Oídio do tomateiro no Brasil: agentes causais, distribuição geográfica e fontes de resistência em Solanum (Lycopersicon)

Abstract

Powdery mildews (PMs) are important plant diseases induced by highly evolved, biotrophic fungi (Ascomycota: Erysiphales). PMs can cause severe yield losses in tomato (Solanum lycopersicum L.) crops, being easily recognized by the white colonies on the aerial plant parts, except fruits. PMs are widely distributed, inducing severe damages to protected crops and in drip-irrigated areas. A complex of three pathogens has been associated with the PMs on tomato and other Solanaceae worldwide: Erysiphe neolycopersici, Golovinomyces lycopersici (both inducing adaxial PM) and Leveillula taurica (abaxial PM). Although fungicides can be used, the use of resistant cultivars is the most efficient and sustainable strategy to control these pathogens. This dissertation aimed to identify and characterize morphologically and molecularly isolates of tomato PM-inducing pathogens in Brazil and to evaluate the reaction of accessions of the genus Solanum (section Lycopersicon) to isolates classified as E. neolycopersici. In Chapter II, 73 isolates were obtained from tomato plants showing PM symptoms/signs, from different areas in four geographic regions of Brazil. Of these, 14 were selected for molecular characterization via DNA sequencing of amplicons of the ITS-5.8S rDNA region obtained with a pair of primers (PMITS1/PMITS2) specific for identification of Erysiphales. A combination of morphological and molecular analyzes confirmed the predominance of E. neolycopersici isolates causing the adaxial PM in Brazil. Nationwide surveys indicated scarlet eggplant (S. aethiopicum var. gilo), potato (S. tuberosum) and Nicandra physaloides as natural hosts of E. neolycopersici isolates. Solanum acanthodes was reported for the first time as an experimental host of this pathogen. In Chapter III, the reaction of 174 accessions and eight experimental interspecific hybrids of the genus Solanum (Lycopersicon) was evaluated to E. neolycopersici isolates. High levels of resistance were detected in a restricted number of wild species accessions. Most of the S. habrochaites accessions showed an immunity-like response to the pathogen. Solanum chilense ‘CNPH 0410’ and S. peruvianum ‘CNPH 0201’ were also immune, corresponding to the first record of these accessions as new sources of PM resistance. Solanum pennellii ‘CNPH 0409’ proved to be highly resistant to E. neolycopersici isolates. The S. arcanum accession ‘LA-2172’ (original source of the Ol-4 gene) showed a significant recovery of symptoms and behaved as resistant. Plants of interspecific experimental hybrids involving susceptible S. lycopersicum accessions and resistant S. habrochaites accessions showed no symptoms, indicating the presence of dominant resistance factors in these wild accessions. In Chapter IV, eight L. taurica isolates (three from tomato, three from Capsicum and two from scarlet eggplant) were obtained from growing areas in the northeast and central-west regions of Brazil. All plants showed characteristic abaxial symptoms and signs of the pathogen. The isolates were characterized morphologically and molecularly by sequencing segments of the ITS-5.8S rDNA region using the PMITS1/PMITS2 primer pair. The fungal structures observed and the molecular characterization of the eight isolates corresponded to those described in literature as L. taurica. In conclusion, the results obtained in the present work indicated that E. neolycopersici (inducing adaxial PM) and L. taurica (inducing abaxial PM) are widely distributed in tomato crops in Brazil. Germplasm accessions identified and characterized with high levels of resistance to E. neolycopersici isolates can be employed in tomato breeding programs aimed at developing cultivars more adapted to tropical and subtropical regions of Brazil.