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Specificity of interactions between eight Lactuca species and 8 Bremia lactucae isolates was studied in seedlings and adult plants of 36 Lactuca accessions plus one L. serriola × L. sativa hybrid. Pathogenicity of the isolates and/or plant susceptibility was expressed by sporulation intensity. A highly compatible relationship was observed in all of L. serriola accession/isolate interactions tested. A differential reaction was found in numerous cases testifying to physiological specialization of the pathogen in a wild pathosystem. Nonspecific nondifferential compatibility (quantitative resistance) can be expected in L. serriola PI 281876. Age dependent resistance (seedlings versus adult plants, and vice versa) and heterogeneity of reactions were also recorded. The comparisonof B. lactucae isolates from L. sativa and L. serriola has shown a significant shift of pathogenicity in favour of L. serriola accessions. A high level of resistance was found in the L. serriola × L. sativa hybrid. The existence of basic incompatibility can be expected in L. saligna and L. virosa as well as in such taxonomically remote species as L. viminea, L. squarrosa and L. biennis. Other taxonomically remote species, i.e. L. dentata and L. alpina, exhibited a compatible reaction, although the reaction of the former was differential. A high level of sporulation was recorded in all accession/isolate interactions of L. alpina.

Twenty-one accessions of 3 wild Lacfuca species which could be hybridised with L. sativa, the cultivated lettuce, were inoculated at different stages of plant development with 3 multivirulent isolates of Bremia lactucae.Nineteen sources of resistance to B. lactucae, not attributable to the previously recognised resistance factors l-l 1 were identified. Two lines of L. serriola showed similar resistance patterns as lines carrying Rl 1. The resistance of some accession was incomplete particularly at the seedling stage and this phenomenon may be race specific.Tests on segregating F, populations of crosses between 2 different L. serriola accessions and L. sativa cultivars showed that the resistance in one line (LSE/lS) appears to be inherited as a single dominant gene, which is sometimes incomplete in expression and allelic to either Dm6 or R7. The segregation patterns for resistance in PI 281876 did not give readily interpretable ratios.To assess the frequency of occurrence in B. lactucae populations of virulence factors to overcome this novel resistance, 11 of the novel sources of resistance were inoculated with numerous collections of the pathogen from the UK, Czechoslovakia and elsewhere and found to show a high level of resistance.

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The early stages of development of Bremia lactucae (lettuce downy mildew) were examined on lettuce cultivars possessing high (Iceberg and Regina di Maggio) and low (Great Lakes and Plenos) levels of field resistance. Germ tubes, appressoria, penetration, primary and secondary vesicles, intercellular hyphae and haustoria were observed 3. 6 and 24 h after inoculation of cotyledons and of leaf discs from adult plants. Differences were observed between cv. Iceberg and susceptible genotypes in the percentage of spores germinating and the incidence and speed of development of infection structures. Secondary vesicles were first observed 24 h and 6 h after inoculation in Iceberg and susceptible genotypes, respectively. The lowest incidence of secondary vesicle formation 24 h after inoculation (48 and/or 43%) was recorded in Iceberg and Regina di Maggio, and the highest incidence (68%) occurred in Plenos. The formation of intercellular hyphae and haustoria was not observed in cv. Iceberg some 24 h after inoculation. There were significant differences in the lengths of germ tubes formed on different cultivars. Those on cv. Iceberg were longer than those formed on susceptible cultivars. The results indicate that the field resistance of B. lactucae may result from mechanisms which are effective in the early stages of infection.