Lactuca georgica, a new wild source of resistance to downy mildew: comparative study to other wild lettuce relatives

Seedlings of 431 accessions representing six and five Armenian natural populations of the wild Lactuca georgica and L. altaica, respectively, and 32, 23, and 20 populations of L. saligna, L. serriola, and L. aculeata, respectively (mostly Israeli natural populations), were screened at seedling stage for resistance to six highly virulent races of Bremia lactucae - the causal of lettuce downy mildew - that were found on several widely grown cultivars and represent the virulence spectrum in the main lettuce production areas in Europe and California. This study is likely the first detailed screening of resistance to B. lactucae races in natural populations of L. georgica and L. altaica. The highest average resistance probability and frequency of highly resistant accessions across races were detected in L. georgica. These were even higher than those shown by the germplasm of known non-host resistance (NHR) species L. saligna, which classified to the secondary lettuce gene pool (LGP-2). Thus, we suggest that L. georgica is a new LGP-2 source of resistance to B. lactucae. Our findings also support previous observations that L. aculeata, a species within the LGP-1, should be prioritized as a source of downy mildew resistance in domesticated lettuce (L. sativa) breeding programs, since some L. aculeata accessions expressed highly resistance across races. For L. serriola, the closest relative of cultivated lettuce, and for L. altaica, also a closer or even a primitive form of L. sativa, probabilities of resistance to each of the investigated B. lactucae races were very low.