Natural occurrence and potential of pathogenic fungi for integrated management of water hyacinth, Eichhornia crassipes (Mart.) Solms-Laubach: Pontederiaceae, in Lake Victoria.

Abstract

These studies were conducted to determine the natural occurrence of pathogenic fungi assciated with water hyacinth in Lakes Victoria and Naivasha in Kenya. The aim was to investigate the potential of integrating some of the most virulent fungal species identified, with Neochetina weevils for control of the water hyacinth in Lake Victoria. Two surveys to collect, isolate and identify fungi associated with diseased water hyacinth plants were conducted in December 1999 (dry season) and May 2000 (rainy season) at different sites in Lakes Victoria and Naivasha. A total of 22 fungal species were isolated and some were found pathogenic to water hyacinth. In Lake Naivasha, 7 species were isolated and identified of which 80% were found to be pathogenic. In Lake Victoria, 22 species were isolated and identified and 69% were found to be pathogenic on water hyacinth. Two new pathogenic taxa of Fusarium and Alternaria were recorded for the first time in the study area. Fungal occurrence was higher during the rainy season recording a total of 17 species as compared to 12 fungal species recorded during the dry season. Pathogencity tests were carried out in the laboratory and screen house at Muguga against water hyacinth to determine the fungi that were virulent on the weed. Alternaria alternata and the new taxa of Fusarium sp. showed the highest mean disease incidence of 50.5% and 40.6% respectively at 2.5% w/v in 0.2% Tween-20 solution, six weeks after infection. Although the disease severity (DS) did not show any significant difference at P=0.05, the new taxa of Fusarium performed better than other species (DS=27.8%). Both the disease incidence and severity were gave a strong negative effect on the water hyacinth plant’s ability to generate new leaves. Correlation analysis between new leaf turnover with disease incidence and disease severity gave correlation co-efficient values of -0.92 and -0.99 respectively. Screen house experiments showed that integration of both fungus Alternaria alternata and the new Fusarium sp with Neochetina weevils increased both the disease incidence and disease severity significantly. Alternaria alternata integrated with Neochetina weevils gave an increase of 31.1% in disease incidence and 16.1% in disease severity. Integrating Fusarium sp. with Neochetina weevils showed an increase of 14.6% in disease incidence and 76.0% in disease severity. The action of the fungi and Neochetina weevils was synergistic and this study recommends integration of the two control methods as part of the management strategy for water hyacinth in Lake Victoria.