Caracterização de fontes de resistência genética contra espécies de Berkeleyomyces e Septoria lactucae em Lactuca sativa

Abstract

Lettuce (Lactuca sativa L.) is the most important leafy vegetable in Brazil and across the world. Lettuce cultivars are grouped into different morphotypes according to their morphological characteristics with special emphasis on the segments ‘Butterhead’, ‘Crispy loose-leaf’, ‘Iceberg’, ‘Cos/Romaine’, ‘Batavian’, and ‘Mimosa’. The yield of lettuce cultivars from the morphotypes ‘Iceberg’ and ‘Butterhead’ have been severely affected by two fungal diseases: Septoria leaf spot (caused by isolates of Septoria lactucae) and black root rot disease (caused by isolates of Berkeleyomyces basicola and B. rouxiae). The present investigation aimed to identify and characterize sources of resistance to S. lactucae as well as against the two Berkeleyomyces species in L. sativa germplasm. Genetic inheritance studies of resistance against isolates of the two Berkeleyomyces species were also performed. Three sets of experiments were carried out at Embrapa Hortaliças in Brasilia-DF during 2021 and 2022. In Chapter 2, forty-two (42) lettuce accessions were evaluated under field conditions (during the rainy season) for reaction to S. lactucae. Nine cultivars categorized as resistant under field conditions were subsequently inoculated with four isolates of S. lactucae under greenhouse conditions. The Area Under the Disease Progress Curve (AACPD) values were significantly lower for the cultivars ‘BRS Mediterrânea’ and ‘Vanda’ (from the ‘Crispy loose-leaf’ morphotype). The other cultivars displayed a tolerance reaction, with intermediate AACPD values, except for the cultivar ‘Rubi’ (control), which displayed moderate susceptibility to the pathogen. In Chapter 3, 68 lettuce cultivars were initially inoculated with an isolate of B. basicola. A second experiment was conducted using 33 cultivars classified as resistant in the first experiment. These accessions were subsequently inoculated with four isolates (two of B. basicola and two of B. rouxiae). A strong correlation was observed between resistance/susceptibility to B. basicola and B. rouxiae. The morphotypes ‘Cos/Romaine’ and ‘Batavian’ showed a higher frequency of resistant accessions. Variation in the reaction to these pathogens was observed among accessions of the morphotypes ‘Crispy loose-leaf’, ‘Mimosa’ and ‘Iceberg’. Most of the lettuces of the ‘Butterhead’ morphotype displayed a susceptible reaction. In Chapter 4, genetic inheritance studies for resistance to both Berkeleyomyces species were carried out under controlled conditions. Different crosses were carried out between the resistant parent ‘La Brillante’ (from the ‘Batavian’ morphotype) and the cultivar ‘Elisa’ (susceptible parent from the ‘Butterhead’ morphotype). Morphological analysis and molecular assays with RAPD markers were used to confirm the hybrid origin of lettuce plants obtained via controlled crosses. Two individual plants were confirmed as F1 hybrids via genotyping and then self-fertilized to obtain two segregating F2 populations. A total of 413 F2 plants were inoculated separately with B. rouxiae (247 plants) and B. basicola (166 plants). Chi-square (χ2) tests at 5% were performed to estimate the best fit of the observed segregation patterns to the predicted Mendelian ratios. Highly contrasting responses were observed between the parents ‘La Brillante’ (resistant) and ‘Elisa’ (susceptible) after inoculation with the two fungi. Segregations in the F2 population displayed a good fit to a dominant monogenic model for resistance against both fungal species. In conclusion, the results obtained in the present work indicate that there are different resistance/susceptibility levels across the lettuce morphotypes in relation to Berkeleyomyces species and S. lactucae. The identification and genetic characterization of novel sources of resistance against these pathogens represents an important contribution to lettuce breeding. The management of these diseases using resistant cultivars can assure higher levels of sustainability in the cultivation of this vegetable in tropical and subtropical regions of Brazil and across the world.