Wheat Scab and Organic Management
by Christina Cowger and Randy Weisz
Head scab of small grains is caused by the fungus Fusarium graminearum, which also infects corn. Scab can occur in all small grains. Wheat and barley are the most susceptible to the disease, oats are a little less susceptible, and rye and triticale are the most resistant.
Infection occurs at or soon after flowering, when fungal spores reach small-grain heads by wind or rain-splash. Once it’s established in a spikelet, the fungus can spread to other spikelets, resulting in heads that are partly green and partly bleached. Superficial pink or orange spore masses can be seen on some infected spikelets.
Early infections can cause kernel abortions, and later infections can cause shriveled kernels (called “tombstones”) that have low test weight. Scab produces toxins in the harvested grain, the most common being DON (deoxynivalenol, or vomitoxin). When DON reaches 2 parts per million (ppm), the grain is no longer fit for human consumption and may not be marketable for flour. When DON reaches 5 ppm, the grain is no longer fit even for swine or dairy cattle feed. The limit for beef cows and poultry is 10 ppm.
Wet weather before, during, and soon after small-grain flowering is the main factor determining whether head scab will be severe. Warm temperatures (59 to 86°F) before and during flowering also favor scab.
Sadly, no single management practice will defeat scab. However, wheat producers who take the following measures will reduce the likelihood of a major scab problem:
- Plant resistant varieties — Especially for organic production, it’s absolutely essential to plant scab-resistant varieties! Many good wheat varieties have moderate scab resistance. The best source on wheat variety resistance to scab is the Wheat Variety Performance and Recommendations SmartGrains Newsletter (www.smallgrains.ncsu.edu). There are also a few barley varieties with some scab resistance (such as Thoroughbred from Virginia Tech).
- Stagger maturities — Spring weather is often not warm and moist for more than a week or two. So scab risk can be reduced by planting at least two wheat varieties from different heading-date classes (for example, one medium variety and one late variety). In that way, head emergence and flowering will be staggered through the spring, reducing the chance that environmental conditions will be conducive to scab in all wheat fields. A second way to force wheat to flower at different times in the spring is to stagger planting dates.
- Check risk and scout –– Just before and during wheat flowering in April or early May, check your scab risk — go to the national risk forecasting web site (www.wheatscab.psu.edu) and choose “Risk map tool.” Two to three weeks after flowering, scout for scab when the contrast between the bleached and green parts of heads is still apparent. Do this before grain heads turn golden.
- Adjust the combine — If scab is severe (10% or more of spikelets are scabby), adjust the combine so that the lightweight diseased grain is blown out the back along with the chaff. This will not remove all the infected grain but can help reduce mycotoxin levels in grain heading to market.
More information on scab management is available at Scab Smart at www.scabsmart.org.