Histological aspects of the response of wild Lactuca spp. and their hybrids, with L. sativa to lettuce downy mildew (Bremia lactucae)

A histological study was carried out on the interactions between a highly virulent isolate (CS9) of Bremia lactucae (lettuce downy mildew) and wild Lactuca spp. and their interspecific hybrids with the cultivated lettuce, L. sativa. Early stages of infection (24 and 48h after inoculation) were investigated in leaf discs from adult plants of 15 genotypes of L. sativa, L. serriola, L. virosa and L. sativa × wild Lactuca spp. The L. sativa cultivars excepted, all accessions and hybrids were completely or partially resistant at the seedling and adult plant stages, according to phenotypic expression. There were significant differences in the length of germ tubes formed 24h after inoculation. Those on the wild Lactuca spp. were significantly shorter than those on the interspecific hybrids derived from them. There was no correlation between length of germ tube and interaction phenotype. Significant differences were recorded among the Lactuca spp. and hybrids in the development and size of infection structures and in plant tissue response. At least four types of resistance response were recognized on a microscopic level. The results provide evidence for different resistance mechanisms in various wild Lactuca spp. that cannot be distinguished by their macroscopic phenotype.