5.2.41
STATUS AND PROGRESS OF BIOLOGICAL CONTROL OF WATER HYACINTH, EICHHORNIA CRASSIPES IN EGYPT
YM SHABANA1, MA ELWAKIL1 and R CHARUDATTAN 2 l Plant Pathology Dept., Faculty of Agriculture, Mansoura University, Egypt; 2Plant Pathology Dept., University of Florida, Gainesville, FL 3261 1, USA Background and objectives Altemaria eichhorniae (isolate Ae,5) is being developed as a bioherbicide agent for water hyacinth < i > Eichhornia crassipes < /i > in Egypt. Aspects of sporulation, phytotoxin production and formulation were studied in an attempt to improve the efficacy of AeS against water hyacinth. Determination of the optimum conditions for production of highly virulent inoculum and determination of the epidemiological requirements for disease incidence and disease severity were also investigated. < b > Results and conclusions < lb > Sporulation: Five methods (modified Walker's, Shahin's, modified Cotty's, Elwakil's, and sodium alginate method) were tested for inducing spore production by AeS. An adaptation of Walker's method [1] was the best with respect to time, materials, and the abundance of spores produced compared to other techniques. Phytotoxin production: Culture media, dextrose level, temperature, light regime, aeration, and pH were tested for their effects on the ability of Ae5 to produce the crimson-red phytotoxic pigments. The maximum production of pigments was obtained when cultures were grown on potato dextrose agar containing 20% dextrose, with an initial pH of 4.5 at 25C to 30C under continuous darkness or diurnal light and without wrapping the culture plates with Parafilm. Results of the phytotoxin bioassay suggest that 10% (w/v) concentration of the partially purified culture filtrate was required for symptom expression on water hyacinth. Culture conditions and epidemiological requirements for high disease severity: The disease was initiated with either conidial or mycelial inoculum of AeS to yield a similar level of disease severity. The ability to use mycelial inoculum instead of conidia is important to the development and large-scale field use because the mycelial inoculum is easier to produce than conidia. Different broth media and cultural conditions were evaluated for the ability to produce highly pathogenic mycelial inoculum. Fresh potato dextrose broth, shake-culturing, and incubating cultures under diurnal light for 1 week followed by continuous darkness for an additional week were most effective. Empirically, mycelia subjected to > or < 6 s of mechanical shear in a blender were less infective than those blended for 6 s. Exposure of inoculated leaves to at least 10 h of dew was conducive to a high level of disease. The addition of a hydrophilic mucilloid as a humectant to the mycelial inoculum augmented the disease level [2]. Formulation: As stated earlier, a major obstacle on the herbicidal efficacy and reliability of Ae5 in field conditions is the uncertain supply of erogenous free water for at least 10 h to enable the fungal propagules to gertninate and infect the weed. Several approaches have been tried to overcome this obstruction, including the use of hydrophilic polymers, vegetable oil suspension emulsions, and invert (water-in-oil) emulsions with varying degrees of success. The latter formulation has averted dew dependence in greenhouse trials [3]. However, these formulations and others must be tested under field conditions to determine their practical utility. It is concluded that for best levels of bioherbicidal efficacy, the spore or mycelial inoculum along with phytotoxic fractions secreted by the fungus (Ae5) during inoculum production, should be formulated in an invert emulsion. The phytotoxic fractions serve as disease-promoting factors while the invert emulsion prevents dehydration and provides moisture for the germinating inoculum. < b > References < /b > 1. Walker HL, 1980. U.S. Sci. and Ed. Adn-@n., Adv. Agric. Tech. Southern series, No. 12. 2. Shabana YM, Charudattan R, Elwakil MA, 1995. Biol. Control 5, 123-35. 3. Shabana YM, 1997. Z. PflKrankh. PfISchutz 104, 231-38.