for the field. Fields with greater than 10% FHB severity are at higher risk for yield losses or elevated DON. Fields with elevated DON should be harvested as early as possible and you may want to consider increasing combine fan speeds and shutter openings to reduce the amount of scabby kernels harvested.
Figure 1. FHB Risk Model for May 19, 2022 for winter wheat rated susceptible to FHB (wheatscab.psu.edu)
Figure 2. FHB Risk Model for May 19, 2022 for winter wheat rated moderately susceptible to FHB (wheatscab.psu.edu)
Figure 3. Frost damage on wheat heads.
Figure 4. Symptoms of Fusarium Head Blight