Manejo da podridão mole em couve-chinesa e alface

Abstract

The soft rot caused by Pectobacterium carotovorum subsp. carotovorum is the main bacterial disease of Chinese cabbage and lettuce in Brazil and the world. Chemical control of this disease has not been efficient, being necessary to search for new alternatives of control. Given the growing importance of soft rot as a limiting factor for the production of these vegetables in the regions of the Agreste and Mata of Pernambuco state, Brazil and the difficulty of effective control measures, the present study aimed to: (i) determine the effect Calcium in the control of soft rot in Chinese cabbage, (ii) select lettuce cultivars as promising sources of resistance to soft rot. Were tested two sources of calcium in different concentrations, calcium nitrate [Ca(NO3)2] a 0.00; 0.15 and 0.3 g L-1 and calcium chloride [CaCl2] a 0.00; 1 and 5 g L-1 applied leaf spraying and soil drench. When applied with both methods, Ca(NO3)2 was effective in controlling soft rot, as it reduced the disease severity by up to 48.5% when sprayed onto the leaves (0.15 g L-1). A significant increase in the leaf calcium content was observed only in the plants that were sprayed with higher doses of Ca(NO3)2 and CaCl2. In all of the calcium treatments, light microscopy analyses revealed an increased number of chloroplasts and improved structuring of the palisade parenchyma, while transmission electron microscopy analyses revealed an increased cell wall thickness that was especially evident for the 0.15 g L-1 Ca(NO3)2 treatment applied by leaf spraying and soil drenching. Of the 41 genotypes tested, 14 were moderately resistant when inoculated with Pcc-C, with severity scores ranging from 3.5 to 4.0; 27 genotypes were susceptible. Eleven of these genotypes, four susceptible and seven moderately resistant were selected to test their resistance stability against three pathogen isolates with different degrees of virulence (Pcc-36, Pcc-A1.1 and Pcc-23). Most of the genotypes evaluated (77%) exhibited the same reaction observed in the selection assays only against isolate Pcc-36: Veneza Roxa was susceptible, while Alface Grega, Mimosa Salad Bowl Roxa, Livia, Livinia, Salad Bowl, Vitória de Santo Antão and Saia Veia were moderately resistant. Vitória de Santo Antão was the only genotype that was also moderately resistant to isolates Pcc-A1.1 and Pcc-23. This genotype can be used as a promising source of stable and durable soft rot resistance.