Fusarium graminearum is the main pathogen causing scab, or head blight, of wheat. Other species involved, depending on the climate and crops grown in rotation with wheat, are F. culmorum, Microdochium nivale, F. avenaceum, F. poae, and F. sporotrichioides. These pathogens affect grain yield and quality due to their ability to produce mycotoxins. Fusa-rium graminearum and F. culmorum produce the trichothecene mycotoxins, deoxynivalenol and 15-acetyl-deoxynivalenol, and zearalenone in North America, or nivalenol, zearalenone, 3-acetyl-deoxynivalenol, 15-acetyl-deoxynivalenol and fusarenon-X in Asia. Deoxynivale-nol is associated with plant disease aggressiveness and may have been responsible for large-scale human poisonings in the last century in China and India. This toxin also causes vomiting and feed refusal in domestic animals and immunosuppression in mice. The closely related nivalenol toxin is toxic to bone-marrow in experimental animals. Zearalenone, a chemically unrelated compound has estrogenic effects in domestic pigs and experimental animals.
Fusarium head blight negatively affects wheat grain quality due both to lower weight of the affected grain and to the reduction in quality that accompanies mycotoxin contamination, and may result in significant economic losses directly to the farmers. CIMMYT, through funding from special grants from the Government of Japan and other donors, provides a global platform for international collaboration on scab research by facilitating the sharing of knowledge and genetically enhanced wheat germplasm and other breeding materials and tools. This global platform capitalizes on the knowledge accumulated on both host plant resistance and genetic improvement of the wheat crop against scab. For example, DNA markers are being mapped and used to incorporate three different types of resistance to the pathogen. These are: (I) resistance to initial infection or penetration, (II) resistance to fungal spread within plant tissues, and (III), degradation of mycotoxins. Chromosome 2D carries quantitative trait loci (QTLs) for type I and type II resistance, which are in the same region as QTLs for heading date and spike length. Although there are a few markers in this chromosomal region, new DNA markers associated with toxin tolerance are being mapped by in silico expressed sequence tag mapping that takes advantage of the synteny of the short arm of wheat chromosome 2D with that of rice chromosomes 4 and 7 (T. Ban, unpublished).
Scab screening with a spike inoculation test remains complex, unstable and low throughput. Easy, stable assessment methods for wheat breeders are being developed at CIMMYT that use the primary leaf. When a drop of a conidial suspension is placed on the wounded portion (~1 mm in diameter) of a leaf, the pathogen can infect and produce an oval lesion. This assay can distinguish resistant and susceptible cultivars (J. Murakami, unpublished). This new screening method, when coupled with the advances in genetic enhancement, should lead to novel resistance sources that carry genetically characterized R-loci. It also should be possible to assess transgressive R-segregating genotypes that combine distinct resistance genes, with the aim of pyramiding disease resistance genes in locally adapted wheat germplasm.
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