ABSTRACT
Epidemiology is the science of plant disease dynamics, their incidence and distribution and their control and prevention. An epidemic is the progress of a disease with time and space and depends on interactions between the pathogen, the host and the environment. If these events are known they may be used as structures and their eff ects may be modelled to predict, or forecast, epidemics particularly when the pathogen does not require a vector for transmission. However, vectors are normally required for the transmission of viruses and the relationships of the vectors with the hosts and the environment are diff erent. So the structure of epidemics of virus diseases follows diff erent and complex patterns. Th ese patterns diff er with the nature of the viruses, their relationship of the vectors, geographical locations and the weather/climate. Because of the complicated nature of these structures, an understanding of ecological concepts of epidemiology is also required for its proper understanding. In the past epidemiology was mostly considered statistically. Statistical epidemiology primarily deals with the spread of disease under a particular set of conditions. An epidemic can be considered as, in a holistic way, taking all ecological components into consideration. Th resh (1987) emphasized the information on (i) host populations (longevity, propagation, mobility, growing season), (ii) strategies of virus spread, (iii) disease progress curves, and (iv) virus strains and variation. Hull (2002) outlined the following factors related to an epidemic: (i) source of the virus, (ii) rate of movement and distribution within the host plants, (iii) severity of the disease, (iv) mutability and strain selection, (v) host range, and (vi) dispersal. Th ese outlines by Th resh and Hull included more or less similar parameters, but it is very diffi cult to quantify them in the fi eld, particularly in tropical and sub-tropical humid or sub-humid climates with multi-culture systems where sources of the virus, availability of susceptible host and movement of vectors are very complex. Most epidemiological studies have been made on arable crops of temperate countries planted in monocultures with dense, regular arrays of uniform
genotypes in contrast to crops and viruses of the tropics. Plumb et al (2000) demonstrated the role of cropping systems and weeds in the perpetuation of the epidemics of diff erent viruses in various crops in the tropics. Th e recent introduction of genetically engineered virus resistant crops throws a new challenge to the ecology and epidemiology of virus diseases as the genetic alterations may bring about changes both in the host and vector systems. Th e key factors for an epidemic are the viruses themselves, their change with time and space, perenation and infectivity, vector activity and climate that bring critical complexities to the events. Th e International Society of Plant Pathologists (ISPP) constituted a separate committee ‘Th e Plant Virus Epidemiology Committee’ in 1981. Th is Committee periodically assesses the development of this most dynamic discipline aiming to bring together researchers working on the epidemiology and control of plant viruses in a global forum to address, from a world perspective, past, current and future thrusts. Th e activities of this Committee have introduced several new dimensions to the concept of the traditional epidemiology such as ‘Molecular Epidemiology’. A reconsideration of plant virus epidemiology has also now become imperative because of the consequences of climate change on global agriculture, with ensuing changes in the cropping systems, hostvector-virus interactions, vector biology, and not least the introduction and movement of viruses and their vectors into new areas. Recently, advanced electronic technologies are being used to record the spread and assure biosafety in some countries. Post-introduction mapping of plant virus spread can be done with GPS and GIS technologies that are currently practised by the USA National Plant Diagnostic Network, which provides a plant disease bio-security system now operating from fi ve hubs in the USA. Gathering temporally and geo-spatially referenced diagnostic data is one of its roles. GIS is proving to be a powerful tool to provide maps that identify production areas with diff erent degrees of risk for specifi c plant virus pathosystems.
