Investigation Of Crop Disease Outbreaks

As with other outbreaks, it is necessary to have a clear understanding of what naturally occurring disease looks like in order to appreciate the unusual. An excellent review of natural plant diseases was given by Rogers;62 for convenience, the major points of that paper are summarized here. Just as in human or animal illness, crop diseases have typical epidemic curves depending on the interaction of the pathogen and its host. Plant pathogens that undergo one life-cycle in one crop growing season are called monocyclic. If there are several pathogen lifecycles in a growing season, a polycyclic epidemic curve is expected.

The intensity of a crop disease epidemic depends on several factors: the virulence of the pathogen, the environment, and the susceptibility of the host plant. Pathogens that infect plants through their leaves and produce spores that are blown by the wind, and pathogens that are spread by flying insects are most likely to trigger rapid epidemics. Development of fungal diseases is greatly affected by weather, being impacted by even small changes in temperature and humidity. Genetic uniformity of a crop predisposes to rapid spread of a pathogen; heterogeneous populations rarely experience epidemic disease.

Sources of naturally occurring plant infections include residual spores surviving in the soil from previous growing seasons, over-wintering of the pathogen on another crop, diseased crop residues that were not properly disposed of, and spread from wild plants.

Most anti-crop biological warfare programs have focused on airborne fungal pathogens of major food crops that are prone to rapidly developing epidemics within a single growing season.62,63 Examples include cereal rusts and smuts, potato late blight, and brown spot of rice. Most soilborne diseases of crops, whether due to viruses, Mycoplasmas, or fungi, are typically slow to spread and would be less effective in a biological attack.

The epidemiology of the major diseases of significant food crops is well enough known to accurately predict the likely development of endemic disease problems within a growing season after taking into account climactic conditions, pathogen reservoirs, and plant types. Recent technological advances such as satellite imagery and mathematical modeling can assist in such analyses.

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