Mancha de Stemphylium: etiologia, gama de hospedeiras e identificação de novas fontes de resistência em Capsicum e tomateiro

Abstract

The family Solanaceae includes many species of agricultural importance such as tomato (Solanum lycopersicum L.) and peppers and sweet peppers (Capsicum spp.), which are widely used as vegetables around the world. However, these plants might be affected by a series of pathogens inducing severe direct and indirect losses. Stemphylium leaf spot deserves special attention, especially in tomatoes, since the disease can reduce crop yield and quality. Symptoms occur on both leaf sides and consist of small dark spots, which enlarge and become grayish brown, compromising the photosynthetic surface. In tomato, the disease is caused by a complex of at least three species of Stemphylium: S. solani, S. lycopersici (= S. floridanum) and S. botryosum. In the genus Capsicum, the disease is caused by three Stemphylium species: S. solani, S. lycopersici, and S. vesicarium. In this context, the objectives of the present work were: (1) To generate information related to the etiology and the host range of the causal agent of Stemphylium leaf spot in Brazil; (2) To identify new sources of resistance in Solanum (Lycopersicon) and Capsicum germplasm and (3) to carry out an inheritance study with a new source of genetic resistance to S. lycopersici in an accession of the wild species S. habrochaites. Phylogenetic analyzes of the ITS-5.8S rDNA and glyceraldehyde-3-phosphate dehydrogenase (gapdh) genomic regions of 89 Stemphylium isolates obtained in species of the Solanaceae family. The isolates were placed in two distinct groups with either S. lycopersici or S. solani. Isolates of S. lycopersici (n=81) were detected in tomato, potato, eggplant, S. paniculatum, and Physalis angulata. Isolates of S. solani (n=8) were detected in S. aethiopicum var. gilo and tomato. The reaction of accessions from 18 plant species was evaluated for two S. lycopersici isolates with 12 of them displaying a susceptible reaction. In search of alternative sources of resistance, 40 accessions of Solanum (Lycopersicon) were inoculated with one S. lycopersici isolate. Twenty-seven accessions were detected with high levels of resistance in the species S. habrochaites (13), S. peruvianum (13) and S. pimpinellifolium (1). Solanum habrochaites ‘PI-127827’ was detected as a new source of resistance to S. lycopersici. A subgroup of resistant accessions showed molecular markers distinct from the Sm locus, indicating potential new resistance genes/alleles. The susceptible cultivar S. lycopersicum ‘Ponderosa’ was used as a female parent in interspecific crosses with ‘PI-127827’. The chi-square test in the F2 generation showed a good fit with a segregation pattern 9 (resistant): 7 (susceptible), indicating a genetic model of resistance under the control of two dominant, independent, and complementary genes. The reactions of the 37 Capsicum accessions were characterized based on epidemiological parameters. The accessions C. annuum ‘Jalapeño TS’ and C. annuum var. glabriusculum ‘CNPH–3839’ showed an immunity-like reaction against two S. lycopersici isolates. In conclusion, new and relevant information was generated that could contribute to the cultural and genetic management of Stemphylium spot in Brazil.