Phenotypic expression and species-specific reaction of Lactuca sativa ‘PI 342444’ to groundnut ringspot virus (GRSV) and tomato spotted wilt virus (TSWV) isolates

Abstract

Currently, diseases caused by tospovirus species are among the most limiting factors for lettuce (Lactuca sativa L.) production in Brazil, where losses can reach up to 100%. In cultivated lettuce germplasm, the highest levels of resistance to tospoviruses were identified in the accession ‘PI 342444’ (= cultivar ‘Tinto’). We carried out controlled inoculation assays aiming to characterize the phenotypic expression and verify the spectrum of ‘PI 342444’-mediated resistance against a panel of isolates of two tospovirus species: Tomato spotted wilt virus (TSWV) and Groundnut ringspot virus (GRSV). Lettuce seedlings were mechanically inoculated with seven isolates (three TSWV and four GRSV). Plants of ‘PI 342444’ did not exhibit symptoms after inoculation with TSWV isolates, whereas severe symptoms were observed on the susceptible lettuce control (cultivar ‘Vanda’). The absence of TSWV infection in the majority of ‘PI 342444’ plants was also confirmed by serological testing. In contrast, all ‘PI 342444’ plants inoculated with GRSV isolates displayed early onset of severe symptoms, indicating that this resistance is not effective against this viral species. Further studies shall be conducted to characterize the genetic factors associated with a putative TSWV-specific resistance observed in ‘PI 342444’. On the other hand, our assays indicated that additional lettuce germplasm screenings searching for sources of resistance effective against GRSV isolates are among the major breeding priorities under Brazilian conditions, since this ‘PI 342444’ resistance-breaking virus is currently the predominant lettuce-infecting tospovirus species in the country.