Organism |
Glomus fasciculatum, Gaeumannomyces graminis tritici, Aphanomyces, Cylindrocladium, Fusarium, Macrophomina, Phytophthora, Pythium, Rhizoctonia, Sclerotium, Thielaviopsis, Verticillium |
Abstracts |
Currently, the world over, especially in developing countries, maintenance of soil fertility and control of plant diseases have become crucial issues in meeting the biomass needs for food, fodder and fuel, as well as preserving a clean environment. An ideal fertile soil is characterized not only by optimum physical properties and chemical constituents conducive for plant growth, but also by microbiological processes that are maintained in equilibrium. More than 90% of land plants are estimated to form arbuscular mycorrhizal (AM) associations with soilborne fungi in the phylum Glomeromycota. They have a wide host range, yet certain host and fungal combinations are more effective from either the perspective of the fungus, i.e. greater spore/hyphae production, or from that of the host, i.e. enhanced growth, nutrient acquisition or pathogen resistance. Besides improving uptake of phosphorus, AM fungi improve plant health through improved resistance to various biotic and abiotic stresses. Of particular importance is the bioprotection conferred to plants against many soilborne pathogens, such as species of Aphanomyces, Cylindrocladium, Fusarium, Macrophomina, Phytophthora, Pythium, Rhizoctonia, Sclerotium, Thielaviopsis and Verticillium, as well as various nematodes by AM fungal colonization of the plant roots. Achieving the effective and sustainable control of plant diseases remains a formidable challenge for all agricultural systems. Despite the continued release of resistant cultivars and pesticides, pathogens still cause crop damages and losses that exceed 12% worldwide. Studies have shown that root rot in wheat caused by S. rolfsii was prevented by the inoculation of Glomus fasciculatum. Reduced quantum of lesioned roots was found in take-all diseases caused by Gaeumannomyces graminis tritici due to G. deserticola in wheat. The association of G. radiatum with apple has been studied in the USA. It was found that soilborne fungi, Cylindrocarpon, Pythium and the parasitic nematode, Pratylenchus spp., were common with replant diseases of apple. In this disease, young trees are stunted and develop fewer branches than healthy trees. The exact mechanisms by which AM fungal colonization confers the protective effect are not completely understood, but a greater understanding of these beneficial interactions is necessary for the exploitation of AM fungi in organic and/or sustainable farming systems. The mechanisms employed by AM fungi indirectly to suppress plant pathogens include enhanced nutrition to plants; morphological changes in the root; increased lignification; changes in the chemical composition of the plant tissues like antifungal chitinases, isoflavonoids, etc.; alleviation of abiotic stress and changes in the microbial composition in the mycorrhizosphere. Bioprotection within AM fungal-colonized plants is the outcome of complex interactions between plants, pathogens and AM fungi. In this chapter, the different diseases of cereals, pulses, fruits and vegetables and the potential mechanisms by which AM fungi contribute to bioprotection against plant soilborne pathogens are discussed. |