Identification, pathogenicity and disease progress of fungi associated with water hyacinth Eichhornia crassipes (Mart.) Solms in Kenya.

Abstract

A survey of plant pathogenic fungi associated with naturally infected water hyacinth tEichhornia crassipesi was conducted at 'Lake Victoria, Lake Naivasha and Nairobi dam in Kenya. Twenty fungal isolates belonging to different genera were isolated. Two Alternaria species designated WH3bl and WH3b2 were found to be pathogenic to the water hyacinth during the preliminary green house studies. The two exhibited different disease symptoms on the water hyacinth. On the basis of conidial measurements, growth characteristics, and pigmentation in culture, the two Alternaria species were identified as A. Alternata and A. eichhorniae. ,1. eichhomiae was found to be ideal for biological control both under green house conditions and in the field. A. eichhorniae caused a severe disease characterised by leaf blight and discrete leaf lesions. Disease symptoms appeared between the 5th and the ill day following inoculation. For A. alternata, symptoms appeared 3 days after inoculation as small yellowish, chlorotic lesions with necrotic brown centres. Later these lesions enlarged gradually and centres turned dark brown with pale yellow margins. When the two pathogens were compared for radial growth in three different solid media, A. eichhorniae did well in water hyacinth leaf decoction agar as compared to potato dextrose agar and V8 agar juice, while A. alternata grew fairly well in all the three. Water hyacinth leaf decoction agar (WHLDA) was the best media for growth and sporulation of A. eichhorniae. Different broth media and cultural conditions were evaluated for the production highly pathogenic inoculum of A. eichhorniae. Inoculum of A. eichhorniae grown on fresh potato dextrose broth (FPOB) for 1 week under diurnal light followed by another 1 week, under continuous darkness produced the highest disease incidence and severity. Histopathological studies were only done for A. eichhorniae. Serial microtome sections of the inoculated leaves showed that the germinating conidia penetrated the leaf in three ways; 1) the germ tube formed an appressorium followed by an infection which punctured the epidermal cells, 2) the germ tube formed an infection peg without an appressorium and penetrated the leaf between the guard cell and epidermal cell and. 3) through the open stomata. Development of the fungus in the host tissue was the same irrespective of the mode of penetration. The fungus ramified through the host tissue both inter- and intra- cellularly destroying the host cells. The air chambers in water hyacinth. which are regular in shape. were seen to be irregular after 72 hours. This was followed by the formation of necrotic spots. Complete death and defoliation of water hyacinth plants infected with A. eichhorniae occurred after the six week and disease development was noticed on uninoculated water hyacinth plants grown in neighbouring plot O.5m away suggesting dispersal of spores by either wind or water. The result of this study suggests A .eichhorniae to be an aggressive pathogen on water hyacinth and hence a potential for development of a bioherbicide for water hyacinth in in Kenya