Detection of Virus and Viroid Pathogens in Plants

Abstract

Plant viruses differ significantly from other microbial pathogens in their simple structure and minute size, method of replication and obligate parasitism. Despite their simple structure, they cause numerous economically important plant diseases that still remain as threat to crop production in several countries. Depending on the biological, morphological, immunological and genomic properties, several approaches have been made for the detection, identification, quantification and differentiation of the viruses. Histochemical methods employing light and electron microscopy have been shown to be useful for detecting and studying the nature of intracellular inclusions characteristic of virus infection. Certain groups of viruses can be putatively identified by the type and nature of inclusion bodies in infected plant cells. Electron microscopy using appropriate stains has been useful to study the ultrastructure of viruses. Immunological techniques have been shown to provide more rapid, sensitive and precise detection of viruses compared to biological methods that depend on inoculation to diagnostic/assay hosts that exhibit characteristic symptoms. Nucleic acid-based techniques represent significant advancement in pathogen diagnostics. They offer more reliable, reproducible, sensitive and specific results for the detection, identification and quantification of the viruses in different host plant species that are symptomatic or asymptomatic. Attempts to detect two or more viruses simultaneously present in infected plants have been successful, resulting in saving time, labor and costs of testing. In spite of all the advantages, many nucleic acid-based techniques have not yet been demonstrated to be applicable in different locations, cost-effective and suitable for large scale use under field conditions. Viroids represent a group of most primitive pathogenic entities constituting exclusively of nucleic acids that are capable of independent replication and inducing diseases when introduced into susceptible plant cells. Biological methods have been useful in detecting the viroids based on their reaction on diagnostic host plants. Nucleic acid-based techniques are the ones that can provide reliable and rapid results for their detection and identification. As it is very difficult to cure the plants already infected by viruses or viroids, the practical option available for the management of diseases caused by them is the removal of infected plants. Such an eventuality will result in devasting adverse effect on crop production. To avoid such a catastrophic possibility, adequate attention has to be bestowed for the early and reliable detection of viruses and viroids in infected seeds and vegetatively propagated materials and eliminate all infected ­materials to build virus/viroid-free nucleus stocks for distribution to growers.