Lettuce genotype resistance to "soft rot" caused by Pectobacterium carotovorum subsp. carotovorum

Abstract

Soft rot, caused by Pectobacterium carotovorum subsp. carotovorum (Pcc), is the main bacterial disease affecting lettuce (Lactuca sativa L.) crops in Brazil and leads to significant yield losses. This study aimed to assess the reaction of lettuce genotypes to soft rot induced by a virulent isolate and the stability of the resistance to three isolates varying in virulence. Using a descriptive ordinal scale ranging from 1 to 9 a classification system was defined: class 1 = resistant (R): severity (Sev) < 1.5; class 2 = moderately resistant (MR): 1.5 < Sev < 2.5; class 3 = susceptible (S): 2.5 < Sev < 3.5; class 4 = highly susceptible (HS): Sev >; 3.5). Of the 41 tested genotypes, 14 were classified as MR and 27 as S when inoculated with a Pcc isolate of intermediate virulence. Eleven of these genotypes (four S and seven MR) were selected to test their resistance stability against three other isolates with an increasing degree of virulence (Pcc36 < Pcc-A1.1 < Pcc-23). Out of the 11 genotypes eight retained the original classification and three moved from S to MR resistant class when challenged with the least virulent isolate. Vitória de Santo Antão was the only genotype classified as MR for all tested isolates and is a promising candidate for durable soft rot resistance breeding.