WOOSTER, Ohio – Hot, humid weather coupled with rain during a critical development stage of Ohio's wheat has caused an outbreak of head scab in some areas of the state – the first major outbreak of the disease in the state in about a decade.
A statewide survey of Ohio wheat fields, which began two weeks ago, has found the incidence of head scab to be moderate to high in 70 percent of the73 fields surveyed in 16 counties.
"Incidence of head scab ranges from 3 percent to 61 percent, meaning that between 3 and 61 heads out of every 100 heads has some level of head scab," said Pierce Paul, an Ohio State University Extension plant pathologist and small grains specialist.
Head scab (Fusarium graminearum), also known as head blight, is a disease that attacks wheat during the crop's flowering stage when environmental conditions are just right. The disease not only affects yields, but the fungal pathogen that causes the disease produces several mycotoxins, the most common of which is known as vomitoxin, that is harmful the humans and animals if ingested.
"We've seen more head scab in some fields than we've seen in the last 10 years," said Paul, who also holds a research appointment with the Ohio Agricultural Research and Development Center. "I think that we are seeing higher incidences of head scab than normal this year partly because of the high level of ear rots we had in corn last year. More spores were carried over from last year's corn to this year's wheat. Add to that favorable weather conditions and the fact that most of our varieties are susceptible, and you have all the makings for an outbreak."
According to the statewide survey, the majority of the fields with greater than 25 percent head scab incidence are in the west-central and northwestern parts of the state. All of the fields with lower than 10 percent head scab incidence were in the southern, southwestern, or northern part of the state.
"However, not every field in those regions were inspected, so it is quite possible that some southern Ohio fields may have higher than 10 percent incidence and west-central and northwestern Ohio fields may have lower than 25 percent incidence," said Paul. "It all depends on variety, resistance, flowering date, and weather conditions during flowering.
Ohio growers potentially impacted by head scab infections must first determine how much scab they actually have in their fields before trying to quantify a yield loss or tearing out their wheat crop and planting soybeans instead.
Paul recommends growers establish a scouting and sampling scheme by:
• Walking the field and examining wheat heads at 30 or more locations across the field.
• At every point, counting the number of heads with scab out of every 40 heads examined.
• Observing the heads to determine how much of each head is scabby and how many spikelets on the head are diseased.
"The more scabby heads you have in the field and the more scabby spikelets you have on the heads, the more scabby kernels you're likely to have and that translates into a higher amount of vomitoxin in the grain," said Paul.
For example, out of the 40 heads examined, if 3 heads are scabby, the percent incidence of head scab is 7.5 percent. If 4 spikelets per each of those scabby heads are scabby, the percent of scabby kernels would be 2 percent. This is assuming that each head has about 15 spikelets and each spikelet produces about 3 kernels. Yield loss on a susceptible variety with 5 percent scabby kernels is about 3 bushels per acre, on average. Vomitoxin would likely exceed 2 parts per million when the percent of scabby kernels is greater than 5 percent.
Paul has provided additional information on head scab and how to measure the amount of scab in the field at: http://bit.ly/dsGqSY.
Paul said that at this stage nothing can be done to control head scab, but growers can take several approaches to minimize losses. They include:
• Scouting fields to make sure that you do indeed have scab, and more importantly, to determine how much scab is there.
• Turning up the air on the combine to blow out scabby kernels.
• Harvesting areas or fields with the most scab first and keep that grain separate from the rest.
• Testing grain for vomitoxin before feeding.
• Plowing under scabby wheat stubble, if you choose to abandon wheat fields with high levels of scab to plant soybean.
In addition, Paul recommends that growers:
• Do not make a decision about their wheat fields before knowing exactly how much scab is present.
• Do not make a decision based on how the field looks from a distance.
• Do not wait until it is too late to scout fields.
• Do not feed grain from fields with scab to livestock before getting it tested for vomitoxin.
• Do not use straw from fields with scab for hay without getting it tested for vomitoxin.
• Do not handle scabby grains without gloves and masks.
Head scab isn't the only factor that may contribute to lower wheat yields this year. The crop has also been impacted by Stagonospora blotch, leaf rust and cereal leaf beetle, and has experienced a reduced grain fill period due to hot weather conditions in late May.
For more information on head scab, refer to the OSU Extension Agronomic Crops Network at http://agcrops.osu.edu.