Positive screening

Positive screening means that the best individuals within an existing, variable population are selected and propagated as a new, improved line. Positive screening is at its most useful with the landraces of subsistence farming.

Robinson (1996) initiated a coffee screening program in Ethiopia, with a view to controlling coffee berry disease (CBD) caused by Colletotrichum coffeanum. The existing coffee crops were genetic mixtures cultivated according to centuries old methods, and yielding only 10% of the commercial coffee yields in neighbouring Kenya. The accidental introduction of CBD in 1970 threatened the Ethiopian coffee industry with ruin. This disease had a continuous pathosystem, and the resistance was horizontal. Its level in individual trees ranged from the minimum to the maximum. Minimum resistance resulted in a total loss of berries some three months before harvest. Maximum resistance resulted in a zero loss of crop at the time of harvest. About one tree in a thousand had maximum resistance, and 650 such trees were identified and propagated with about 1000 seeds of each. Further selection for homozygosity (Coffea arabica is autogamous, and is usually cultivated as pure lines), yield, cup quality, etc., reduced these selections to about fifteen pure lines that could be cultivated without any crop protection chemicals whatever. Virtually unlimited amounts of seed were available within eight years of initiating the project.

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