Source: Pierce Paul, Alissa Kriss, Dennis Mills, Katelyn Willyerd, Ohio State University
Based on a statewide survey of wheat fields over the past 10 days, the incidence of head scab ranges from about 3 to 61 percent. This means that between 4 and 53 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. The conditions have been favorable in a number of locations this spring. 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. For a comparison between Fusarium Head scab and Stagonospora Glume Blotch the two photos.
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 exactly 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 percent. If in another field, 3 out of the 10 heads have 5 spikelets diseased, the incidence is still 30 percent. However, the severity of scab is quite different between the two fields. Severity has a closer relationship to how much yield will be lost as well as potential levels of DON. Although this is not always a straight correlation. To get a handle on how severe disease is we begin to look closer at the spikelets within each head and how many of those are scabby.
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 percent 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 percent of the kernels). The second field with 10 percent of the kernels scabby will likely have more vomitoxin than the first field with 2 percent scabby kernels, even though the incidence is 30 percent 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 than your neighbor’s 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.
This is the most important. 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 will 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 scout field to make sure that you do indeed have scab, and more importantly, to determine how much scab is there.
Walk fields and examine heads at multiple (30 or more) locations spread out across the field. At every point, count the number of heads with scab out of every 40 heads you examine. Also observe the heads to determine how much of each head is scabby, that is, how many of the spikelets on the head are diseased. Assuming that each head has about 15 spikelets and each spikelet produces about 3 kernels, use the table below to help you estimate what percentage of your grain may be scabby. For example if you examine 40 heads and 3 are scabby, the incidence will be 7.5 percent. If the scabby heads have an average of 4 scabby spikelets, then the percent scabby kernels will be about 2 percent.
The incidence of scab will likely be highest where there is more corn/wheat residue on the ground close to source of inoculum. It will be higher where the moisture takes longer to dry off. If an area was impacted check the fields that are not tilled or have trees surrounding the field preventing air flow.
Table 1. Estimate of average percent scabby kernels based on scab incidence and the assumption that each head has 15 spikelets and each spikelet produces 3 kernels. These are assumptions based on some research data. The results will vary based on the variety, disease severity and the environmental conditions that will occur over the next few weeks. This table is a preliminary estimate and guide to use for when fields should be not harvested. All fields with scab should be tested for DON levels.
Vomitoxin will likely exceed 2 ppm in the red area when the percent scabby kernel is greater than5 percent (red). Vomitoxin levels can be deceptively high in some years at low scab levels (blue), especially if rains for several days between flowering and harvest.
2- Do turn up the air on the combine to blow out scabby kernels.
These kernels are lighter than non-infected or healthy kernels.
3- Do harvest areas or fields with the most scab first and keep that grain separate from the rest.
4- Do get grain tested for vomitoxin before feeding.
5- 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.
6- Do continue to read the C.O.R.N newsletter for more on head scab and vomitoxin.
Disclaimer: Dr. Paul, Field crops disease team and The Ohio State University will not be held liable for the information presented in this table. This is just meant to be a guide. The association among scab incidence, percent scabby kernels, and vomitoxin contamination will vary with variety and weather conditions and as such may be different from that shown here. DO NOT use this information as a substitute for vomitoxin testing. Sending sample to a certified lab is the best way to tell how contaminated the grain is.