Further work on the genetics of race specific resistance in lettuce (Lactuca sativa) to downy mildew (Bremia lactucae)

Abstract

SUMMARYData are presented on the segregation of resistance to four races of Bremia lactucae in the F2 progenies of crosses involving 15 resistant and various susceptible lettuce cultivars. Most of these data and those recently published by other workers fit the systematic model for the genetics of race specific resistance to B. lactucae proposed by Crute & Johnson(1976).Seven different dominant resistance genes of major effect were found. There was also evidence of a pair of dominant genes with complementary effect, one of which may sometimes be effective on its own. Two of the genes may be linked and another may have two different alleles for resistance at the same locus. The resistance conferred by these genes is specified in relation to two British, two Dutch and four French races of the fungus. Resistance genotypes are proposed for 16 cultivars.

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

“…The characteristics and inheritance of the presently recognised factors for specific resistance to B. lactucae in lettuce are well documented (CRUTE & JOHNSON, 1976a;JOHNSON et al, 1977JOHNSON et al, ,1978NORWOOD & CRUTE, 1979). Race specific resistance is under the control of major genes and is associated with different degrees of host cell necrosis depending on both R-factor, environment and genetic background (CRUTE & NOR-WOOD, 1978).…”

“…Since the early 1920's over 200 cultivars carrying some resistance have been bred or identified but resistance has always proved highly race specific. Commercial cultivars carry different combinations of resistance factors (R-factors) most of which are single dominant genes, designated Dm (JOHNSON et al, 1977(JOHNSON et al, , 1978NORWOOD & CRUTE, 1979). Factors of unknown quality continue to be referred to by their R prefix.…”

An association between resistance to root aphid (Pemphigus bursarius L.) and downy mildew (Bremia lactucae Regel) in lettuce

“…Resistance to certain isolates used is contributed in some cultivars by other resistance genes in addition to Dm-6. Additional to the interactions with key isolates of B. lactucae, Dm-6 is distinguishable from some of the other racespecific resistance genes because it conditions the incomplete resistance response, described by CRUTE & NORWOOD (1978). The isolate 74/T which was virulent on all resistance factors Rl-10 was used as a control rendering all cultivars tested fully susceptible.…”

“…Recently, CRUTE & JOHNSON (1976) interpreted data on specific resistance to downy mildew in lettuce cultivars in terms of a gene-for-gene type of relationship (PERSON, 1959). This hypothesis has since been shown to be valid by the genetical studies of JOHNSON et al (1977JOHNSON et al ( , 1978. CRUTE & JOHNSON'S (1976) studies indicated that Avoncrisp and Avondetiance differed from most other lettuce cultivars in their resistance to isolates of B. lactucae by virtue of the presence of a race-specific resistance factor they called R6.…”

“…CRUTE & JOHNSON'S (1976) studies indicated that Avoncrisp and Avondetiance differed from most other lettuce cultivars in their resistance to isolates of B. lactucae by virtue of the presence of a race-specific resistance factor they called R6. this factor was later identified as a single dominant gene (Dm-6) (JOHNSON et al, 1977(JOHNSON et al, , 1978. Subsequently, several other lettuce cultivars carrying gene Dm-6 have been identified using the appropiate B. lactucae isolates and have been tested for resistance to lettuce root aphid.…”