Judith M. Norwood

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.

A demonstration of the inheritance of field resistance to lettuce downy mildew (Bremia lactucae Regel.) in progeny derived from cv. Grand Rapids

SummaryThe inheritance of field resistance to downy mildew in lettuce derived from cv. Grand Rapids was studied in F2, F3 and F4 progeny from a cross with the more susceptible cv. Lobjoits Green Cos. The results for 32 randomly produced F3 families and the F2 population corresponded closely although the absolute level of resistance varied from season to season according to disease pressure and environmental conditions. The field resistance character showed a quantitative pattern of inheritance but the rapid response to selection at F3 implied that only a few genes may be involved. F4 families raised from families selected from the two extremes of the F3 distribution fell into distinct categories with resistance levels corresponding to that of their F3 parents. An association between resistance and morphological features of the resistant parent was present at F2 but not in subsequent generations when individuals and families which were as resistant as cv. Grand Rapids and yet bore little morphological resemblance to this cultivar were readily recovered.

Gene-dosage effects on the relationship between Bremia lactucae (downy mildew) and Lactuca sativa (lettuce): the relevance to a mechanistic understanding of host—parasite specificity

The expectations for the reaction phenotypes of four models proposed to explain the genetic regulation of cultivar-isolate specificity in gene-for-gene relationships were different when gene-dosage effects (incomplete dominance) were taken into account. To test these hypotheses, F 2 individuals from two crosses involving the segregation of gene R6 for resistance in Lactuca sativa (lettuce) to Bremia lactucae (downy mildew) were inoculated with isolates homozygous ( AA or aa ) or heterozygous ( Aa ) at the matching avirulence locus. Assessment of pathogen development revealed evidence for gene-dosage dependence in the expression of both resistance and avirulence. However, these effects were only observed in incompatible genotype combinations and the data only satisfactorily fit the hypothesis that specificity is regulated by the interaction between specific alleles for resistance ( R ) and avirulence ( A ). The implications of gene-dosage dependence to a mechanistic understanding of specificity are discussed.

Infection of lettuce by oospores of Bremia lactucae

A procedure which increased the frequency with which sexual progeny were recovered is described for Bremia lactucae. This facilitated the observation of penetration resulting from such infection.

The use of sexual progeny of Bremia lactucae Regel to improve the efficiency of selection for race-specific resistance genes to this fungus in lettuce

Resistance to Bremia lactucae REGEL (downy mildew) is an important attribute in lettuce cutivars. Race-specific resistance, governed by major genes, provides the basis for most commercial breeding programmes in that the control achieved although ephemeral, results in disease free crops. The screening procedures are well established, simple and are readily accommodated in selection programmes. In addition, the presence of particular resistance gene combinations may provide the proof of distinctness necessary for cultivar registration under EEC regulations. Genetic studies suggest that there are at least eleven race-specific R-factors available in cultivated lettuce (CRUTE & JOHNSON, 1976; JOHNSON et al., 1977; 1978) however, the number of different combinations of R-factors represented in commercially important cultivars is small (Table 1) considering the potential that exists with this number of available genes. Probably no cultivar carries more than three R-factors in combination. Some R-factors (e.g. R5, R9 and RlO) are present in commercially valueless cultivars and have not been exploited while others are rendered ineffective so frequently (e.g. RI) that incorporation into breeding material is worthless. Genetic linkage and allelism has also played a part in restricting the gene combinations employed (NORWOOD & CRUTE, 1980). Probably the most important determinant of which Rfactor combinations are developed and exploited is the virulence characteristics of the isolates used to screen the breeding material. These isolates should be fully representative of the pathogen population that a cultivar is likely to encounter when released commercially. This importance of the isolates used in screening is demonstrated by examining the recent history of European downy mildew resistant lettuce cultivars (CRUTE & DIXON, 1981). The majority of these have originated in the Netherlands and until 1977, selection was based on resistance against six Dutch B. lactucae races (NLl-6) (BLOK & VAN BAKEL, 1976). From the virulence phenotypes given in Table 2 it can be seen that resistance to all the races can be contributed by the resistance factor combination R2 + R3 + R7. Since these R-factors were already present uncombined in commercially