Mechanisms of Pathogenicity of Three Fungi Which Cause Dry Rot in Potato Tubers

Kebabti, Abdelkader (1996) Mechanisms of Pathogenicity of Three Fungi Which Cause Dry Rot in Potato Tubers. PhD thesis, University of Glasgow.

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Four isolates each of F. coeruleum and F. avenaceum and three isolates of Cylindrocarpon destructans were isolated from dry rot infected potato tubers. Pathogenicity tests showed that F. coeruleum and F. avenaceum were strongly pathogenic to potato cvs Maris Piper and Record but C. destructans caused slow spreading lesions only. For all isolates of each species the higher the number of spores inoculated the faster the lesions established and the faster the lesions expanded within the tuber. An analysis of the size of the dry rot lesions produced 24 days after inoculation by each isolate revealed significant differences in pathogenicity between isolates of each species in both cultivars. Isolates Fc1 and Fc2 of F. coeruleum were slightly more pathogenic than the other two isolates of F. coeruleum and isolates Fa1 and Fa2 of F. avenaceum were sUghtly more pathogenic than the other two isolates of F. avenaceum. Although all isolates of C. destructans were only weakly pathogenic Cd2 was the most pathogenic. For aU three fimgi cv Maris Piper was shghtly more susceptible than cv Record. Small but significant differences were found between isolates of each species in spore germmation on the surface of tuber slices. Differences in the rates of germ tube growth between different isolates were significant for F. coeruleum and F. avenaceum, but not for C. destructans. The rates of lesion development into tuber slices were about the same for all isolates of F. coeruleum and for all isolates of F. avenaceum, but a significant difference was found between the isolates of C destructans. The differences between the three species were supported by an analysis of chitin on the surface and m the internal tissues of the sHces. Much larger amounts of chitin were present on the external surfaces of the slices than in the intemal tissues for all three species and much more chitin was found with F. avenaceum and F. coeruleum than with C. destructans An electron microscopic study of the colonisation of tuber tissues by both F. coeruleum and F. avenaceum showed that the middle lamellar region of the cell wall was relatively resistant to degradation compared to the primary wall and remained intact for a considerable time after the tissue was infected. The resistance of the middle lamellar region to degradation explains the dry-rot nature of the lesions caused by Fusarium species. All three species produced pectin and pectate degrading enzymes during the early stages of growth in tuber tissue medium and in living tuber tissue. The activities of pectin lyases and hydrolases were higher than those of pectate lyases and hydrolases, and lyase activdties were greater than hydrolase activities. Both endo and exopectic lyase and hydrolase activities were found. In F. coeruleum, enzyme production was found to be strongly inducible by substrate and inhibited by simple sugars. Strangely the pectic enzymes induced in synthetic media, containing pectin or polypectate substrates and in potato broth medium did not macerate potato tuber tissue. However, macerating activity was found in extracts from autoclaved tuber tissue and from living tuber tissue. It is probably that the main enzymes responsible for maceration of tuber tissue were enzymes other that pectinases, such as glucanases, or any of a number of other polysaccharide degrading enzymes such as galactanases, xylanases, arabanases or cellulases. All three species produced toxic substances when growing in tuber tissue mediiun and in living tuber tissue. The activity of the toxic substances produced by F. avenaceum and C destructans in living tuber tissue, or in autoclaved tuber tissue was only slightly reduced by boiling but the activity of that produced by F. coeruleum in living tuber tissue was almost conletely destroyed. Clearly the toxins produced in living tuber tissue by F. avenaceum and C. destructans are different from those produced by F. coeruleum. F. ceoruleum did produce heat stable toxins when grown in Baker's medium containing glucose as the sole carbon source but not when grown in Czapek-Dox medium with single sugars or pectic substances as carbon sources and nor when grown in potato broth, but these toxins are likely to be artefacts of the culture condition since heat stable toxins were not found in other substrates including living tuber tissue. The results of this study suggest that the isolates of all three species cause dry rot m potato tuber tissue by a complex interaction of various factors, including the ability to produce multiple forms of a range of cell wall degrading enzymes and toxins but that the mechanisms of pathogenicity of each fungus are different.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: D D Clarke
Keywords: Microbiology, Plant pathology
Date of Award: 1996
Depositing User: Enlighten Team
Unique ID: glathesis:1996-75699
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Dec 2019 09:15
Last Modified: 19 Dec 2019 09:15

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