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the distribution of an infectious plant disease over considerable land areas (a farm, raion, oblast) during a specific period of time. The following diseases are often epiphytotic: rusts and smuts of bread grains, phytophthoric disease of potatoes, apple scab, cotton wilt, and snowy and common leaf cast.
In the past epiphytotics caused great damage. Phytophthora caused enormous losses in the potato harvest in Ireland in the 1840’s. The sunflower harvest in Russia during the 1860’s was damaged greatly by rust, and stem rot seriously hurt the wheat harvest in Amur Oblast in 1923. Epiphytotics have become less common as a result of advances in land cultivation, in the forecasting of widespread plant diseases, and in the measures used to combat the diseases.
Epiphytotics develop under favorable conditions from isolated nidi of infection. Their development is fostered by the accumulation of nidi of rapidly spreading infections, by weather conditions that encourage reproduction of the pest and development of the disease, and by the presence of a sufficient number of receptive plants. Phytopathogenic microorganisms move from their reserves and infect a large number of plants. The development of several generations of causative agents leads to the appearance of new, larger nidi of infection. The region of infection expands, and an epiphytotic situation develops. Epiphytotics develop rapidly or slowly, with periodic outbreaks occurring under favorable situations. The amount of time needed for their development depends on the type of disease, the characteristics of the causative agent, the host plant, and external factors.
Various aspects of the epiphytotic process are studied by epiphytotiology, which is a relatively new scientific discipline. The effect of epiphytotics can often be weakened if a relationship between their development and specific factors can be determined. For example, changes in the populations of the causative agents and the host plants, which are the foundations for the development of epiphytotics, are taken into account when forecasting plant diseases, when developing crop varieties that are resistant to infectious diseases, and when introducing new resistant varieties into crop rotations. Measures to combat epiphytotics depend on the characteristics of each individual disease.