Various Pathogens for the Control of Waterhyacinth (Eichhornia crassipes)
Several highly virulent and host-specific pathogens attack waterhyacinth in different parts of the world. Of these, the following are leading candidates that could be developed as bioherbicides.
Acremonium zonatum: This fungus causes necrotic zonate leaf spots, 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 associated in the field with infestations of the waterhyacinth mite Orthogalumna terebrantis. This pathogen is represented by several highly virulent strains in different parts of the world.
Figure 1
Alternaria eichhorniae: Two species of Alternaria, A. eichhorniae and A. alternata, have been recorded on waterhyacinth. One or both of these species have been reported from Australia, 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 waterhyacinth. Alternaria eichhorniae has been shown to have good potential as a bioherbicide agent. It causes discrete necrotic foliar spots (oblong, 2-4 mm length) surrounded 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-colored compounds in culture, including bostrycin and deoxybostrycin that are phytotoxic to waterhyacinth leaves. The extent of naturally occurring variability in virulence in this pathogen is not clear.
Figure 2
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 recently, two species of Cercospora, C. piaropi and C. rodmanii, have been recognized as pathogens of waterhyacinth, but now the two species have been merged into one species, C. piaropi. Cercospora piaropi has been reported on waterhyacinth from throughout the present range of this plant. This pathogen causes small (2 to 4 mm diameter) necrotic spots on laminae and petioles (Fig. 3). The spots are characterized by pale centers 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 waterhyacinth 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 waterlogged, and sink or disintegrate.
Figure 3
Myrothecium roridum: This fungus causes a teardrop-shaped leaf spot (up to 1 x 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 center of the spot covered with discrete white and black conidial masses. Myrothecium disease of waterhyacinth has been reported to occur in India, Malaysia, Indonesia, possibly Mexico, and some western African countries. Although this fungal species has a worldwide distribution, the typical Myrothecium disease has not been recorded on waterhyacinth in the Americas. The extent of variability in virulence in this pathogen is therefore not clear. Some recent studies suggest that some Myrothecium spp. can be used as broad-spectrum bioherbicides against several weeds, a finding that has implications to the development of M. roridum for waterhyacinth control.
Figure 4
Rhizoctonia solani: Disease symptoms caused by this fungus may resemble damage caused by a contact chemical herbicide (e.g., diquat). Symptoms consist of irregular, necrotic spots, broad lesions, and general blighting of leaves (Fig. 5). Unlike chemical damage, the brown necrotic areas are usually surrounded by noticeable, thin, water-soaked margins of darker brown color than the rest of the lesion. Rhizoctonia disease has been reported on waterhyacinth from the southeastern United States, Brazil, Mexico, Panama, Puerto Rico, India, Malaysia, and Indonesia. This fungus is usually very aggressive and destructive, capable of killing waterhyacinth plants rapidly and completely. The extent of variability in virulence of R. solani pathogenic to waterhyacinth is not clear, but isolates collected in the USA, Panama, and Brazil have been found to be equally virulent. Despite its high virulence and destructive capabilities, R. solani has never been seriously considered as a bioherbicide agent because of its reputed wide host range.
Figure 5
Uredo eichhorniae: This rust fungus occurs in southern Brazil and Argentina, and occasionally in Uruguay. It is known only in its uredial spore stage (Fig. 6). Being a rust fungus, it is likely to be highly host-specific and therefore a desirable classical biological control agent. However, several aspects of its biology are poorly known. For instance, since this fungus was first described from the Dominican Republic, a tropical island, it is unclear why it does not occur beyond its present range of distribution in the subtropical to temperate regions of South America. It is possible that it is adapted to slower-growing plants in the temperate fringes of waterhyacinth's distribution, and its original finding in the Dominican Republic is due to a misidentification of the host plant. Nevertheless, the occurrence of this pathogen only inside the native range of waterhyacinth (South America), but not outside (e.g., Asia, Africa, or Australia), is consistent with the ecological theory of coevolution of rust fungi with their host plants.Currently, studies are ongoing to determine the life cycle and disease cycle of U. eichhorniae under field conditions. The biological control potential of this fungus is also under evaluation. These studies should help to establish the suitability of U. eichhorniae as a classical biocontrol agent for waterhyacinth and set the stage for its eventual release into the USA and other countries. The addition of this rust pathogen to the existing suite of biocontrol agents is likely to improve the prospects for a sustainable, long-term biological control of waterhyacinth.
Figure 6 |
