I. R. Crute

The location and characteristics of novel sources of resistance to Bremia Lactucae Regel (downy mildew) in wild Lactuca L. Species

SummaryTwenty-one accessions of 3 wild Lactuca 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 1–11 were identified. Two lines of L. serriola showed similar resistance patterns as lines carrying R11. The resistance of some accession was incomplete particularly at the seedling stage and this phenomenon may be race specific.Tests on segregating F2 populations of crosses between 2 different L. serriola accessions and L. sativa cultivars showed that the resistance in one line (LSE/18) 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.

The identification and characteristics of field resistance to lettuce downy mildew (Bremia lactucae Regel)

SummaryEighty-one accessions of three Lactuca species which showed no recognisable race specific resistance to Bremia lactucae when tested in the laboratory as seedlings, exhibited different degrees of susceptibility when exposed to natural field infection. As a group, crisp genotypes had less mildew and a slower rate of disease development than other types of lettuce. Wild forms of Lactuca sativa and Lactuca serriola were particularly susceptible. In a further trial, the low field susceptibility of three lettuce cultivars (Iceberg, Batavia blonde de Paris and Grand Rapids) was confirmed. Disease development on cv. Iceberg was compared to that on the highly susceptible cv. Hilde in experiments where the two cultivars were grown either in close proximity or in isolation. The absolute level of attack on cv. Iceberg depended upon disease pressure and differences between the two types only became apparent approximately 8 wk after sowing.