Head scab is not the only cause of bleached wheat heads

By Pierce Paul and Dennis Mills, Ohio State University Extension

Head scab is showing up in Ohio wheat.

We have received several reports of bleached wheat heads in fields across the state. The distribution of symptoms in the affected fields ranges from individual bleached heads scattered throughout the field to huge sections of fields or entire fields with bleached heads. Timing of symptom development ranges from one to three weeks after flowering. In some instances, bleached heads are empty (blank). Such a wide variety of patterns and symptom characteristics is causing considerable confusion among producers as to whether they are dealing with head scab or some other problem. Scab does indeed cause bleached heads, but it is not the only cause of this type of head disorder. Along with head scab, take-all, hail, frost, flooding, and injuries caused by insects (wheat stem maggot) may all lead to bleached or white discoloration of wheat heads.

Useful information to help you determine whether you are dealing with scab include 1) the weather condition shortly before and during flowering, 2) the timing of symptom development after flowering, 3) the bleaching pattern on the head and the plant, and 4) the distribution of affected heads in the field. First of all, we have had head scab favorable weather in several parts of the state this season. It has been wet and humid during flowering in some areas, and these conditions are highly favorable for head scab. Infection by the scab fungus occurs at flowering and early grain fill, resulting in symptom development between 18 and 21 days after flowering. However, under wet, humid conditions, symptoms may develop earlier. Scab symptoms appear as partially (one or a few spikelets) or completely bleached heads, usually borne on green, healthy stems. With flood damage, wheat stem maggot injury, or take-all disease, completely bleached heads are usually borne on bleached stems, as a result of premature death of the entire plant, not just the head. With scab, symptoms are usually seen scattered throughout the fields, with partially or completely bleached heads intermingled with healthy-looking heads. This is contrary to what is usually seen with damage caused by flooding or take-all, where all heads in certain sections of the field or even entire the field may be affected. Although scab may cause heads to be partially or completely blank, scab infected heads normally produce grain, however, these are usually small and lightweight, with a pinkish-white discoloration. Completely blank heads are very common with frost and flooding injuries.

Adding to the confusion is the fact that some of the bleached heads are showing up in fields that were treated with a fungicide for head scab control, leading some producers to believe that they should not be seeing scab after having their fields treated. Remember, even the best fungicides (Prosaro and Caramba), applied at the right time (at flowering) will not provide 100% scab control. Fungicides are about 50% effective against scab. Think about this in a very simple way; a field that would have had 50% of the spikes infected without a fungicide treatment, will end up having about 25% of the spike infected after fungicide application, not 0%.

It is important to be able to tell scab apart from other problems, since scab infection is usually associated with vomitoxin contamination of grain, which does not occur with damage caused by take-all, hail, frost, flooding, or insects. Separate grain handling, processing, storage, and feeding guidelines need to be followed when dealing with vomitoxin contaminated grain.

For more from the OSU Extension CORN Newsletter, visit corn.osu.edu.

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