Cause Several species of Fusarium can cause scab, including: F. graminearum, F. avenaceum, F. culmorum, and Microdochium nivale (formerly Fusarium nivale). These fungal species easily reside in soil and can grow there under a wide range of conditions. They operate as plant pathogens and can colonize plant residues and live as saprophytes (without a living host). They also can infect roots of numerous plant species without causing aboveground symptoms, which means their population can increase in a field and one may never notice. In small grains, these fungi also can cause seed decay, seedling blight, and leaf spots as well as root and foot rot. Spores produced on plant residue on the soil surface can ride on wind to the wheat head and subsequently invade parts of it. Rain, irrigation, or heavy dew promotes the high humidity necessary for spore germination and penetration. The longer the wheat plants stay wet during flowering and early seed development, the greater the chance of infection and spread in the plant. The most favorable conditions for infection are prolonged periods (36 to 72 hours) of high humidity. Generally, wheat is most susceptible at the flowering stage. Scab-infected grain can contain the mycotoxins known as vomitoxin and zearalenone. Vomitoxin (deoxynivalenol) may cause vomiting and feed refusal in swine. Zearalenone is an estrogenic mycotoxin and may cause infertility in domestic animals. Scab in grain does not mean it has mycotoxins; however, scabby grain should be analyzed for vomitoxin and zearalenone. Scab was severe in eastern and south-central Idaho and Washington in 1982 and 1984.
Symptoms Dark brown spots develop on glumes shortly after the developing head is infected. Entire spikelets become blighted, and infection may spread to other spikelets. In warm, humid weather, pink mold will be visible. A peach-color spore mass may be at the base of infected glumes. The fungus may spread throughout the spike and down into the culm, leading to head death, which causes premature bleaching. Only part of a head may be prematurely bleached. Infected florets that do not die produce poorly filled grain.
Cultural control Environmental conditions play a key role in scab development, but certain cultural practices reduce pathogen survival, lowering the risk of scab infection.
- Rotate fields out of cereals, grass seed, or corn for at least one year. Avoid crop rotations in which wheat follows other susceptible plants. Especially avoid planting wheat after corn or sorghum.
- If irrigating, manage water application to minimize high-humidity periods.
- Tilling to bury infested crop residues helps decrease risk. In minimum or no-till practices, spread and distribute chaff and other residue to speed decomposition, reducing pathogen populations.
- Increasing the combine's airflow removes many scabby kernels, which are shriveled and light.
Chemical control Seed treatments containing Dividend, Mertect, or Thiram reduce Fusarium contamination on seed used to plant a subsequent crop, but the treatment has no effect on head blight.
- Miravis Ace (Group 3 + 7) at 13.7 fl oz/A. Apply between Feekes 10.3 and 10.5.4. Harvest interval is 7 days for forage and hay. 12-hr reentry.
- Prosaro at 6.5 to 8.2 fl oz/A at early flower (Feekes growth stage 10.5). Do not apply within 30 days of harvest. 12-hr reentry.
- Other Group 3 formulations of propiconazole (Tilt) or prothioconazole (Proline 480 SC) are labeled for wheat head blight suppression only, not control.
References Bai, G., and Shaner, G. 1994. Scab of wheat: prospect for control. Plant Disease 78:760-765.
Hollingsworth, C.R. 2004. Fusarium Head Blight, University of Minnesota, Crookston. 4 pp.
Wiese, M.V. 1987. Compendium of Wheat Diseases, 2nd ed. St. Paul, MN: APS Press.