Big vein is a disease of lettuce; symptoms include vein clearing and delayed head formation. Big vein virus is vectored by a fungus that inhabits lettuce roots. We developed a greenhouse screening procedure to evaluate cultivars and breeding materials for resistance, which is relative and based on the proportion of plants in a population that show no symptoms at an arbitrary time after inoculation. We have screened 744 cultivars in preliminary tests. Of these, 49 showed promise for resistance. Eleven of the promising cultivars showed consistent resistance in replicated greenhouse and field trials. In breeding populations, reaction of single plants is not as reliable an indicator of resistance as the reaction of plant groups. Therefore, F3 families are more useful than the F2 for screening. Field test rankings of materials selected with this method are consistent with greenhouse evaluations.
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