Fusarium head blight (FHB) or head scab, caused by the fungus Fusarium graminearum and related species, is one of the most important diseases of wheat, oats, barley, spelt and other small grain crops. However, scab is not only a disease of small grains, it also affects many other grass species including foxtail, quackgrass, crabgrass and bluegrass. In addition, scab is not the only disease caused by F. graminearum. If scabby wheat seeds are planted, seedling blight may develop, plus the scab fungi also cause root and crown rot diseases in small grains; ear, stalk and root rot of corn; and seed rot and damping-off of soybean.
|Fig. 1. Wheat spike at the flowering growth stage. Note the extruded anthers.|
The severity of scab infection varies greatly from year to year. Infection occurs primarily during the flowering stage (Fig. 1) and shortly afterward when wet, humid weather prevails. Two to three days of rainfall or extended periods of high relative humidity during flowering and early grain-fill favor infection. If the weather is dry during this critical period, the crop will likely be scab-free. In Ohio, during years of favorable weather, the incidence of infected heads may be as high as 100 percent in some fields. In these cases over 50% or more of the spikelets may become damages. Scab also causes floret sterility, poor test weights due to shriveled grain and yield loss. In general, oats are less susceptible to scab than wheat and barley.
Scab is important not only because it reduces yield, it also reduces the milling and baking quality of wheat flour, malting quality of barley, and feeding value of grain. The fungi causing scab produce mycotoxins, most notably vomitoxin (deoxynivalenol or DON), during colonization. Vomitoxin may accumulate at high levels in the harvested grain and may cause vomiting and feed refusal if scabby grain is consumed by livestock. Pigs are particularly sensitive while ruminants are more tolerant to vomitoxin. Levels of vomitoxin exceeding 1 ppm and 5 ppm (parts per million) are unfit for human and livestock consumption, respectively. Wheat with vomitoxin greater than 2 ppm may lead to price discounts or rejection at grain elevators. There is a near zero tolerance for vomitoxin in malt barley (i.e. barley used for making beer).
|Fig. 2. A, Wheat spike showing bleached florets affected by scab; B, scab symptoms on barley spike. Insert showing close-up of discolored spikelet (courtesy A. Friskop).|
Symptoms and Signs
The earliest and most conspicuous symptom of scab occurs soon after flowering. Diseased spikelets turn light-straw colored and have a bleached appearance due to premature death of tissues. Healthy spikelets on the same head retain their normal green color (Fig. 2A). One or more spikelets may be bleached, or the entire head may be diseased. When the fungus infects the stem immediately below the head the entire head may die. Characteristic scab symptoms on wheat usually develop about 18 to 21 day after flowering, the growth stage at which infection typically occurs, but may be seen earlier if conditions are consistently wet and humid during the days after flowering. Diseased spikelets of oats are ash-grey and those of barley are light brown (Fig. 2B).
Within 7 to 10 days after symptom development pinkish to salmon-colored spore masses and mycelium (called sporodochia) may form on the margin of the glumes of individual spikelets, especially near the base of the spikelet (Fig. 3A). The pink spore masses are easiest to see early in the morning before the dew dries. Infected kernels are generally shrunken, wrinkled, and light-weight, with a rough, scabby appearance (Fig. 3B). These kernels range in color from light-brown to pink to grayish white. The extent of shriveling and discoloration of the kernels depends on when and where infections occur and the weather conditions following infection. If the fungus invades and kills the rachis or main axis of the spike, the spikelets above that point die, even if they are not colonized by the fungus. The result is no grain at all or small, shriveled kernels that are lost during the threshing process. Towards the end of the season, heads with diseased spikelets may become speckled with dark purplish-black fruiting bodies (perithecia) of the fungus if the weather remains cool and moist (Fig. 4). These perithecia are signs of the sexual stage and the overwintering structures of the fungus.
Fig. 3. A, Pink to salmon-colored spore mass at the base of spikelet and B, Shriveled, wrinkled, light- weight, and discolor kernels from diseased heads
Disease Cycle and Epidemiology
The fungi causing scab are all members of the genus Fusarium, the asexual spore forming stage. In Ohio, the principal pathogen is Fusarium graminearum (the sexual stage is Gibberella zeae), but F. avenaceum (G. avenaceum) and F. culmorum (no known sexual state) have also been reported. These fungi overwinter and survive between crops in infected grain and grass stubble, chaff, and cornstalk residue left on the soil surface. They survive as asexual spores (conidia), mycelium, and perithecia within which are borne the sexual spores (ascospores). These fungi continue to grow and produce spores from harvest until the residues decompose in the soil.
|Fig. 4. Dark purplish-black fruiting bodies (perithecia) of the fungus on mature wheat heads.|
Conidia are produced profusely during warm, moist weather on corn and small grain residues. Ascospores produced within perithecia are forcibly discharged and carried by air currents to the flowering spikelets where infections occur. Ascospores and conidia may also be splash-dispersed to spike from in-field crop residue. Both spore types germinate in free water on the surface of the spikelet and invade the flower. Infections are most serious when the anthers are exposed during flowering. Early symptoms may develop in as little as three days after infection when temperatures range from 77 to 85 degrees F (25-30 degrees C) and humidity is high. Conidia from sporodochia on diseased spikes can be blown by wind to flowering heads in neighboring fields or splash-dispersed to heads on late-developing tillers in the same field where new (secondary) infections occur. This process may continue as long as the spikelets are susceptible and moist weather conditions prevail, leading to secondary spread within and across fields. Primary and secondary infections may all result from long distance spread of air borne conidia and ascospores. In most years, ascospores and conidia are produced on the heads too late in the season to cause secondary infections on the same heads or even in the same field. However, spore-bearing structures usually persist in crop residues and contaminate seeds, serving as primary inoculum for the following year’s crop.
Scabby kernels may be dead, or else germinate weakly. If the seedling manages to emerge from the soil, it frequently dies before it becomes established. Seedling blight is first noticed when infected plants appear light to reddish brown in color and may be covered with a mass of pink or whitish mold. If seedlings survive, they generally lack vigor, and frequently send up only a few tillers with small heads. A light to reddish brown root or crown rot may develop as the weakened plants mature.
Management of Fusarium Head Blight
- Plant moderately resistant varieties. There are no varieties of wheat, oats or barley varieties with very highly level of resistant to scab, but there are a few that can be considered moderately resistant. On these varieties the development of the disease may be restricted to one, or only a few, florets per head. Compared to susceptible varieties, moderately resistant varieties usually have up to 50% less scab and vomitoxin. In the field, some varieties appear more resistant than others because they flower earlier or later. These varieties may look resistant because they escape infection-favorable conditions during anthesis. Differences in susceptibility may also be due to physical barriers to infection of spikelets. Visit the web page of the Ohio Wheat Performance trial web page for a list of varieties with moderate scab resistance.
- Plant cereals as far away as possible from old corn fields if stalk residues are left on the soil surface. No-till wheat seeded in old corn residues greatly increases the chance of scab. If conventional tillage is used, clean, deep plowing of all infested stubble and straw of cereals and weed grasses, corn stalks and rotted ears is recommended. Complete coverage of crop residues reduces head blight infection by reducing inoculum levels. Manure containing infested straw or corn stalks may harbor the fungus and should not be put on fields planted to small grains. When possible, plant wheat following a legume crop (soybean) and maintain a rotation with 2 to 3 years between wheat crops. Although soybean is also affected by F. graminearum it is still better to plant wheat after soybean than after another small grain crop or corn.
- There are a few fungicides with good efficacy against scab. There should be applied at flowering (Fig. 1) in wheat and at heading in barley and in combination with moderate resistance. When applied to susceptible varieties, the best fungicides provide about 50% control of scab and 45% reduction of vomitoxin. However, for both wheat and barley, percent scab and toxin reduction, when compared to an untreated susceptible variety, increases substantially (60–75%) when a fungicide is combined with moderate resistance. If rain or some other factor prevents application from being made at flowering, applying the fungicide up to 6 days after flowering may still provide good scab and vomitoxin control. A scab prediction/forecasting system (www.wheatscab.psu.edu) is available for use as a guide to determine if the risk of scab is high enough to warrant a fungicide application. Consult your extension educator and specialist for more on scab management.
Managing Seedling Blight
- Avoid using grain from scab-affected fields for seed. However, if scabby grain is used, thoroughly clean the seed to remove lightweight kernel. Seed treatment fungicides are available that will improve germination and control seedling blights. However, seed treatment will not control head scab, foot rot or crown rot resulting from Fusarium infection.
- Delay planting of winter cereals until the soil temperature is 60 degrees F (16 degrees C) or below to reduce chances of seedling blight. Planting should always be after the Hessian fly free date. Spring seeded grains should be sown as early as possible when soil temperatures are relatively cool.
- Sow seed in a well prepared seed bed with adequate fertilizer to ensure good root growth and vigorous plant development.
- Rotate small grains and corn with legumes, and leave at least one year between cereals, grass or corn before planting small grains.
Managing Mycotoxin Contamination
- When a high percentage of heads in a field are affected by scab, precautions should be taken to reduce mycotoxin contamination of the harvested grain. Mycotoxin contamination is usually highest in the more heavily diseased kernels. Adjusting the combine to blow out the smaller, shriveled kernels will help reduce mycotoxin levels.
- Dry harvested grain to 5 percent moisture as soon as possible after harvesting. The Fusarium fungi cannot grow or produce mycotoxins in grain at this moisture level.
- Harvest and store suspect grain by itself and do not mix with good quality grain. Mixing contaminated grain with good grain will only yield a poor product that may be difficult to sell.
- Have suspect grain analyzed by a laboratory for contamination with mycotoxins, especially for deoxynivalenol (DON), before feeding to livestock.