SCREENING FOR RESISTANCE TO BIG-VEIN DISEASE OF LETTUCE (LACTUCA-SATIVA)

A methods was developed for screening of breeding populations and evaluation of genotypes of lettuce for resistance to big vein. First and rapid screening of plant populations or large numbers of genotypes was in greenhouse trials by inoculation of the substrate in which the seedlings to be tested were grown with viruliferous zoospores of Olpidium brassicae. Further testing with limited numbers of genotypes was in container tests, using dry soil with viruliferous resting spores as inoculum. No single criterion or simple and early parameter was found to represent fully the final effect of the disease on yield and quality at the time of harvest. Effect on head weight was not correlated with disease incidence or sympton severity. The final effect on promising lines and cultivars could only be studied in field experiments after zoospore inoculation of seedlings in the greenhouse or on naturally infested soil. All 55 lettuce genotypes tested, including American 'resistant' crisphead cultivars and breeding lines, developed symptoms of big vein in either test, but there were considerable differences in length of incubation time, sensitivity, and capacity to recover from disease. Loss of head weight was considerable in all nine genotypes tested for yield reduction after early inoculation, and for the crisphead cultivars also after late inoculation. Cos lettuce 'Lobjoit's Green' and 'grasse' lettuce 'Little Gem' are promising parental genotypes for breeding for resistance, as are two entries of Lactuca virosa. © 1990.