Isolation, Identification And Pathogenicity Of Alternaria Alternata In Water Hyacinth, Eichhornia Crassipes (Mart.) Solms.

Water hyacinth [Eichhornia crassipes (Mart.) Solms.] has become one of the most invasive aquatic weeds in the world. At present, mechanical control (e.g. hand-pulling and use of draglines) and chemical control (herbicides) have been used to control water hyacinth in most countries infested by this weed. However, these control measures have their disadvantages in practical application. To develop a more sustainable and effective control strategy for management of water hyacinth, further research is needed to theoretically and practically integrate available approaches. Biological control, such as introducing natural enemies, pathogens and plant allelopathy, is taken to be one of the most effective and environmentally acceptable approaches. This paper addresses a pathogenic strain, black spot pathogen, Alternaria alternata (Fr.: Fr.) Keissler, with high virulence against water hyacinth, which was isolated from naturally diseased plants in Zhangzhou, South Fujian in 2007-2008. Major results of experiments with this pathogen are summarized as follows:1 Isolation and identification of pathogenic strains to water hyacinthA pathogenic strain with high virulence against water hyacinth was isolated from naturally diseased plants collected from Zhangzhou, Fujian province. The strain was named black spot pathogen to water hyacinth, Alternaria alternata (Fr.: Fr.) Keissler, as firstly reported in China.2 Biological characteristics of Alternaria alternataThe optimum culture medium for mycelial growth of A. alternata was PSA. Fresh phytic materials were better than dried substance. Mannitol, glucose and sucrose were suitable for mycelial growth as carbon source. Monosaccharide was the best carbon source. The weak alkaline and neutral nitrogen could distinctly promote the growth of mycelium while acid nitrogen did inhibit it. A. alternata could grow from pH 6 to 8 and the optimum pH was at 7. The optimum temperature for mycelium growth was 27℃. The best cultural condition for mycelial growth was 24 h illumination with 15000 LUX.3 The safety of Alternaria alternataWe inoculated A. alternata on 16 species of normal field vegetables, arable crops and weeds in Fujian to determine host specifically and crop safety. Inoculation didn't result in symptoms on the plants, except water hyacinth and water lettuce. We conducted that the fungus was safe to test plants.4 The effect of Alternaria alternata to physiological and biochemical characteristics, and growth of water hyacinthAfter inoculation of water hyacinth with A. alternata, the contents of soluble protein and malondialchehyche (MDA), and the activities of catalase (CAT) and nitrate reductase in plants, increased significantly, while the activities of superoxide dismutase (SOD) and peroxidase (POD), and the contents of chlorophyll and reducing sugars, the rates of photosynthesis and transpiration in plants, decresed significantly. But the pathogenic strain showed no significant effect on the contents of soluble sugars of water hyacinth.The A. alternata induced increase of the activity of nitrate reductase of water hyacinth, which accelerated the nitrate translocation within the plant, further hindering the composition and decreasing the content of chlorophyll. The activities of SOD and POD decreased followed by the depressed ability of antioxidation and scavenging reactive oxygen. The biofilm of roots and leaves were damaged by the accumulation of MDA. The structure of chlorophyll was destroyed, which restrained the process of photosynthesis. The decreased photosynthesis rate led to decline of transpiration rate. Finally, the biochemical changes of water hyacinth significantly decreased the coverage rates and biomass, while these decreases were positively correlated with the density of A. alternata infections.