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
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