Variation for Resistance to Verticillium Wilt in Lettuce (Lactuca sativaL.)

Abstract

Hayes, R. J., Vallad, G. E., Qin, Q.-M., Grube, R. C., and Subbarao, K. V. 2007. Variation for resistance to Verticillium wilt in lettuce (Lactuca sativa L.). Plant Dis. 91:439-445.Host resistance offers the most cost-effective method of Verticillium wilt control in lettuce (Lactuca sativa). In 2004 and 2005, 107 and 22 lettuce cultivars, respectively, were screened for resistance in a field infested with Verticillium dahliae, and disease progress on resistant and susceptible cultivars was determined. Greenhouse experiments were conducted to evaluate 16 cultivars for resistance to a race 1 and a race 2 isolate. Significant differences for resistance were observed within cultivated lettuce. In susceptible cultivars, disease levels increased through the season, whereas disease in resistant cultivars remained constant. Resistance in greenhouse tests was dependant upon the race used. Seven cultivars were resistant to race 1, whereas all were susceptible to race 2. Cultivar reactions to race 1 in greenhouse and field experiments were correlated, indicating the utility of greenhouse evaluations. The identification of resistance in diverse lettuce types is beneficial to the breeding process. However, because of the existence of resistance-breaking race 2 isolates, this resistance may not be durable. Alternatively, targeted releases of race-1-resistant cultivars to fields with only race 1 pathogen genotypes may extend the life of these cultivars.

Colonization of Resistant and Susceptible Lettuce Cultivars by a Green Fluorescent Protein-Tagged Isolate of Verticillium dahliae

“…Principal among them are its well-defined taproot system, rosette pattern of growth, the acuteness of symptom development that coincide with vegetative maturity, the development of microsclerotia in basal leaves prior to plant death, and the seedborne nature of the pathogen (56,58). Of even greater importance to the disease management is the presence of two genetically distinct races and several lettuce cultivars that can differentiate the two races (27,46,57). This being the only characterized race structure for V. dahliae other than that described for tomato (1,48).…”

“…Resistance conferred by the Ve genes in tomato, restricted the colonization of incompatible, race 1 isolates of V. dahliae and V. albo-atrum to the roots, allowing only limited spread to stem and foliar tissues (25,55). Several potential sources of resistance to V. dahliae were recently identified in a screen of heirloom and cultivated lettuce germplasm (27). Resistance appeared complete and specific against most isolates of V. dahliae pathogenic on lettuce (57).…”

Diversity, Pathogenicity, and Management of Verticillium Species

“…Genetic resistance to Verticillium wilt was described in alfalfa, cotton, potato, tomato, strawberry, sunflower, oilseed rape, lettuce (65,109,129), and other crops. Characterization of resistance at the genetic and physiological levels is best characterized in tomato, potato, and cotton.…”

“…Salinas were designated as race 1 (162). Although some lettuce cultivars, breeding lines, or accessions show some form of resistance or tolerance to race 2 of the pathogen (R. J. Hayes, personal communication), resistance to race 2 of the pathogen has not been identified in lettuce, and brings into question the durability of race 1 resistance (65). Race 1 resistance in lettuce is conditioned by a single dominant gene (64), but whether the same factor(s) control(s) race 1 resistance in both lettuce and tomato is unknown.…”

“…Race 2 steadily supplanted race 1 in various regions of the world because of the extensive use of race 1-resistant cultivars (41). Two races were also described in lettuce (65,162), and germplasm with resistance to race 1 was identified (65). However, there remains no source of resistance to race 2 in either tomato or lettuce.…”