This African virus is transmitted by leaf hoppers that are gregarious. As a consequence, the spread of the virus within a crop is limited and, only a low percentage of plants are diseased. This low frequency of disease exerts no selection pressure for horizontal resistance. The few diseased plants are so susceptible that they are usually killed, and the population as a whole remains susceptible. Occasionally, a much higher proportion of plants become infected, and the disease is then very destructive.
This disease has two important lessons for breeding for horizontal resistance. First, it is essential to select plants that have few symptoms but that are known to be infected, otherwise chance escapes will be chosen without any genetic advance in resistance. Second, inoculation is desirable to ensure as uniform a distribution of parasitism as possible. With this virus, disturbing the leaf hoppers every day, so that they eventually inhabit every plant, achieves such a uniformity. Major staple
A major staple is a crop that has a high yield per man-hour, and per unit area; that is reliable from season to season; that produces a food that can be stored; and a food that is easily cooked. A major staple liberates a significant proportion of the population from food production, and they become available for other specialised activities, such as arts and crafts, medicine, architecture, and all those attributes of a sophisticated civilisation, which can be defined as the growth of cities. There are only three major staples in the world. These are wheat, rice, and maize. Every ancient and modern civilisation was based on one of these three crops, and any area or society that lacked them failed to produce a major civilisation. See also: Minor staple. Malarial mosquitoes
These mosquitoes provide good examples of unstable insecticides, such as DDT.
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