Graduate Thesis Or Dissertation
 

Sensitivity of wheat genotypes to a toxic fraction produced by Cephalosporium gramineum and correlation with disease susceptibility

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  • Cephalosporium stripe, caused by the soil-borne ascomycete Cephalosporium gramineum, is becoming an increasingly important disease of winter wheat (Triticum aestivum) in several areas of the world, especially where stubble mulch is practiced to maintain soil moisture and prevent erosion. As cultural control of the disease is infeasible and no fungicides are registered, the development of resistant cultivars offers the best hope for disease control. Selection of resistant genotypes remains problematic due to the requirements of evaluating adult plants in variable field environments. The symptoms of cephalosporium stripe suggest the involvement of pathogen-produced toxins, and the toxin called graminin A has previously been isolated from C. gramineum. The goals of this thesis were to determine if insensitivity of wheat genotypes to a toxic fraction produced by C. gramineum is associated with resistance to cephalosporium stripe, and to evaluate the potential use of this toxic fraction to screen wheat genotypes for disease resistance. A method was developed to mass-produce a toxic fraction of C. gramineum by modifying the method of K. Kobayashi (Kobayashi and Ui 1977). Large volumes (9 L) of broth medium were inoculated with C. gramineum and incubated for 35 days. The culture filtrate was then extracted four times with ethyl acetate, which eliminated need for the most time-consuming step of rotary evaporation. Reversed phase fractionation of the crude extract was used to isolate the toxic fraction. Leaves from fourteen-day-old wheat plants were excised and placed in scintillation vials containing the toxic fraction. Exposure to a concentration of(60μl/ml) of the toxic fraction for 72 hours produced distinct wilting symptoms that allowed us to distinguish wheat genotypes in a repeatable manner. Degree of wilting was measured on a continuous 1-5 scale, where 1 = no wilting and 5 = a fully wilted leaf. Twenty wheat genotypes, belonging to four distinct germplasm groups (common, club, durum, and synthetic) were included in three runs of the toxin assay. Wilting reactions for the 20 genotypes were very consistent among runs. Sensitivity of the individual genotypes to the toxic fraction did not differ significantly within germplasm groups, but all differences among the means of the four germplasm groups were highly significant (P<0.001) based on linear contrasts. Seventeen winter wheat genotypes representing the common, club, and durum germplasm groups were planted in infested fields at two locations. The percentage of tillers showing whitehead symptoms (early maturity and reduced grainfill) was significantly correlated with wilting symptoms measured by the toxin assay. The common and durum genotypes varied more for disease susceptibility than for toxin insensitivity. However, all pairwise linear contrasts between means of the whitehead percentages for the three germplasm groups were highly significant (P<0.001). Failure of the toxin assay to identify some of the moderately resistant common wheat genotypes suggests that additional mechanisms of resistance are operative. As no toxin-insensitive genotype showed susceptibility to cephalosporium stripe in the field experiments, however, the toxin assay can be used as an initial screening procedure to reduce the number of genotypes to be tested in the field.
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