As recommended by a lettuce breeder, five susceptible and five
resistant butterhead and batavia lettuce cultivars or lines were
selected (Table 1). Plants were grown in a greenhouse under
semi-controlled environmental conditions (T = approx. 20 C). Natural
light was supplemented by sodium-vapour lamp (SON-T Plus
600W; Philips Deutschland GmbH, Hamburg, Germany) to give a
total daily amount of 10–14 mol photons m2 for 16 h.