الفهرس 
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Abstract
The high frequencies of Fusarium oxysporum and F. solani and their ability to cause considerable losses during seedling stage, strongly suggest that they are the most important pathogenic Fusarium spp. involved in the etiology of seedling blight and root rot of flax. Of seventy-three nonpathogenic isolates of Fusarium spp., twenty-seven (37%) showed antagonism (cross protection) against a highly pathogenic isolate of F. oxysporum. Flax cultivars and pathogenic isolates played a determinant role in the efficiency of cross protection. There was no association between susceptibility of cultivars to the pathogenic isolates and their response to the nonpathogenic isolates. On the other hand, cross protectionwas evidently more effective against the highly pathogenic isolates. Low concentrations of a nonpathogenic isolate resulted in the highest protection wheras high concentration did not result in protection. Thepathogenicity of 12 isolates of Fusarium spp. (8 isolates of F. oxysporum and 4 isolates of F. solani) was evaluated on 20 falx cultivars under greenhouse conditions. Analysis of variance (ANOVA) showed very highly significant (p=0.000) effects of cultivar, isolate, and cultivar x isolate interaction for preemergence damping-off, postemergence damping-off, and survival. In the case of dry weight, isolate was the only significant source of variation (p=0.000). Statistically significant interaction between flax cultivars and Fusarium isolates suggests that physiologic specialization exists within F. oxysporum and F. solani pathogenic on flax. AVOVA also implies that resistance of the tested cultivars is a mixture of both vertical and horizontal resistance and there are significant differences among cultivars in both types of resistance. Similarly, pathogenicity of the tested isolates is also a mixture of virulence and aggressiveness, and the isolates significantly differ in both types of pathogenicity. Cluster analysis divided the isolates into groups, based on their virulence on 20 flax cultivars; however, grouping the isolates was not related to their geographic origin nor the species, cluster analysis was also used to group the cultivars based on their vertical resistance to 12 isolates of Fusarium isolates. Cluster analysis placed the American cultivars and the Egyptian cultivars in two distinctly separated unrelated subclusters; however, the Egyptian cultivar Giza 7 was a notable exception because it was placed with the American cultivars. The Dutch cultivars showed a peculiar behavior because two of them were placed with the American cultivars and one placed with the Egyptian cultivars. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of proteins and polyacrylamide gel electrophoresis (PAGE) of isozymes (malte dehydrogenase, peroxidase, and esterase) were employed as biochemical tools to differentiate between pathogenic and nonpathogenic isolates within F.oxysporum and F. solani. Proteins and esterase isozymes were able to differentiate between pathogenic and nonpathogenic isolates. The application of double diffusion (DD) technique, by using the antisera of the tester isolates of F.oxysporum no.1 and F.solani no 14, was not a reliable method to differentiate between pathogenic and nonpathogenic isolates within F.oxysporum and F.solani. however, the antiserum of F.moniliforme isolate 18 was partially successful in differentiation between the nonpathogenic F.oxysporum no.10 and some of the pathogenic isolates of F.oxysporum.
Keywords: Flax, Fusarium spp., pathogenicity, cross protection, electrophoresis of proteins, and isozymes, and double diffusion.