Pure line

A cultivar of a seed-propagated, inbreeding species in which all the individuals are effectively identical and are almost homozygous. A pure line thus 'breeds true to type'. It is produced by self-pollinating the best heterozygous individual in a mixed breeding population for several generations. In each generation, the progeny show a reduced variability, and the process is repeated 4-6 times until no further variability is apparent. See also: Single seed descent. Pyrenophora

This is the Latin name of the perfect (i.e., sexual) stage of many species of Helminthosporium. Pyrethrins

Natural insecticides extracted from the flowers of Chrysanthemum cinerariifolium, which is a native of Dalmatia. This plant is now cultivated in a number of countries and the extracted pyrethrins are used mainly in household aerosols. This insecticide is completely non-toxic to mammals, and it leaves no toxic residues. It also has a very rapid 'knockdown' effect. It has been used for centuries by local people in Dalmatia, who put dried pyrethrum flowers in their bedding to control fleas and bed bugs, and no resistance has been known to develop to it in any species of insect. It is thus a stable insecticide.

At present, this very safe insecticide is too expensive to be used routinely on food crops. However, with improved yields of both flowers and pyrethrin content, and the development of a mechanical system of harvesting, the price might be brought down sufficiently to allow its use on food crops. This would form an excellent project for a plant breeding clubs.

If the price of natural pyrethrins could be brought down sufficiently, by high yields and high pyrethrin content, and mechanical harvesting, the market for crop protection is virtually unlimited. This insecticide would be widely used both by organic farmers, and the producers of those fruits and vegetables in which the actual sprayed surface is eaten by people. Pyrethrum is also a potential replacement crop for tobacco farmers.

Natural pyrethrins have several advantages over synthetic insecticides. First, as already mentioned, they are stable; they do not break down to new race of the insect. Second, their mammalian toxicity is extremely low and this is one of the safest insecticides available. Third, they break down to carbon dioxide and water after twenty-four hours of exposure to sunlight. Fourth, they leave no residues whatever. Finally, they are a very powerful insecticide. Their chief disadvantage is their cost, and the overall objective of amateur breeders should be cost reductions sufficient to make natural pyrethrins competitive with synthetic insecticides for crop protection. A second disadvantage is that pyrethrum paralyses insects, but they are likely to recover, unless the insecticide is formulated with a synergist such as piperonyl butoxide. Such additives may preclude its use by organic farmers. However, sesame oil is a natural synergist.

In order to make pyrethrum a commercial success, a simple machine for mechanical harvesting will have to be developed. The harvested flowers must be carefully dried without overheating, and then sold to an extraction factory. Pyrethroids

Synthetic pyrethrins. Unlike natural pyrethrins, pyrethroids are unstable and are liable to break down to new strains of an insect pest. Pyrethrum

See: Chrysanthemum cineriifolium.

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