Novel sources of resistance/tolerance to orthotospoviruses in Lactuca germplasm identified via field exposure to viruliferous vectors and in greenhouse mechanical transmission assays

Abstract

The ‘spotted wilt’ disease (induced by a complex of orthotospoviruses) is one of the main biotic constraints of lettuce (Lactuca sativa L.) production in South America. Genetic resistance to orthotospoviruses is the most practical and sustainable management strategy due to the low efficiency of chemical control of their thrips vectors. Thus far, conventional breeding sources of resistance have been detected in the genus Lactuca only to tomato spotted wilt orthotospovirus (TSWV) and Impatiens necrotic spot orthotospovirus (INSV). However, the major sources of TSWV and INSV resistance proved to be highly susceptible to Brazilian isolates of groundnut ringspot orthotospovirus (GRSV), which is the main orthotospovirus under Neotropical conditions. Herein, we evaluated the reaction to GRSV and TSWV isolates of a diverse germplasm collection (comprising 65 Lactuca accessions) under open-field conditions with natural GRSV inoculum. Nine accessions with superior levels of performance under field conditions (incidence below 20%) were subsequently selected for a set of experiments using controlled mechanical inoculation with three GRSV and two TSWV isolates. Sources of immune-like resistance were detected only for TSWV isolates but not for GRSV in greenhouse assays. However, three accessions displayed significantly lower GRSV incidence even under high temperature regimens. Additional experiments are necessary to verify the presence of vector-interfering traits in the accessions that displayed exclusively field-resistance/tolerance. This is the first study reporting the detection of novel sources of simultaneous field resistance/tolerance against GRSV and TSWV isolates, thus representing potential donors of genes for incorporation into elite inbred lines and cultivars of lettuce.