Studies on some aspects of the biology of Cercospora Henningsii Allesch in relation to the Epidemiology of Brown Leap Spot Disease of Cassava (Manihot Esculenta Crantz.) and Tree Cassava (Manihot Glaziovii Muell.-Arg.)

Cercospora henningsii Allesch, causal agent of brown leaf spot of cassava (Manihot esculenta Crantz) and tree cassava (Manihot glaziovii Muell. -Arg.), produces two types of spores named as the macro-conidia and the micro-conidia. (As the names suggest, the former are bigger in size (20 - 120 x 5 - 7.5 μm) and the latter are smaller (8.75 - 17.5 x 3.75 - 7.5 μm). Isolates from each of the varieties of cassava and tree cassava could not be differentiated on the basis of the shapes of their spores. There was, however, a slight variation between the sizes of each type- of spore from the test plants as statistical analysis showed. The micro-conidia were observed to have been formed from the macro- conidia through budding and fragmentation. The frequency of occurrence of each type of spore depended on the time of the year (that is, the season) during which lesions were taken for studies. More micro-conidia were harvested from lesions of each test plant during the wet periods of the year. However, during the dry harmattan periods, fewer numbers of spores were harvested of which the macro—conidia were more frequent. Sporulation of the fungus is sensitive to disinfectants (for - example, mercuric chloride - alcohol solution which was used to sterilize the lesions reduced sporulation). The longer the lesions were exposed to the disinfectant, the fewer the number of spores harvested; macro-conidia were found to be more frequent after lesions have been kept in mercuric chloride - alcohol solution for long periods of time. The micro-conidia became more frequent as the incubation periods on lesions became longer. Successive crops of spores, more frequently the micro-conidia, could be harvested from lesions when they were washed and reincubated for successive periods of time. The lesions, therefore, have a high sporulating potential. Lesions sporulated from 40-100% relative humidity but sporulation was best at 95% and 100% humidity and poorest at 40% and 60% humidity. Latent period of germination for micro-conidia was 4-6 hours and 5-7 hours for the macro-conidia. Diverse forms of appressoria were formed at the apices of germ tubes of the micro-conidia whilst those of macro-conidia had fewer but simple Knob-like forms of appressoria. Germinated spores of each type could produce as many as three germ tubes. Long germ tubes were produced from germinated spores of each type and some of them were as long as 90Ojm when the spores were incubated at 100% humidity for 24 hours. Germination was better on host leaf surfaces than on glass surfaces. Branching of germ tubes could also be stimulated better on host leaf surfaces than on glass surfaces. Conidia germinated from 80-100% humidity. It was, however, best at 100% humidity and percentage germination was better for micro-conidia than for macro- conidia at this level of humidity. Germination of conidia of isolates from cassava and tree cassava was similar. Incubation period on cassava varieties during self-inoculation studies was shorter (12.6 days) whilst that on tree cassava was longer (22.6 days) when leaf-borne conidia of high spore density were used for inoculation through the contact method. Incubation periods were also shorter when abaxial or lower surfaces of red and white varieties of cassava were inoculated using inocula with higher densities. In much the same way, the number of inoculated spots which got infected depended Ofl the concentration of inocula, the variety of host plant and the leaf surface inoculated. More inoculated spots on the varieties of cassava got infected than on tree cassava. During cross-inoculation tests, inocula from cassava could infect tree cassava. Similarly, isolates from tree cassava could infect both varieties of cassava. Tree cassava is a possible source of inoculum for cassava.

Source: KNUST