ARS | Publication request: Evaluating the Potential Utility of Partial Resistance to <i>bremia Lactucae</i> from the Lettuce Cultivars 'grand Rapids' and 'iceberg'.

Submitted to: Phytopathology
Publication Acceptance Date: March 8, 2004
Publication Date: August 20, 2004
Citation: Grube, R.C. Evaluating The Potential Utility Of Partial Resistance To Bremia Lactucae From The Lettuce Cultivars 'grand Rapids' And 'iceberg'. Phytopathology. 2004. V. 94. P. S36.

Technical Abstract: Single dominant Dm genes that confer race-specific resistance to downy mildew (Bremia lactucae) have historically had short effective life spans once deployed in lettuce cultivars. Partial or 'field resistance' (FR) has been investigated as an alternative and possibly more durable host resistance strategy. Most studies of FR, including ongoing mapping efforts, have focused on the cultivar 'Iceberg'. We sought to evaluate the utility of the cultivar 'Grand Rapids' as a source of FR, particularly in comparison to 'Iceberg'. Field trials were conducted in the Salinas Valley of California over a three-year period to examine responses of both cultivars to endemic populations of B. lactucae. Both cultivars remained nearly free of symptoms, with a level of resistance comparable to cultivars with effective Dm genes. Both FR cultivars were crossed with susceptible cultivars and evaluated for response to B. lactucae. Susceptibility of F1 progeny established that FR was recessive, and similar segregation of FR in F2 and other populations suggested similar inheritance for both cultivars. A large proportion of 'Grand Rapids' x 'Iceberg' F2:4 families were susceptible, implying that FR genes in the two cultivars were not allelic. Based on the susceptibility of cultivars with known genes in field experiments as well as on seedling screens using characterized isolates, FR from Grand Rapids could not be attributed to any known Dm genes. Our results suggest that 'Grand Rapids' may be useful as a source of one or more novel B. lactucae resistance genes.