Alyssa Collins, Extension Plant Pathologist (Penn State); collins@psu.edu
Just like waiting on the Polaroid photos of old, once wheat heads flower, it is a much less delightful waiting game to see if (and how much) head scab develops in your crop. It takes about 18 to 21 days for the symptoms of scab to become visible, even though the plant was infected at flowering. 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 sometimes 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. Scab on barley is less obvious, but you may notice the light orange sporulation here, too (Figure 3). There is also some risk of post-flowering infection in both crops if conditions continue to be humid.
It is important to note that the level of head scab in your field does not always match the level of toxin (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, and 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 applied a timely spray on their crops, or those that planted scab-resistant varieties, 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 50-70% reduction in disease severity and, ultimately, vomitoxin production. If a spray was applied prior to flowering, disease control will be even less. In years of high disease pressure, even resistant varieties will show some symptoms.
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. 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. If an insurance adjustment is needed, you will have to follow their specific directions for sampling and testing prior to harvest.
Figure 1. Head scab on wheat.
Figure 2. Scope of infection patterns possible with head scab.
Figure 3. Head scab on barley.