Identification of stable sources of rate-reducing resistance to Septoria lactucae in lettuce germplasm

Abstract

Septoria leaf spot, caused by Septoria lactucae, is one of the major open-field diseases of lettuce (Lactuca sativa) in tropical and subtropical regions. However, there are few available studies dealing with the identification of useful sources of resistance to this disease. Here, a genetically diverse collection of 42 lettuce accessions was evaluated for Septoria leaf spot reaction under open-field conditions (natural inoculum) during the rainy season. The experimental design was randomized blocks with three replications (16 plants each). Although no immunity-like source was detected, 21 accessions displayed superior levels of rate-reducing resistance with significant lower Area Under Disease Progress Curve (AUDPC) values, encompassing seven accessions of the ‘Crispy green loose-leaf’, eight of the ‘Crispy purple loose-leaf’, two of the ‘Cos/Romaine’, two of the ‘Loose-leaf super crispy’, one accession of the ‘Butterhead’, and one accession of the ‘Iceberg’ morphotype. A subgroup of nine accessions with the highest levels of field resistance was also inoculated under greenhouse conditions with four S. lactucae isolates (collected in distinct geographic areas) in two assays at two distinct phenological stages. The cultivars ‘BRS Mediterrânea’ and ‘Vanda’ displayed lowest AUDPC values at the late vegetative/reproductive phenological phase. The remaining accessions displayed intermediate values of AUDPC, with the exception of the cultivar ‘Rubi’ (control), which exhibited moderate levels of susceptibility. The availability for lettuce breeding programs of genetic sources with higher and stable levels of resistance to S. lactucae will enable the development of more adapted commercial cultivars, increasing the sustainability of this crop across Neotropical regions.