As is the case for human and animal diseases, a knowledge of the epidemiology of plant disease can provide important information for its treatment and control, and can lead to the development of forecasting systems [see Shaw 2001; Zadoks and Schein 1979]. In the case of fungal plant diseases, particularly those affecting agricultural crops, the two main areas that need to be determined are the mechanism of spread of the disease, and the specificity and host range of the infecting organism. Fungi are transmitted to, and between, plants by a number of different mechanisms. Many fungi are spread through the soil, some growing from previously infected debris in or above the soil. Others can be transmitted as spores and other propagules through water droplets, or directly as airborne particles. Some plant pathogens may exist on secondary hosts, such as Fusarium oxysporum (Armstrong and Armstrong 1958) or on weeds [see Terry and Parker (2001)]. If these hosts are present in or near a crop, they may then act as a reservoir that allows a crop disease to be carried over successive plantings. A few plant pathogenic fungi such as Ophiostoma species [see Brasier (1991)] can be spread by insects and other vectors, and some such as Sclerospora graminicola (Shetty et al. 1980) remain in the seeds of infected plants, and cause disease in subsequent generations. If the mechanism of transmission is known, this knowledge may be important in the development of control or treatment strategies. Such knowledge is particularly important in the selection of planting material, the preparation and maintenance of planting areas, and the establishment of crop successions and rotation [see Maude (1996)].
In addition to knowledge of the mode of transmission of a plant pathogen, it is also important to be able to determine exactly what is being transmitted in terms of fungal populations. In some cases a single fungal species may consist of a number of different host specific populations. An example of this is the vascular wilt pathogen F. oxysporum, where around 170 different forms (referred to as special forms) have been identified. Each of these special forms shows preferential or specific pathogenicity to different hosts, and so F. oxysporum special form cubense will cause vascular wilt of banana, but would not be expected to cause significant disease on oil palm. Therefore, in order to monitor what fungi are present, and may pose a risk to a crop, it is necessary to know specific details of their pathogenicity. This situation becomes further complicated if there are subpopulations within the pathogen that show differential pathogenicity, either in terms of the degree of damage or the particular cultivars attacked. Such populations are generally described as races, and the ability to differentiate these can make a significant difference to their control. A further factor in considering populations in fungal plant pathogens is whether the population is comprised of meiotic or mitotic forms. Some pathogenic fungi, such as Fusarium species, occur almost exclusively in a mitotic (or imperfect) form. In this state, the variability within the population can be assumed to be relatively low, particularly if the disease is the result of a single introduction. Some fungal pathogens, such as Phytophthora and the rusts and smuts, are however present on the plant in a meiotic (or perfect) form, and this allows for variability to be introduced into the population at each generation [for example see Duncan et al. (1998)].
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