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All types of plants can be attacked by disease so that a knowledge of the early signs of infection and of the way in which a disease can be combated is valuable. New varieties of plants are being produced all the time by various methods of selection and inbreeding. Highly bred plants are, however, not necessarily highly resistant to disease. Where one particular type of plant is grown in quantity, often on the same ground year after year, an outbreak of disease can cause serious losses. This trouble is more likely to arise in glasshouses and probably in flower beds that are replanted each year with the same or similar types of plants. Treatment of the soil in early spring or late autumn when the site is vacant will often be necessary. The term "plant disease" refers to anything - except insect damage - which may check the growth of a plant, cause abnormal growth or death. Included, therefore, are parasitic organisms, unsuitable soil, incorrect temperature, injury from fumes and sprays or excessive liming. Fruits and vegetables - such as apples, pears, potatoes and carrots-continue to live even when stored and can suffer from various troubles which either reduce their food value or destroy them entirely. Plant diseases may be divided into two main groups: non-parasitic diseases, which are not infectious and parasitic diseases, which are infectious. The latter may be subdivided into two classes: fungous and bacterial diseases; virus diseases. |
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