November 4, 2012
The weather was beautiful this past week for baling hay, and looks like the crops are almost all in around Delaware County. I’ve been out scouting the wheat fields this past week, and I’ve seen head scab in a few fields and I’ve seen surprising good wheat. The wet and humid spring has made it favorable for head scab. All areas around the state are reporting of head scab in fields. The severity has varied around the state. The reports of affected fields range from individual bleached heads scattered throughout the field to huge sections of fields or entire fields with the bleached heads. The affected fields in Delaware County have just scattered affected wheat heads. Most of our fields missed the worst of it.
We start to see the development of these symptoms in one to three weeks after flowering. Bleached heads can contain no seeds or have just have some blank pods in the head. There has been confusion among producers as to whether they are dealing with head scab or some other problem stemming from this wide variety of patterns and symptoms. Scab does indeed cause bleached heads, but it is not the only cause of this type of head disorder. According to Pierce Paul, OSU Extension Specialist in Plant Pathology, the following information can help you determine whether you are dealing with scab: 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.
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 the entire field may be affected. Paul says that 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,” said Paul. Remember, even the best fungicides, applied at the right time will not provide 100 percent scab control. Fungicides are about 50 percent effective against scab. Think about this in a very simple way: A field that would have had 50 percent of the spikes infected without a fungicide treatment will end up having about 25 percent of the spike infected after fungicide application, not 0 percent.
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. Paul says that separate grain handling, processing, storage and feeding guidelines need to be followed when dealing with vomitoxin contaminated grain.
Rob Leeds is an OSU Extension Educator for Agriculture/NR.