Fifteen years of verticillium wilt of lettuce in america’s salad bowl: a tale of immigration, subjugation and abatement.

Authors: ATALLAH, ZAHI - University Of California; Hayes, Ryan; SUBBARAO, KRISHNA - University Of California

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2011
Publication Date: 4/12/2011
Citation: Atallah, Z.K., Hayes, R.J., Subbarao, K.V. 2011. Fifteen years of verticillium wilt of lettuce in america’s salad bowl: a tale of immigration, subjugation and abatement. Plant Disease. DOI:10.1094/PDIS-01-11-0075.

Interpretive Summary: Lettuce is a popular leafy vegetable that is globally cultivated. The US ranks second in production, with coastal California producing half of the US supply. In 1995, the plant disease Verticillium wilt caused by the soil living fungus Verticillium dahliae was identified as a disease of lettuce in coastal California. The fungus and disease have subsequently spread throughout the Monterey County lettuce production district. In addition to lettuce, the fungus can cause disease on about 400 plant species. The symptoms of Verticillium wilt of lettuce typically strike at or near harvest maturity, and include a greenish black discoloration inside the root, and foliar wilting, yellowing and death. In many infested fields, complete loss of the crop can occur. The fungus produces long-lived microsclerotia that are deposited into the soil that can infect subsequent crops. In many infested fields, complete losses can occur when soil inoculums densities are greater than 100 microsclerotia / g. This threshold for disease is relatively high among crops that are susceptible to the fungus. In infected lettuce plants, the fungus produces long-lived microsclerotia that are deposited into the soil to infect subsequent crops. The amount of microsclerotia returned to the soil from lettuce can vary greatly, but could be as high as several million per infected plant in conditions optimal for the fungus. The fungus can enter and survive in the seed of lettuce and other crops grown in rotation with lettuce. The distribution and planting of infecting seed may increase the spread of novel V. dahliae strains into new and existing lettuce production districts. New fungal strains may resist or overcome current control efforts, such as resistant cultivars. Soil fumigation is currently the most widely used control method. However, due to the high fumigation costs, fumigated fields are typically planted to strawberry to recover the costs associated with fumigation. This is typically followed by two to three crops of lettuce, at which time V. dahliae inoculum densities will have recovered to pre-fumigation levels. Lettuce cultivars with resistance to Verticillium wilt represent the best long-term control method, and two races of V. dahliae were described on lettuce (race 1 and race 2). Complete resistance (plants that have no symptoms) to race 1 is available, and is being bred into commercial acceptable cultivars adapted to coastal California. Widespread production of race 1 resistant cultivars may lead to a selective sweep in favor of race 2 strains, to which there is no known complete resistance. Therefore, there is an urgent need to identify lettuce germplasm exhibiting high levels of resistance to race 2 of V. dahliae.

Technical Abstract: Lettuce is a popular leafy vegetable that is globally cultivated. The US ranks second in production, with coastal California producing half of the US supply. In 1995, Verticillium wilt caused by the soil borne fungus Verticillium dahliae was identified as a disease of lettuce in coastal California, and the pathogen and disease have subsequently spread throughout the Monterey County lettuce production district. In addition to lettuce, the fungus can cause disease on about 400 plant species. The symptoms of Verticillium wilt of lettuce typically strike at or near harvest maturity, and include a greenish black vascular discoloration, and foliar wilting, chlorosis and necrosis. Symptoms on leaves typically display a characteristic V-shape pattern. In many infested fields, complete losses can occur when soil inoculums densities are greater than 100 microsclerotia / g. This threshold for disease is relatively high among host crops. In infected lettuce plants, the fungus produces long-lived microsclerotia that can remain in the soil to infect subsequent crops. The amount of microsclerotia returned to the soil from lettuce can vary greatly, but could be as high as several million per infected plant in conditions optimal for the fungus. The fungus is seed transmitted in lettuce and other crops grown in rotation with lettuce, which may increase the spread of novel V. dahliae genotypes into new and existing lettuce production districts. New fungal strains may limit the effectiveness of control efforts, such as developing resistant cultivars. Soil fumigation is currently the most widely used control method. However, due to the high fumigation costs, fumigated fields are typically planted with strawberries to recover the costs associated with fumigation. The strawberries are followed by lettuce crops until V. dahliae inoculum densities recover to pre-fumigation levels, which is typically two to three crops of lettuce. Host resistance to Verticillium wilt represents the best long-term control method, and two pathogenic races of V. dahliae were described on lettuce (race 1 and race 2). Complete resistance to race 1 is available, and is being bred into commercial acceptable cultivars adapted to coastal California. Widespread production of race 1 resistant cultivars may lead to a selective sweep in favor of race 2 strains, to which there is no known complete resistance. Therefore, there is an urgent need to identify lettuce germplasm exhibiting high levels of resistance to race 2 of V. dahliae.