Wheat scab or fusarium head blight

Wheat scab or Fusarium head blight (FHB) is a devastating disease for wheat production worldwide, the causal organism is Fusarium graminearum Schevabe [(telomorph: Giberella zeae Schw. (Petch)].



Arthur, in 1891, reported that a wheat field which was expected to yield 35–40 bushels/acre yielded only 8 bushels/ acre in 1890, a season in which there was a severe epidemic of Fusarium head blight (FHB). The damage attributed to FHB has been well documented periodically throughout the past 120yr. The disease has frequently caused low to severe wheat crop losses in the United States, and with increased frequency and severity coinciding with the recent widespread adoption of reduced soil tillage for purposes of soil conservation and reduced input costs of crop production.



The widespread FHB (scab) epidemic causing extensive damage in the wheat and barley production areas of the Northern Great Plains of the U.S. is well documented. In 1993, scab in spring wheat caused losses of approximately $80 million in South Dakota alone. It is a floral-infecting disease caused by the fungus Fusarium species, with Fusarium graminearum Schwabe, telomorph Gibberella zeae (Schw.) Petch, as the predominant causal organism in the U.S. Infected wheat florets and spikelets are often destroyed. The fungus readily colonizes florets, spreading through the rachis to adjacent spikelets. The fungus produces mycotoxins, including deoxynivalenol (DON), causing Fusarium infected grains to be toxic to animals and humans. Deoxynivalenol (DON) has been linked to livestock feed refusal and depression of the immune system, nausea, and vomiting in humans. Concerns over food safety have led the FDA to impose a strict 1 µg g-1 (1 ppm) standard for finished wheat food products.



Symptoms

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. One or more spikelets may be infected, or the entire head may be diseased. When the fungus infects the stem immediately below the head the entire head may die. Infected spikelets of oats are ash-grey and those of barley are light brown.


Several days after infection masses of pink to salmon-colored spores and mycelium may form on the margin of the glumes of individual spikelets, especially near the base of the kernel. The pink spore masses are easiest to see early in the morning before the dew dries. Infected kernels are generally shrunken, wrinkled, and light in weight, with a rough, scabby appearance. These kernels range in color from light-brown to pink to grayish white. The extent of shriveling and discoloration of the kernels depends on the time of infection 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. The result is no grain at all or small, shriveled kernels that are lost during the threshing process. Heads with diseased spikelets may become speckled with dark purplish-black fruiting bodies (perithecia) of the fungus if the weather remains cool and moist until harvest. These perithecia are a sign of the sexual stage, the Gibberella stage of the fungus.



Mangement

1. There are few varieties of wheat, oats or barley highly resistant to scab, but in greenhouse tests some varieties restrict the development of the disease to one, or only a few, florets per head. In the field, some varieties appear more resistant than others because they flower earlier or later than other varieties, or because they shed their anthers more quickly than other varieties. These varieties look resistant because they have escaped infection by avoiding rains that supply free water on the surface of the heads for germination of the spores. Differences in susceptibility may also be due to physical barriers to infection of spikelets.


2. 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.