The reactions of lettuce cultivars to physiologic races of Bremia lactucae are interpreted in terms of a gene-for-gene relationship between pathogen and host. The hypothesis takes into account the parentage of cultivars and the origins of their resistance, the characteristics of the resistance reactions and data available from detailed genetical analysis of various race/cultivar combinations. Cultivars are classified with respect to ten postulated resistance genes and B. lactucae races are defined by the virulence genes present. The practical significance of these studies is discussed in relation to both future lettuce breeding programmes and to the choice of cultivars availabIe to counteract any given local race situation. I N T R O D U C T I O NSeveral methods have been employed in attempts to control lettuce downy mildew caused by Bremia lactucae Regel, and the use of resistant cultivars has been one of the most popular. Over the last 50 years more than 120 cultivars of lettuce carrying resistance to this fungus have been discovered or specially bred. All resistance to B. lactucae reported to date is race specific and usually appears to be controlled by single dominantIn common with most other instances of race specific resistance, that in lettuce has readily been overcome by the occurrence of new races of the pathogen. Such races were first noted by Jagger (1926) and since then about thirty different isolates of the fungus have been studied from this point of view (Table I).These races have usually been denoted by combinations of letters and numbers, the former designating the place of origin and the latter the sequence in which they were defined. For example, NL 1-6 are six Dutch races and W 1-4 are four British races originally identified at Wellesbourne. This method of nomenclature is meaningful only when accompanied by the reactions of the particular isolate on a long list of cultivars and is complicated because various differential cultivars have been used by different workers.So far, the available data have not been used to indicate the genetic constitution of lettuce cultivars or the races of the fungus. This has been impeded by the mass of A P B 83 5 as the primary event in resistance to fungal parasites. Nature, London 249, 186-187. NET-, D. (1971). Identification of races of downy mildew (Bremia lactucae Regel) in Israel, and breeding for resistance. Lactuca sativa and on other composites. Annals of Applied Biology 82, 433-450. and variation within races of the pathogen. Horticultural Research 13, 89-95. 411-416.

SUMMARYLaboratory methods are described for the culture and observation of Bremia lactucae on lettuce.Mycelial growth of the fungus within the cotyledons of the cultivar Cheshunt Early Giant progressed to a minimum density at which sporulation occurred, and then continued to a maximum density coinciding with peak sporulation. The timing and intensity of sporulation was influenced by the concentration of the inoculum. Seedlings often became systemically infected and this, together with intensity of sporulation, was a good indicator of fungal growth and hence host susceptibility.The severity of disease development on four susceptible lettuce cultivars (Cheshunt Early Giant, Paris White, Trocadero Improved and Crackling Ice) decreased with age and development of the seedlings at inoculation. The reasons for this are discussed.

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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Failure to control Bremia lactucae (lettuce downy mildew) with metalaxyl in an intensive lettuce‐producing region of Lancashire at the end of 1983 was shown to be due to the occurrence of a high level of resistance to this fungicide (isolates capable of growth at < 100 μg/ml metalaxyl). During most of 1984, metalaxyl‐resistant isolates were obtained from numerous sites but all within a 20‐km radius of the initial outbreak. Thereafter, at the end of 1984 and during 1985, metalaxyl‐resistant isolates were recovered from most major lettuce‐producing regions in the UK with protected crops more affected than field crops. AH metalaxyl‐resistant isolates tested were identical in their response to fungicide, sexual compatibility type (B2) and virulence phenotype, probably representing a clone from a single origin. The resistant pathotype was virulent on resistance factors R 1‐10 and 12‐15 but lacked virulence for R 11 and 16‐18. This was also the most common virulence phenotype among sensitive isolates collected at the same time. Cross‐resistance to other phenylamide fungicides was demonstrated but isolates resistant and sensitive to phenylamide showed a similar response to the unrelated systemic fungicides propamocarb and fosetyl‐Al. An F1 sexual progeny isolate from a cross between a phenylamide‐sensitive and a phenylamide‐resistant isolate (presumed heterozygous at the locus or loci regulating response to phenylamide fungicides) exhibited an intermediate response to phenylamide fungicides. No isolates of this type were obtained from the field. At the high concentrations affecting spore germination, phenylamide fungicides exhibited lower activity against a resistant isolate compared with a sensitive isolate. The findings are discussed in relation to future control strategies, the population biology of the fungus and possible directions for lettuce breeding programmes.