Fusarium ear blight
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Effect on US economy
From an economic standpoint, it is one of the major cereal diseases, being responsible for significant grain yield reduction in wheat and oats. From 1998-2000 the Midwestern United States suffered $2.7 billion in losses following a Fusarium head blight epidemic. .
Fusarium Head Blight emerged in the past decade as a widespread and powerful enemy of American agriculture. This disease, also known as ‘Scab’, inflicts yield and quality losses on farms in at least 18 states. Food industries throughout the U.S. incur losses from the cost of dealing with the toxin-contaminated grain that often accompanies scab infection. Combined losses to all steps in the food system are difficult to estimate, but the bill at the farm-gate alone is estimated to exceed 9.0 billion dollars since 1990.
US Wheat and Barley Scab Initiative
The goal of the U.S. Wheat and Barley Scab Initiative (USWBSI) is to develop as quickly as possible effective control measures that minimize the threat of Fusarium head blight (scab), including the reduction of mycotoxins, to the producers, processors, and consumers of wheat and barley.
Since 1997, federal, state, and private sector scientists have worked closely with growers, input providers, millers, and food processors from across the country to design and fund just such a system. The result of these efforts is the U.S. Wheat and Barley Scab Initiative (USWBSI). In one sense, the Initiative is a self-organized "contact group" on scab. The goal of the U.S. Wheat and Barley Scab Initiative is to develop as quickly as possible effective control measures that minimize the threat of Fusarium head blight (scab) to the producers, processors, and consumers of wheat and barley. The USWBSI is guided by a Steering Committee that includes growers, farm organizations, food processors (e.g., millers, bakers, pasta manufacturers, and brewers), scientists (from Land Grant universities, USDA, and private companies), and consumer groups. Nine members of the Steering Committee also serve on an Executive Committee. The Steering and Executive committees are advised by a series of six research committees composed of recruited volunteers from the scientific leaders of the U.S. wheat and barley research communities. The Initiative’s Networking and Facilitation Office, based at Michigan State University, facilitates the work of these committees. That office also promotes communication among the varied parties interested in scab, and represents the Initiative’s only ‘fixed’ asset.
Effect in human and animal health
Many ear species (including F. graminearum) produce mycotoxins—fungal chemicals that are harmful to animals. These chemicals may operate in nature to disable plant defense mechanisms or to defend the fungus against other microorganisms. The major toxin produced by F. graminearum in association with FHB in wheat and barley is deoxynivalenol (DON). DON is sometimes called vomitoxin because of its deleterious effects on the digestive system of swine and other monogastric animals. DON disrupts normal cell function by inhibiting protein synthesis. Humans consuming flour made from wheat contaminated with DON will often demonstrate symptoms of nausea, fever, headaches, and vomiting.
DON contamination is measured in parts per million (ppm). DON levels in FHB-infected wheat are frequently quite high (>20 ppm). The USDA recommends that DON levels in human foods not exceed 1 ppm. However, individual grain buyers may have lower tolerances of DON in purchased grain. The FDA has various guideline levels of DON permissible in livestock feed: ruminant animals, such as feeder cattle, are the most tolerant, while swine have the highest sensitivity to DON in livestock feed, with pigs refusing feed containing 1ppm of DON.
The mycotoxins produced by F. graminearum may pose a serious threat to human and domestic animal health. Grain that has been infected with the fungus may become incorporated into our staple diets. Strains of the fungus from different countries produce different toxins, some potentially more potent and dangerous than those from strains currently in the United States.
The Fusarium ear blight is due to a Fusarium fungus. There are five major species of Fusarium:
- Fusarium avenaceum (Gibberella avenacea)
- Fusarium culmorum
- Fusarium graminearum (Gibberella zeae)
- Fusarium poae
- Microdochium nivale (Monographella nivalis, formerly Fusarium nivale)
The aggressive form of headblight is caused by Fusarium graminearum. Fusarium graminearum is an ascomycete, producing sexual spores in a sac known as an ascus (plural asci). The asexual stage of the fungus produces spores called macroconidia, and the sexual stage produces spores called ascospores.
In wheat, Fusarium infects the head (hence the name “Fusarium head blight”) and causes the kernels to shrivel up and become chalky white. Additionally, the fungus can produce mycotoxins that further reduce the quality of the kernel.
Infected florets (especially the outer glumes) become slightly darkened and oily in appearance (29). Conidiospores are produced in sporodochia, which gives the spike a bright pinkish color (30). Infected kernels may be permeated with mycelia and the surface of the florets totally covered by white, matted mycelia.
(A) Resistant cultivars Since 1990, an extensive research endeavor has focused on development and use of resistant cereal cultivars and integrated pest management systems for the control of Fusarium head blight. Thousands of plant lines are subjected to artificial inoculation with F. graminearum. Those lines having reduced fungal growth and low levels of seed contamination with the mycotoxin DON are selected and advanced in additional breeding trials.
To date, sources of resistance conferring complete resistance to FHB have not been identified in wheat. Quantitative Trait Loci (QTL) composed of one or more genes, such as Fhb1 derived from the Chinese wheat cultivar Sumai 3, have been identified in wheat. However, these genes confer only partial resistance to FHB, and many of the initial sources of resistance were not well adapted to most of the grain production regions of the U.S. While some success has been made in transferring FHB resistance from such exotic sources into adapted cultivars, identification and deployment of FHB resistance already present in local native germplasm and cultivars is providing another means to achieve this goal. Ultimately, control of FHB, to meet the very low DON limits in wheat grain, will require an integrated approach including development of cultivars having multiple resistance genes and use of fungicides.
Fusarium head blight resistance is a complex trait,. Two to five major genes, plus several minor genes, have been reported from various sources of FHB resistance (). QTLs for FHB resistance have been mapped to almost all wheat chromosomes when different mapping populations were investigated. In Sumai 3, QTLs for FHB resistance have been identified on 3BS, 5AS, 6AS, 6BS, and 3BSc, a QTL region proximal to the centromere on 3BS,. In Wuhan 1, the QTLs for FHB resistance were mapped to 2DL and 4B, QTLs on other chromosomes were also reported including those on chromosomes 2A, 2B, 3A, 3B, 5D, 6D, 5B, 4A, 1B, and 7A in wheat germplasm from Europe, Brazil, and Asia. But only the QTL on 3BS from the Chinese cultivar Sumai 3 consistently showed a major effect on Type II resistance across different genetic backgrounds and environments. Other QTLs for FHB resistance exhibited a minor effect, and their expression varied significantly with genetic backgrounds and the environments where the disease was evaluated. Therefore, Sumai 3 has been extensively used as a major source of resistance to FHB in breeding programs worldwide. However, heavy use of narrow FHB resistance sources may increase selection pressure on the pathogens to wear away the effectiveness of the resistance genes involved. New FHB resistance germplasm are desired to broaden the genetic diversity of FHB resistant sources and improve the level of wheat resistance to FHB.
(B) Agronomic management practices Crop sequence (what crops were planted and when) and tillage (soil incorporation of crop residues) have been shown to affect the incidence of FHB. In recent years, decreases in tillage are thought to have contributed to regional scab epidemics by increasing levels of inoculum available for infection. Since the risk of FHB depends on a viable inoculum source, the management of cereal debris on the soil surface may or may not impact the level of FHB. The relative contribution of inoculum from local and distant sources is not yet fully understood. In regions where there is a significant source of airborne inoculum, local management of the disease (on a single farm) may not be effective
(B) Chemical control Chemical controls, such as fungicides, provide partial control of FHB and associated mycotoxin contamination. A number of foliar fungicides have been used to manage FHB in some areas and are applied around the period of wheat flowering. In many areas, fungicides are rarely used for FHB control because of high cost, variable efficacy, and the erratic nature of FHB epidemics. Research continues to identify fungicides that are more effective for the control of FHB.
(c) Biological control Several investigators are focused on finding affordable and environmentally compatible biocontrol agents for the management of FHB. Biocontrol agents could play an important role in organic cereal production. In conventional production, such agents may extend protection of spikes past the flowering stage after fungicides can no longer be applied. Certain strains of spore- producing bacteria (such as Bacillus species) and yeasts (such as Cryptococcus flavescens) show some promise for the control of FHB and the reduction of mycotoxin contamination, however there effectiveness at field level is not well known.
(d) Integrated management Integrated management of FHB may one day be achieved by the combined application of biocontrol agents and fungicides to flowering wheat and barley varieties with partial resistance. Disease forecasting models may help to optimize FHB management by targeting fungicide and biocontrol applications.The online Fusarium Head Blight Risk Assessment Tool (http://www.wheatscab.psu.edu/riskTool_2010.html) may be used to gauge the relative risk of FHB in wheat fields in the U.S. A public outreach program called Scab Smart provides U.S. wheat and barley growers with the latest information on integrated management tools that can be applied in their section of the country. Disease Forecasting models help producers determine the risk of FHB infection at the flowering period of wheat, and thus help optimize FHB management by having fungicides applied only when models indicate that the risk of FHB infection is present, based on current weather parameters. The spring wheat model also has the option for the producer to choose the level of resistance present in the wheat variety grown. Level of resistance affects the FHB risk.
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Return of an old problem: Fusarium head blight of small grains
Fusarium head blight in Canada
United States Wheat and Barley Scab Initiative
Fusarium Head Blight Risk Assessment Tool