The majority of wheat in the Mid-Atlantic has reached or is beyond flowering at this time. There is also some risk of post-flowering infection if conditions continue to be humid. Fields that flowered 2 to 3 weeks ago will be starting to show the symptoms of head scab now. These symptoms are easiest to see before the wheat turns straw-colored. Bleached spikelets will be apparent in an otherwise green head, and upon closer examination, an orange-pink cast can be seen. In Figure 1 we see some infected heads in the field and Figure 2 shows the scope of infection patterns possible with this disease.
It is important to note that the level of head scab in your field does not always match the level of DON in your harvested wheat, but it is generally a good indicator. This has to do with the biology of the fungus. Also, many spikes that were affected by scab may not fill, subsequently the lightweight infected grain may be removed with the chaff. Start scouting fields about 3 weeks after flowering to determine the level of infection. Heads that are infected after the flowering stage may not have time to show symptoms before maturity.
Even those farmers that got a timely spray on at-risk wheat may see some symptom development. This is because even the best products applied at the perfect time (at the onset of flowering) do not give 100% protection. At best, these fungicides can offer a 50-60% reduction in disease severity and, ultimately, vomitoxin production. If a spray was applied before flowering, disease control will be even less.
If you find you have more than 25% of your heads affected by scab, consider harvesting it using a high fan speed on your combine, which helps to clean out the lighter, infected kernels (which are highest in vomitoxin). Another option is to attempt to segregate scabby fields from clean ones during harvest. Fields often have levels of infection that vary on the edges or from field to field based on planting date and flowering date.
This document provides a list of providers for mycotoxin quick tests. Check with your crop insurance agent to understand the proper procedure for harvest and testing if you suspect you may have a problem with vomitoxin this year.