Head Scab Hits Ohio Wheat
The following article was written by OSU Extension Pathologist Pierce Paul –
A survey of wheat fields across the state shows that the incidence of head scab ranges from about 4 to 60 %, meaning that between 4 and 60 heads out of every 100 heads have some scab. Head scab is a disease caused by the fungus Fusarium graminearum or Gibberella zeae (two names for the same fungus). Scab develops best when wet, humid weather occurs when the wheat is flowering. We have had quite a bit of those conditions this year. The disease causes shriveled and lightweight kernels, reducing grain yield and test weight. In addition, the fungus also produces a toxin called vomitoxin that is harmful to humans and livestock. So, the effects of scab can be devastating, lower yields, lower test weights, and toxin contamination = price discounts or complete grain rejection at elevators.
At this stage, nothing can be done to control scab, but several approaches can be used to minimize losses. As you scout fields and make decisions or recommendations, here are a few DOs and DON’Ts:
1. Do not make a decision before you know how much scab is out there. Incidence is a very good measure of disease, but when using incidence one needs to be carefully not to overestimate or underestimate how serious the problem really is. A wheat head has on average 15 spikelets. If you look at 10 heads and 3 out of the 10 heads have a single spikelet diseased, then the incidence is 30%. If in another field, 3 out of the 10 heads have 5 spikelets diseased, the incidence is still 30%. Now if you consider that each spikelet will give you 3 kernels. Each set of 10 heads will give you 450 kernels (10x15x3). In the first case, 9 (3 heads each with 1 scabby spikelet x 3 kernels/spikelet) of the 450 kernels will be scabby (2% of the kernels). In the second case, 45 (3 heads x 5 scabby spikelets x 3 kernels per spikelet) of the 450 kernels will be scabby (10% of the kernels). The second field with 10% of the kernels scabby will likely have more vomitoxin than the first field with 2% scabby kernels, even though the incidence is 30% in both fields.
2. Do not make a decision based on how the field looks from a distance. Scabby heads contrast nicely with green leaves, making the field look more scabby than it really is. In addition, we have quite a bit of glume blotch (another disease of the head) this year, and you will not be able to tell the difference between glume blotch and scab from a distance. Not because you know for a fact that your neighbor has scab you should assume that you have it too. Your variety may be more resistant to scab or even if the varieties are the same, you and your neighbor’s fields may have flowered at different times.
3. Do not wait until it is too late to scout fields, fields are turning. Scabby heads and maturing heads both take on a straw color. So if you wait until next week you may think all the heads are scabby or all are healthy because they all look bleached and straw colored.
4. Do not feed grain from fields with scab to livestock before getting it tested for vomitoxin. Animals, particularly swine, may have serious health problems if fed grain with high levels of vomitoxin.
5. Do not use straw from fields with scab for hay without getting it tested for vomitoxin. Yes, straw from field with high levels if scab also becomes contaminated with vomitoxin and may cause the same problems caused by feeding scabby grain.
6. Do not handle scabby grains without gloves and masks.
1. Do turn up the air on the combine to blow out scabby kernels. These kernels are lighter than wholesome kernels.
2. Do get grain tested for vomitoxin before feeding.
3. Do plow under scabby wheat stubble, if you choose to abandon wheat fields with high levels of scab to plant soybean. Scabby wheat on the soil surface means more spores available to infect corn and cause Gibberella ear rot. Yes, the same fungus causes both diseases. Remember last year? Let’s break this cycle.
4. Do read the next C.O.R.N newsletter for more on head scab and vomitoxin. Available at http://corn.osu.edu
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