In the temperate regions, it is normally possible to achieve only one breeding cycle each year, with the cross-pollination and seed production undertaken in a greenhouse, during the winter, and the on-site screening conducted in the field, during the summer. With ambitious breeding targets (e.g., horizontal resistance that is complete and comprehensive against all locally important parasites), this could require 10-15 breeding cycles during the same number of years.
If, however, we could squeeze two breeding cycles into each year, the total breeding time would be halved. This would necessitate a 180-day breeding cycle, with each cycle divided into a 90-day seed production cycle, and a 90-day screening cycle. This would require a lot of careful planning, and hard work, but it is feasible. It would be achieved by considerable over-lapping of the cycles.
For example, about five times the required number of parents could be selected about two thirds of the way through the screening cycle (i.e., after 60 days of screening). These would be grafted on to tomatoes (sown well in advance) on the clear understanding that 80% of them will be discarded when the final selections are made. By the end of the screening cycle, the selected parents will be grafted, flowering, and ready for cross-pollination. This would add 30 days to the seed production cycle, which would then be effectively 120 days.
One breeding cycle would obviously have to be conducted during the winter and, in most temperate countries, this would mean a fairly large heated greenhouse, as well as the induction of both insect infestations and disease epidemics. Technical help may be needed here. The screening cycle would begin by sowing the true seed immediately after it is becomes available at the end of the seed production cycle, including seed taken from somewhat immature fruits.
In the temperate regions, one 180-day breeding cycle should start in spring, and allow an on-site screening in the summer. The second breeding cycle would start in the autumn, and this would necessitate a winter screening in the greenhouse.
Obviously, this winter screening would not be an on-site screening but, as this happened in every alternate breeding cycle, the small reduction in breeding efficiency would be acceptable, in view of the huge gain in time. (But see also: Tuber resistance).
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