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Scouting and Quantifying Fusarium Head Blight in Small Grains

By Alyssa Collins and Paul D. Esker
 
Given that in many areas of the state barley headed during the first two weeks of May along with wheat flowering occurring during the past two weeks in most areas, now is the time to start to think about scouting for Fusarium head blight (scab).
 
Typically head scab symptoms takes about 18 to 21 days 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 (Figure 1) will be apparent in an otherwise green head, and upon closer examination, an orange-pink cast can sometimes be seen. It is really important to know that symptoms can be variable in the head, as evidenced in Figure 2 that illustrates a range of symptoms and locations on heads. In barley, scab is less obvious and often has a darker appearance on spikelets, but you can also see a light orange sporulation here at times (Figure 3). There is also some risk of post-flowering infection in both crops if conditions continue to be humid.
 
Figure 1. Fusarium head scab on wheat showing bleached spikelets.
 
Figure 2. Range of patterns often seen on small grains with Fusarium head scab.
 
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. Heads that are infected after the flowering stage may not have time to show symptoms before maturity.
 
Figure 3. Fusarium head scab on barley.
 
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.
 
As you make assessments, 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.