Head scab or fusarium head blight (FHB), is a fungal disease that can occur on all small grain crops grown in North Dakota. The disease is seen most commonly in spring wheat, winter wheat, durum, and barley, that can cause significant reductions in yield and quality.
Yield loss occurs from floret sterility and the productions of shriveled, light-test-weight kernels. Quality reductions occur from fungal toxins, specifically mycotoxins, that are produced in infected seed. The toxins are unacceptable for certain end uses such as animal feed and human consumption. Dockage can also occur at the point of sale.
You may wonder what you need to look for to identify if your crops are experiencing scab. Common symptoms in wheat and durum include bleaching of the head. Bleaching may occur on a portion of the head or the entire head and is most noticeable in a susceptible variety. The fungus may also infect the stem immediately below the head causing a brown to purplish discoloration of the stem tissue. Signs of Fusarium infections are pink to salmon-orange spore masses often seen on infected spikelets and glums during prolonged periods of wet weather.
The fungi persist and multiply on infected crop residues of small grains. During wet and humid weather conditions, spores of the fungi are windblown or splashed onto the heads of cereal crops. Spores can come from within a crop or can be blown from surrounding crops sometimes long distances away.
Wheat and durum crops are susceptible to infection from heading to the hard dough stage but are most susceptible at flowering (when anthers are present). Spores of the causal fungus may land on the exposed anthers at flowering time and then grow into kernels, glumes, or other parts of the head.
For spring barley, infections occurs once the head breaks through the leaf sheath since it flowers when the head is in the boot stage. Infection in either crop may continue until close to grain maturity under favorable environmental conditions for the organism.
The most favorable conditions for infection are prolonged periods (48-72 hours) of high humidity and warm temperatures (75 to 85 F). However, infection can occur at cooler temperatures when high humidity persists for longer than 72 hours. Early infections may produce air-borne spores, which are responsible for the secondary spread of the disease, especially if the crop has uneven flowering due to late tillers.
Triazole fungicides (fungicide resistance group 3) are recommended for scab suppression because they are locally systemic and have been shown to reduce scab and DON. A well-timed fungicide may help reduce Fusarium damage. Reductions in scab severity of 50 to 60 percent can be achieved when fungicides are applied at early flowering for wheat and durum, and at full head in barley.
Applying a fungicide four to seven days after early flowering in wheat (full head in barley) still can suppress FHB. When making late applications, make sure to consider the harvest restriction interval for the fungicide that is used.
Application studies have shown that spray coverage and disease control with these fungicides is improved when the sprays are directed at an angle forward and backward toward the grain head, or with single nozzles directed toward the grain head, all at a 30-degree angle from the horizontal.
FHB-infected grain may contain fungus-produced toxic substances called mycotoxins. The most common mycotoxin associated with Fusarium-infected grain in the northern Great Plains is deoxynivalenol or DON (vomitoxin). This mycotoxin may cause vomiting and feed refusal in nonruminant animals, such as pigs.
The presence of this toxin may result in substantial price discounts at the market and even refusal to purchase if DON toxin levels are high. In the case of barley used by the malting and brewing industry, DON level requirements are very low, generally less than 0.5 part per million (ppm; milligrams/kilogram).
The presence of Fusarium-infected grain does not automatically mean mycotoxins are present. The occurrence, amount and kind of mycotoxins may depend on several factors, including environment, species of fungus present, severity of infection and the variety or kind of crop. Fusarium-infected grain may be tested for DON and other mycotoxins at properly equipped laboratories.
Contact a veterinarian or feed specialist for further information on safe livestock feeding levels. The risk of human exposure to DON ingestion is minimal under the Food and Drug Administration (FDA) guidelines, but producers and elevator operators need to be aware that moldy grain can cause allergy and breathing problems. Producers and elevator operators should wear a good-quality dust mask when working around grain with high amounts of scab or other molds.