We've been lucky in most of Pennsylvania that our weather conditions have not been conducive to the development of wheat diseases this season. We are probably looking at a pretty clean harvest. Around now, the vast majority of wheat in the state is past flowering by at least a week or two. It's a good time to start checking your crop for flag leaf and head disease symptoms to get an idea of how your disease control approach worked and what kind of quality you can expect at harvest.
Fields that flowered two to three 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 (vomitoxin) 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 applied a timely spray to 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, DON 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 DON). 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, flowering date, and variety.
Check with your crop insurance agent to understand the proper procedure for harvest and testing if you suspect you may have a problem with DON this year.
Other Diseases Visible During Grain Development
Some additional disease symptoms you might see on your heads while you're scouting include Stagnospora glume blotch (more common in our area) and black chaff (less common). The symptoms of these diseases are less distinct than those of head scab. Both of these give more of a dark brown/black to purplish appearance to the glume. They usually affect parts of each spikelet on a head instead of discrete groups of spikelets here and there.
Depending on severity of these diseases, your yield and quality may be affected. The fungus that causes the Stagnospora blotch also often causes leaf spots on the flag leaf, which can impact grain fill. Another common leaf spot seen at this time is Septoria leaf blotch. If you treated with a fungicide at flowering for head scab, you will also get some control of Stagnospora and Septoria (but not black chaff). At this time, most growers are too close to the harvest restrictions for our effective fungicides to do any treatments.
There is a nice publication that can help you identify head and grain diseases of wheat.