Microbial Plant Pathogens-Detection and Disease Diagnosis:

pp 5-199


Detection of Fungal Pathogens in Plants

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Among the microbial plant pathogens, fungus-like and fungal pathogens have well developed thallus consisting of hyphae, asexual and sexual reproductive structures. The morphological characteristics of these structures and various kinds of spores produced by them have been the basis of identification up to genus/species level and classification of these pathogens into family, order and class. However, the formae speciales, strains, varieties or biotypes within a morphologic species have to be identified using other characteristics such as pathogenicity, biochemical and immunological properties or nucleotide sequences of the genomic DNA. Isozyme analysis, vegetative compatability group (VCG) analysis and electrophoretic mobility of cell wall proteins have been shown to be useful for the detection of strains of some fungal pathogens. The usefulness of immunoassays for early detection and precise identification has been significantly enhanced following the development of enzyme-linked immunosorbent assay (ELISA) and monoclonal antibodies which exhibit greater sensitivity and specificity compared with Appendix 1 based methods which are laborious and time-consuming. Nucleic acid-based diagnostic techniques depending on the variations in the nucleotide sequences of the pathogen DNA have become the preferred ones, because of their greater speed, specificity, sensitivity, reliability, and reproducibility of the results obtained, following the development of polymerase chain reaction (PCR). Several variants of PCR and commercial kits for on-site adoption under field conditions, away from the laboratory, are now available, providing the results in a short time. The possibility of detecting two or more pathogens simultaneously has become bright after the development of DNA array technology. A wide range of diagnostic techniques can be applied for detection, identification and quantification of fungal pathogens present in the infected plants, propagative plant materials and postharvest produce. Speed, specificity, sensitivity and cost-effectiveness are the primary factors that may determine the suitability and choice of the diagnostic tests.