ARS | Publication request: Selection for resistance to Verticillium wilt caused by race 2 isolates of Verticillium dahliae in accessions of lettuce (Lactuca sativa L.).

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 23, 2011
Publication Date: February 1, 2011
Citation: Hayes, R.J., Maruthachalam, K., Vallad, G.E., Klosterman, S.J., Subbarao, K.V. 2011. Selection for resistance to Verticillium wilt caused by race 2 isolates of Verticillium dahliae in accessions of lettuce (Lactuca sativa L.).HortScience. 46(2):201-206.

Interpretive Summary: The disease Verticillium wilt of lettuce caused by the soil dwelling fungus Verticillium dahliae can severely damage lettuce crops, rendering them unharvestable. Developing cultivars of lettuce that resist the disease is the most economical and environmentally friendly control method. The fungal strains that attack lettuce exists as two races (races 1 and 2), and resistance is only known against race 1. Resistance to race 2 has not been reported, and repeated use of race 1 resistant cultivars by lettuce growers will likely select for and increase the frequency of race 2 strains. Resistance to race 2 strains needs to be identified for this reason. In this research, two separate populations totaling 314 distinct collections of lettuce known as Plant Introductions (PI) were evaluated for resistance to Verticillium wilt caused by a race 2 V. dahliae isolate. No PIs with complete resistance was identified, although PIs with partial resistance were selected. Four PIs (169511, 171674, 204707, and 226641) were selected for further testing in three replicated greenhouse experiments, and demonstrated significantly improved resistance compared to susceptible control cultivars. The results indicate that lettuce possesses genetic variation for partial resistance to a race 2 isolate of V. dahliae. The resistant PIs selected in this research are morphologically different, and no association between rate of bolting and resistance was found. PIs with partial resistance may be useful for breeding new lettuce cultivars with resistance to Verticillium wilt caused by race 2 isolates of V. dahliae.

Technical Abstract: Verticillium wilt of lettuce caused by Verticillium dahliae can cause severe economic damage to lettuce producers. The pathogen exists as two races (races 1 and 2) in lettuce, and complete resistance to race 1 is known. Resistance to race 2 isolates has not been reported, and production of race 1 resistant cultivars will likely increase the frequency of race 2 strains. The objective of this research was to select lettuce accessions for resistance to race 2 isolates of V. dahliae. Two independent populations totaling 314 randomly sampled Plant Introductions (PI) were evaluated for Verticillium wilt disease incidence (DI) caused by V. dahliae isolates VdLs17 in one unreplicated and two replicated greenhouse experiments. Selection for PIs with reduced DI was conducted between each experiment and plant stems were plated on semi-selective media to identify colonized plants that remained non-symptomatic. No accession with complete resistance was identified, although accessions with partial resistance were selected. Genetic variation for the frequency of V. dahliae colonized plants that remain symptomless was detected. Four PIs (169511, 171674, 204707, and 226641) were selected for further testing in three replicated greenhouse experiments, and demonstrated significantly lower disease incidence than the susceptible control cultivars. The results indicate that cultivated lettuce has genetic variation for partial resistance to a race 2 isolate of V. dahliae. The resistant PIs selected in this research are morphologically diverse, and no dependence between rate of bolting and resistance was found. PIs with partial resistance may be useful for breeding lettuce cultivars with resistance to race 2 isolates of V. dahliae.