No. 13/June 20, 1997
Scab of Wheat
Wheat scab is beginning to show up in many fields in southern and south central Illinois. Although at a low level, this disease has prompted some calls about the appearance of white heads in the field. Scabbed heads are commonly found scattered throughout a field; and the numbers will vary, depending on the genetic resistance of the plant and the time of flowering. Because wheat scab infects only the open flowers when weather conditions favor the disease, it is quite common to see differences among wheat varieties planted on the same farm if the flowering times are different.
Scab (also known as head blight, pink mold, or white-heads) is caused by several fungi in the genus Fusarium. The principal pathogens are Fusarium graminearum, F. avenaceum, and F. culmorum (synonymous name for all three is F. roseum f. sp. cerealis); and F. nivale. These fungi produce only asexual spores with limited genetic diversity. The sexual stage is known as Gibberella zeae.
Scab infects all small-grain cereal crops, corn, and many other grasses, including spelt, emmer, cheat, wildrye, several foxtails, quackgrass, crabgrass, and bluegrasses. Other plants that may be attacked include clovers, alfalfa, pokeweed, sweet potato, and several members of the parsley family. In Illinois, the fungi that cause scab and related diseases are most widespread and damaging on wheat, barley, rye, and corn. Scab can be severe in wheat sown in the residue from a previous host crop such as corn.
The Fusarium fungi may cause a blast of the spikelets (usually called head blight or scab); a rot of the root, crown, or foot; and stem blight. The fungi also cause a seedling blight and a rot of the stalk, root, and ears on corn. The stand, yield, quality, and feeding value of the grain may all be seriously affected by scab. The disease has been reported from most of the humid and semihumid areas of the world where cereals and corn are grown. The severity of scab infection on cereal grains varies greatly from year to year. Severe infection occurs during the flowering stage and shortly afterward, when the weather is warm and moist. If the weather is dry after the head emerges, small grains are nearly scab-free.
Symptoms. In the field, the earliest and most conspicuous symptom of scab infection occurs soon after flowering. Diseased wheat spikelets become a bleached, light-straw color and ripen prematurely, while healthy spikelets are still a normal green. Diseased wheat kernels are grayish brown and lightweight. Scabinfected rye kernels are dark brown to carmine red. If the entire head is infected, barley spikes are dwarfed; diseased spikelets are compressed rather than spreading. Scab-infected spikelets of oats are ash gray; those of barley, light brown. One or more spikelets may be infected, or the entire head may appear prematurely bleached. The dead "whiteheads," which are sterile or contain only partially filled seed, are often rapidly colonized by secondary black molds.
During warm and moist weather, masses of light pink to salmoncolored mold (composed of mycelium and "summer" spores or conidia of the fungi) may form on infected glumes and lemmas of the spikelets, especially near the base of the kernel. This is an excellent diagnostic sign. Infected kernels are generally shrunken, wrinkled, and light in weight, with a rough and flaky-to-scabby appearance. These kernels range in color from light brown to pink or grayish white, depending on the time of infection and the weather conditions.
Occasionally, the fungus may girdle and kill the spike or rachis of a grain head, causing the parts beyond to die. The result is no grain at all or small, shriveled kernels that are separated out in the process of harvesting or threshing. If the weather remains warm and moist, spikelets on heads infected early become speckled by harvest time with superficial, spore-producing bodies (perithecia) that are a dark purplish black.
Disease cycle. The scab fungi overseason and propagate in and on the soil as spores, mycelium (threadlike mold growth), and fruiting structures known as perithecia, which produce ascospores. Light pink to orangecolored masses of conidia or "summer" spores are produced in tremendous numbers during warm, moist weather. Ascospores (often called "winter" spores) produced within the perithecia are discharged into the air during warm, moist weather in the spring and early summer. Air currents and rainsplash carry the ascospores and conidia to the young spikelets. The spores germinate in a film of moisture and first invade the flower parts, frequently spreading to the glumes and other parts of the head. Infections are most frequent and serious at anthesis. At this time, anthers and pollen may serve as a food base for pathogens. Blight symptoms develop within 3 days after infection, when temperatures range between 77 and 86 degrees F and moisture is continuous. In areas that otherwise are too dry for scab development, sprinkler irrigation may predispose plants to the disease.
Conidia and ascospores may also fall on crop residues. These spores soon germinate, resulting in mycelium, which produces large numbers of conidia. Both the ascospores and conidia are capable of producing infection if they land on the head of a cereal or grass plant during or shortly after the blossoming period. Within 7 to 10 days after infection, salmon pink masses of conidia form at the base of the diseased spikelets. These conidia are carried by the wind to the heads of other cereal and grass plants, in turn producing new, secondary infections. The process is repeated as long as the spikelets are susceptible and the moist weather prevails.
Secondary infections may result from the long-distance spread of airborne conidia. Ascospores are usually produced too late in the season to function as secondary inoculum but persist in crop residue and can contaminate seed.
Seedling infections result primarily from seedborne mycelium and spores. When the soil temperature is above 60 degrees F, seedlings from clean seed may become infected from mycelium in decaying crop residues on or in the soil. Head and stem infections occur independently of seedling blight and root rot (because the fungi cannot grow for any distance within the cereal plant). However, the fungi may live for several years within infected kernels.
H. Walker Kirby, Associate Professor, Department of Crop Sciences, (217)333-8414