Moses, Emmanuel; (1997) Cassava stem tip dieback: Identification and virulence factors of the causal organism. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

A fungus with morphological features corresponding to the group species Colletotrichum gloeosporioides was isolated from cassava shoots showing dieback symptoms in Ghana. When four locally-grown cultivars were inoculated with isolates of the fungus they developed symptoms which consisted of discrete dark brown lesions on the stems, defoliation and stem dieback. Koch's postulates were completed by re-isolating the fungus from the inoculated plants. Biochemical characteristics of the cassava pathogen were compared with those of four authentic Colletotrichum species, C. gloeosporioid.es, C. acutatum, C. capsici and C. lindemuthianum. All the organisms were able to utilize nitrite except C. lindemuthianum. Aesculin, tributyrin, pectin. Tween 80, inositol, and sorbitol were used by all five organisms. They all also hydrolysed starch, casein, and gelatin, degraded cellulose, and cleaved 4-methylumbelliferone β-D-glucoside, 4-methylumbelliferone β-D-galactoside and methylumbelliferone xyloside. C. capsici differed from the other species in that it was able to degrade lignin. The identity and taxonomic position of the cassava pathogen was confirmed by sequencing Domain 2 of ribosomal DNA and comparing the nucleotide sequences with those of other members of the genus whose identities are well established. Out of 193 nucleotides analysed the cassava isolate differed from authentic C. gloeosporioides isolates by only one, confirming that the cassava pathogen was a form of C. gloeosporioides. Fruits of tomato, banana, and pepper developed lesions within 2 days of inoculation with the cassava pathogen, suggesting that it may have a wide host range. Cells isolated from leaves of cassava were killed by culture filtrates of the fungus. Fractionation by acetone precipitation and partitioning of the supernatant with ethyl acetate showed that the majority of the activity was in the ethyl acetate fraction. When this was separated by flash chromatography on silica gel three compounds were isolated. These shared a prominent peak at m/z 355 as well as several other peaks suggesting that they all belonged to the same family of compounds. The acetone precipitate macerated cucumber mesoderm tissue and had pectate lyase activity.

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