Mean Fv/Fm of leaf discs of plants of susceptible (A, ‘Neckarriesen’) and resistant (C, ‘Analena’; D, ‘Veronique’) butterhead and susceptible batavia (B, ‘Maravilla’) lettuce
cultivars during 14 d after inoculation with Bremia lactucae (Bl:18). Each leaf disc was classified according to success of infection; successful infection corresponds to
categories A and D; contaminations corresponds to categories B and D, absence of contamination corresponds to categories A and C (c. f. Table 2).