PATHOGEN TO CONTROL WATER HYACINTH

Acremonium zonatum

This fungus causes an easily identified necrotic zonate leaf spot characterised by spreading lesions,  most noticeable on the upper laminar surface (Fig. 1). On the lower surface, which is normally protected from direct sunlight, the area directly under the spot may have a sparse, spreading layer of white fungal (mycelial) growth. Each spot may be small (2 mm diameter) to large (> 3 cm diameter) and the spots may coalesce, covering most of the lamina. The zonate pattern may not be evident in new infections when most spots are small. This disease has been reported from Australia, USA, and many countries of Asia, Central America, and South America. It is often asso-ciated in the field with infestations of the water hya-cinth mite Orthogalumna terebrantis. This pathogen is represented by several highly virulent strains such as the ones found in Mexico by Martinez Jimenez and Charudattan (1998).

Alternaria eichhorniae

Two species of  Alternaria, A. eichhorniae  and A. alternata,  have been recorded on water hyacinth. One or both of these species have been reported from Aus-tralia, Bangladesh, Egypt, India, Indonesia, and South Africa. Alternaria alternata appears to be a weak,opportunistic parasite, whereas A. eichhorniae is a highly virulent, host-specific pathogen of water hya-cinth.  Alternaria eichhorniae has been shown to have good potential as a bioherbicide agent (Shabana et al.1995a,b; these proceedings). It causes discrete necrotic foliar spots (oblong, 2–4 mm long) sur-rounded by a bright yellow halo. Blighting of the entire leaf lamina can be induced by using mycelial inoculum and  providing prolonged, 100% relative humidity (Fig. 2). In culture,  A. eichhorniae produces several bright red compounds in culture, including bostrycin and deoxybostrycin that are phytotoxic to water   hyacinth leaves (Charudattan and Rao 1982). The extent of naturally occurring variability in viru-lence in this pathogen is not clear. More details about this pathogen are given by Shabana (2001).

Cercospora piaropi  (=  C. rodmanii )

Symptoms caused by Cercospora  spp. may be easily confused with those of many other foliar pathogens, including many opportunistic, weak parasites. Until now, two species of  Cercospora,  C. piaropi  and  C. rod-manii, have been recognised as pathogens of water hyacinth, but recently Tessmann et al. (2001) have merged the two species into an emended  C. piaropi. Cercospora piaropi  has been reported on water hya-cinth from throughout the present range of  the weed. This pathogen causes small (2–4 mm diameter) necrotic spots on laminae and petioles (Fig. 3). The spots are characterised by pale centres surrounded by darker necrotic regions. Occasionally, the spots may appear in the shape of ‘teardrops’ that coalesce as the leaf matures, causing the entire leaf to turn necrotic and senescent. In fact, the senescence is accelerated by the Cercospora  disease, and the disease can rapidly spread across water hyacinth infestations, causing large areas of the weed mat to turn brown and necrotic. Under severe infections, the plant may be physiologically stressed, lose its ability to regenerate, become water- logged, and sink or disintegrate.  Tessmann et al. (2000) compared 60 isolates of  Cer- cospora  species isolated from water hyacinth leaves showing symptoms of  Cercospora  infection which were collected from the USA, Mexico, Venezuela, Brazil, South Africa, and Zambia. They found the iso- lates to be variable in pigmentation in culture, spore morphology, and virulence. Virulence of the isolates was also variable: isolates ranged from being nearly avirulent to highly virulent and capable of causing leaf death. These traits were independent of the geographicorigin of isolates. The isolates were then tested to see if the species concept based on conidial and cultural morphology and virulence, as used by Conway (1976)  in his designation of  C. rodmanii  into a separate species, might agree with a species concept developed with the help of molecular markers.

Myrothecium roridum

This fungus causes a teardrop-shaped leaf spot (up to1  ×  5 cm), rounded on the side facing the petiole and tapering to a narrow point in the direction of the laminar tip (Fig. 4). Older leaf spots turn necrotic with dark brown margins, with the centre of the spot covered with discrete white and black conidial masses. Myrothecium disease of water hyacinth has been reported to occur in India, Malaysia, Indonesia, possibly Mexico, and some western African countries. Although this species is worldwide in distribution, the typical myrothecium disease has not been recorded on water hyacinth in the Americas. The occurrence of variability in virulence in this pathogen is therefore not clear. Some recent studies suggest that some Myrothecium species can be used as broad-spectrum bioherbicides against several weeds, a finding that has implications for the development of M. roridum for water hyacinth control.

Rhizoctonia solani

Disease symptoms caused by this fungus may resemble damage caused by a desiccant type of chemical herbicide (e.g. diquat). Symptoms consist of irregular, necrotic spots, and broad lesions (Fig. 5). Unlike chemical damage, the brown necrotic areas are usually surrounded by noticeable, thin, water-soaked margins of darker brown colour than the rest of the lesion. Rhizoctonia disease has been reported on water hyacinth from the southeastern United States, Brazil, Mexico, Panama, Puerto Rico, India, Malaysia, and Indonesia. This fungus is usually very aggressive and destructive, capable of rapidly killing water hyacinth plants. The extent of variability in virulence of  R. solani  pathogenic to water hyacinth is not clear, but isolates collected in the USA, Panama, and Brazil have been found to be extremely virulent (R. Charudattan, unpublished data; R.A. Pitelli, University of the State of Sao Paulo, Jaboticabal, Brazil, pers. comm.).

 

 

Source :  

Charudattan. 2001. Biological and integrated control of water Hyancinth, Eichornia crassipes. Jornal Center for Aquatic and Invasive Plants