Brown spot disease of sugarcane caused by Cercospora longipes (butler)

The objective of this project was to investigate the biology, epidemiology and control of brown spot disease of sugarcane caused by Cercospora longipies so as to provide basic information which would enable cane breeders to recognize cultivators resistant or immune to this disease. In East African, only three species of Cercospora have been recorded, the Cerocospora stage of Eriosphaeria sacchari which causes purple spot on leaf blade, C.koepkei causing yellow spot on leaves and C. longipes which causes brown spot on leaves. Studies in the greenhouse indicated that C.longipes has a restricted host range: only Saccharum officinarum (Sugarcane), S. spontaneum, S.robustum and Sorghum vulgare (Sorghum) were infected. A spore concertation of 2 x 10 per ml was optimal for pathogenicity tests on sugarcane. Cultural studies of the East African C. longipes indicated that it conforms in conidial and conidiophore measurements to those made by Edgerton (1958) in U.S.A but, both East African and United States conidia and conidiophore are larger than those shown that (i) C. longipes grows over a wide range of temperature (10-30 degrees). Of the temperatures tested, 25 degrees was the best for growth, (ii) Ultraviolet irradiation enhances sporulation (iii) spore germination is favored by light and high relative humidity of above 95%. Epidemiology studies showed that water (rain or dew) helps to dislodge conidia from conidiophores; the dislodged conidia later become airborne or are dispersed by rain splash. It was found that the incidence of the disease is highest immediately after the peak of the rainy seasons. Spores were found to tolerate desiccation for periods of longer than 96 hours and it was shown that the pathogen is carried over seasons on cane leaf trash. Studies with juice extracts expressed form resistant and susceptible varieties indicated no detectable fungi static substances present in resistant canes. Crop loss studies under the conditions which prevailed at the trial sites indicated that brown spot causes a loss in cane yield of at least 14% and in terms of sugar production, the loss is up to 18%. It was also shown that brown spot disease has little detrimental effect the sugar quality part on brix, pol and purity which are the sugar quality parameters. Spraying one a month, at the rate of 2.50 kg/ha of Dithane M 45 in 900 litres was the best for controlling the disease. However, under high rainfall and humidity, sporulation and germination of C. longipes were enhanced; in such a situation higher rates of fungicides gave better coverage since the fungicide remained on the cane leaves for longer periods than the lower rate. A field method for screening cane cultivators for resistance to brown spot was devised. The method entailed the planting of ‘test cultivators’ in alternate rows with cultivators known to be highly susceptible to brown spot. Plants were assessed for disease incidence periodically during the growing period by using a set of standard diagnosis showing various degrees of severity of leaf spotting. Results from disease resistance trials indicated a range of reactions form tolerant, like B 3172 and M 423-51 to highly susceptible cultivators such as CP 29/305 and PR 975. No variety was found immune to brown spot disease. A comparison of laboratory and field screening methods showed that the figures for laboratory rating were, in most cases higher than the corresponding ratings for the same variety assessed for field trials. This was attributed to a higher inoculum pressure and an ideally controlled environment in the laboratory. Inter-generic hybrization of Saccharum and sorghum promises to yield cultivators resistant to brown spot as in nature C. longipes rarely infects sorghum.