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

Abstract

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.

Aspects of the Interactions between Wild Lactuca Spp. and Related Genera and Lettuce Downy Mildew (Bremia Lactucae)

“…Some other R-factors were not assigned to a linkage group (Table 5). From other studies (Lebeda, 1983(Lebeda, , 1986Norwood et al, 1981;Bonnier et al, 1992) it is evident that numerous additional resistance genes might be present in L. serriola accessions. The occurrence of race-specificity will also probably be a very common phenomenon in the L. serriola -B. lactucae pathosystem (Lebeda, 1984b(Lebeda, , 1986Lebeda and Petrželová, 2001).…”

“…Norwood et al (1981) found that it was not possible to clearly interpret the segregation data from progeny because of the sparse sporulation, frequent occurrence of necrosis and difficulty in classifying the infected seedlings into discrete categories. Current thoughts are that this resistance is based on some major gene(s) and modifiers (Table 5).…”

“…Norwood et al (1981), Lebeda (1983Lebeda ( , 1984cLebeda ( , 1986 and recognised that accessions PI 281876 and PI 281877 in the seedling stage were infected by some B. lactucae isolates, however the intensity of sporulation was mostly very low and in some interactions followed by an expression of a necrotic response. According to Norwood et al (1981) it is improbable that the resistance of these accessions is governed by the same factors since PI 281877 showed extensive necrosis, whereas the response of PI 281876 was more discrete. This characteristic response of this accession was recently confirmed by more detailed histological study (Lebeda and Pink, 1998).…”

Nonhost wild lettuce as a donor for resistance to downy mildew in cultivated lettuce

“…The broad resistance spectrum of L. saligna was described in a couple of studies since 1976 (Netzer et al 1976;Norwood et al 1981;Bonnier et al 1992), but has not been graphically visualized. To illustrate the host and nonhost classification in Lactuca species, we have visualized a large dataset obtained from the Dutch Centre for Genetic Resources (CGN) of 55 four Lactuca species and their B. lactucae infection scores at seedling stage, supplemented with some seedling disease test data for three accessions not tested by CGN.…”

“…The cultivated host species L. sativa is cross-compatible with the nonhost species L. saligna. Since 1976, several studies indicate that L. saligna accessions are totally disease-free (Netzer et al, 1976;Norwood et al, 1981;Lebeda, 1986;Gustafsson, 1989;Bonnier et al, 1992;Petrželová et al, 2011;Van Treuren et al, 2011). Histology studies suggested that germinated conidia in L. saligna were arrested before normal hyphae formation (Lebeda et al, 2008;Zhang et al, 2009b).…”