Screening for lettuce (Lactuca sativa L.) big-vein resistance in the F 2 generation is highly inefficient. Efficiency
improves in the F 3 and following generations with continued inbreeding. Traits useful in ascertaining resistance are
0% of plants showing symptoms and percentage of plants showing symptoms at a given date. Breeding
lines identified as resistant in greenhouse screening have proved resistant under field conditions. Forty-nine cultivars have
been identified in preliminary testing as potentially resistant. Of these, 11 have been confirmed as resistant in greenhouse
and field tests.