wheat scab fusarium head blight

 Fusarium head blight (FHB), caused by the boletus Fusarium graminearum and connected species, is one of the greater important diseases of wheat, oats, barley, spelt and other small crops of grain. However, a scab is not just a disease of small grains. It also affects many other types of grasses, including foxtail, quackgrass, crabgrass and bluegrass. In addition, scabies is not the only disease caused by F. graminearum. If the wheat seed is planted, seedling blight can occur, as well as scab fungi that also reason root and crown rot diseases in small crops; ear, stalk and root rot of corn; and seed rot and wilting of soybeans.

Wheat crops during the flowering period. Note the extruded anthers.

The severity of yeast infections varies greatly from year to year. Infection occurs mainly during flowering and shortly after when the weather is wet and humid. Two to three days of rain or high humidity during flowering and early grain infection. If the weather is dry during this troublesome period, it is possible that the crops will not be white. In Ohio, during years of good weather, infected heads may be as high as 100 per cent in some areas. In these cases, more than 50% or more of the spikelets can become damaged. Scab also causes floret sterility, low test weight due to dry grain and yield loss. In general, sorghum is less susceptible than wheat and barley.

Rust is important not only because it reduces yield but also the milling and baking quality of wheat flour, the germination quality of barley, and the nutritional value of the grain. Mould-causing fungi produce mycotoxins, particularly vomitoxin (deoxynivalenol or DON), during colonization. Vomitoxin can accumulate to high levels in harvested grain and can cause vomiting and regurgitation when the white grain is consumed by livestock. Pigs are very sensitive when ruminants tolerate vomiting. Levels of vomitoxin greater than 1 ppm and 5 ppm (parts per million) are unsuitable for human and livestock consumption, respectively. Wheat with vomitoxin greater than 2 ppm can lead to a price reduction or rejection in grain elevators. There is zero tolerance for vomitoxin in malted barley (i.e. the barley used to make beer).

Signs and Symptoms

The first and most obvious sign of whiteness appears immediately after firing. Diseased spikelets become lighter in colour and bleached due to premature cell death. Healthy spikelets on one head retain their normal green colour (Figure 2A). One or more spots may turn white, or the entire head may be affected. The entire head can die when the fungus infects the stem just below the head. Tuberculosis symptoms on wheat usually appear about 18 to 21 days after budding, a growth stage in which infection is common but can be seen earlier if conditions remain wet and moisture in the days after flowering. Diseased spikelets of oats are grey, and those of barley are light brown.

Within 7 to 10 days after the symptoms develop, pink to salmon-coloured masses of spores and mycelium (called sporodochia) may form on the edge of the buds. Of individual spikelets, particularly near the base of the spikelet. Most pink spores are easily seen in the morning before the dew dries. Infected fruits are usually shrivelled, wrinkled, and light-weight, with a rough, scabby appearance. These fruits range in colour from light brown to pink to bluish-white. The degree of the kernels' browning depends on when and where the diseases occur and the weather conditions after infection. If the fungus attacks and kills the rachis or principal axis of the spike, the spikelets upon that point die, even if the fungus does not colonize them. The result is that absolutely no grains or small dry grains are lost during threshing. Towards the end of the season, the heads with diseased spikelets may be spots and dark fruits of the fungus (perithecia) of the fungus if the weather is always cool and humid. These perithecia are signs of the sexual ways and structures of overwintering fungi.

Management of Fusarium head blight

1. Plant moderately resistant varieties. There are no wheat, oats or barley varieties that have a very high level of resistance to mildew, but several can be considered moderately resistant. In these species, the development of this disease can be limited to one or only a few flowers per head. Unlike susceptible strains, moderately resistant strains typically have 50% less toxin and vomitoxin. Some varieties seem more resistant in the field because they bloom earlier or later. These species may appear resistant because they escape the acceptable conditions of infection during anthesis. The difference in susceptibility may be due to physical barriers to the infection of the spikelets. Visit the Ohio Wheat Performance test page for a list of moderately resistant varieties.
2. Sow grain as far as possible from old cornfields if stubble is left on the ground. Uncultivated wheat seed in old corn residue increases the chance of becoming white. If normal soil is used, clean, deep cultivation of all infested straw and grain grass and weed grass, corn stalks and rotten ears. Complete covering of crop residues reduces head lice infection by reducing inoculum levels. Manures containing straw or corn stalks may harbour fungus and should not be applied to small grain fields. Plant wheat following a soybean crop and maintain a rotation of 2 to 3 years between wheat crops. Although soybeans are also affected by F. graminearum, it is still more delicious to plant wheat after soybeans than other small grains or corn.
3. Several fungicides have good effects against the fungus. It should be used during flowering with wheat and when there is a head of barley with moderate resistance. When applied to susceptible species, the best fungicides provide about 50% control of mildew and a 45% reduction in vomitoxin. However, for wheat and barley, the percentage reduction of the toxic weed, compared to the non-treated type, increases significantly (60-75%) when the herbicide is combined with the resistance to the moderate method. If rain or other factors prevent fungicide application until 6 days after flowering, it may provide good control of downy mildew. A powdery mildew prediction system (www.wheatscab.psu.edu) is available for use as a guide to determine whether powdery mildew risk is high enough to warrant a fungicide application. Contact your extension educator and specialist to learn more about whitefly management.