The early stages of interaction between effective and non-effective race-specific genes in Lactuca sativa, wild Lactuca spp. and Bremia lactucae (race NL16)

The early stages of Bremia lactucae development and the tissue response of nine genotypes of Lactuca sativa and closely related wild species (L. serri ola, L. saligna, L. virosa) were microscopically examined on leaf discs fro m adult plants 3, 6, 12, 24, 36 and 48 h after inoculation. Intensity of sp orulation on the cotyledons and the leaf discs was recorded for 14 days aft er inoculation to obtain information on the phenotypic expression of host r eaction. The experimental set of Lactuca spp. accessions was composed to re present different reactions to a highly virulent race NL16 of B. lactucae. The set represented three susceptible genotypes with ineffective race-speci fic resistance genes and six genotypes with different resistance mechanisms . Analysis of the histological components of resistance gave data concerning the differences in expression in genotypes with different resistance. Resis tance mechanisms associated with early hypersensitive reaction (HR) and arr est of the pathogen immediately following formation of the primary infectio n structures was found in the L. virosa accessions. The proportion of infec tion sites with necrotic epidermal cells and extent of necrosis was the hig hest in L. serriola and L. virosa accessions where subepidermal necrosis (S EN) also occurred. The mechanism of L. saligna resistance seems to be based on restriction the pathogen in the latter stages of infection, however, di ffering between accessions with expected race-specific resistance and no ef fective race-specific resistance (non-host?).