Verticillium wilt is a destructive disease of lettuce (Lactuca sativa L.) in the Salinas Valley of California, a region that accounts for 50% of the U.S. lettuce production (Monterey County Crop Report, 2012). The disease is caused by the soilborne fungus V dahliae and is a threat to crops grown in California such as strawberry, artichoke, tomato, and lettuce. The pathogen attacking lettuce exists as two races (Gurung et al., 2014). Complete resistance to race 1 was identified in several heirloom lettuce cultivars (cvs.) (Hayes et al., 2007) and resistant iceberg germplasm was bred using ‘La Brillante’ as a parent (Hayes et al., 2011a, 2015). It is expected that widespread production of race 1 resistant cvs. will cause race 2 strains to increase in frequency and developing resistance to this strain is a priority for the lettuce industry. Although race 2 isolates are largely limited to the Pajaro Valley, a few have been found in the Salinas Valley (Gurung et al., 2014). Adequate levels of resistance to race 2 are not known (Hayes et al., 2011b) and alternative approaches to breed crops that tolerate the disease or escape economic damage could be useful to complement existing resistance.
Verticillium dahliae penetrates the secondary roots of lettuce and moves into the taproot through the vascular system (Vallad and Subbarao, 2008). In susceptible plants, the fungus moves into the foliage of the plant causing wilting, chlorosis, and necrosis, and finally death. These symptoms progress acropetally, from base to apex (Fradin and Thomma, 2006). In most crops, reduced yields of fruit, seeds, or tubers from verticillium wilt occurs indirectly, through reduced photosynthesis as a result of leaf wilting and death. Economic damage in lettuce occurs immediately whenever chlorosis is expressed on leaves, a characteristic unique to leafy vegetable and ornamental crops.
Cultivated lettuce contains a diversity of market types that exhibit unique leaf characteristics and plant architectures. The popularity of each type is usually geographically dependent; in the United States, romaine and iceberg types are predominant. Although V. dahliae causes wilt in all types of lettuce, the economic damage to iceberg lettuce is the most severe due to its unique architecture and development (Vallad et al., 2006). Iceberg cvs. form a solid, spherical head. The heading process begins when outer leaves cup to form a sphere, an event that is genetically and environmentally dependent (Still, 2007). The head becomes solid as new leaves grow and fill the inside of the head. When plants are infected with verticillium wilt, the older and outer leaves of the iceberg head are the first to show symptoms. As the iceberg plant reaches maturity, these outer leaves that wrap around the lettuce head become the most severely wilted, leading to plant collapse and death. Healthy looking crops can often turn diseased in as little as a week. In some situations, growers may harvest iceberg crops before peak maturity but before severe symptoms occur. Conversely, iceberg cvs. that delay the onset of symptoms past peak harvest maturity could be useful to reduce economic damage from verticillium wilt. Similar characteristics are known in other lettuce diseases, but have not been pursued for verticillium wilt (Simko et al., 2014).
The majority of U.S. iceberg lettuce is produced year round in the southwestern United States. In Coastal California, lettuce is harvested from April to October. Later in the year (October to November), the production moves to the San Joaquin Valley and then the winter production is concentrated in the low desert of California and Arizona near the border with Mexico. Proper heading is a major determinant of iceberg cv. adaptation, and is a process influenced by genotype × environment interactions (Simko et al., 2014). Because of this, iceberg lettuce cvs. are generally bred to be narrowly adapted to specific locations and production times. Production of cvs. outside of the environment for which they were bred often results in poor heading and low yields (Simko et al., 2014).
The objectives of this research were to determine: 1) the genetic diversity for onset of FS in iceberg lettuce and 2) the adaptation of cvs. with delayed onset of symptoms to Coastal California production environments.
BlackhurstF.M.WoodR.K.S.1963Verticillium wilt of tomatoes—further experiments on the role of pectic and cellulolytic enzymesAnn. Appl. Biol.528996
BrunnerE.DomhofS.LangerF.2002Nonparametric analysis of longitudinal data in factorial experiments. Wiley New York NY
FradinE.F.ThommaB.P.H.J.2006Physiology and molecular aspects of verticillium wilt diseases caused by V. dahliae and V. albo-atrumMol. Plant Pathol.77186
GayosoC.PomarF.Novo-UzalE.MerinoF.Martinez de IlarduyaO.2010The Ve-mediated resistance response of the tomato to Verticillium dahliae involves H2O2, peroxidase and lignins and drives PAL gene expressionBMC Plant Biol.10232
HayesR.J.MaruthachalamK.ValladG.E.KlostermanS.J.SimkoI.LuoY.G.SubbaraoK.V.2011aIceberg lettuce breeding lines with resistance to verticillium wilt caused by race 1 isolates of Verticillium dahliaeHortScience46501504
HayesR.J.MaruthachalamK.ValladG.E.KlostermanS.J.SubbaraoK.V.2011bSelection for resistance to verticillium wilt caused by Race 2 isolates of Verticillium dahliae in accessions of lettuce (Lactuca sativa L.)HortScience46201206
HayesR.J.McHaleL.K.ValladG.E.TrucoM.J.MichelmoreR.W.KlostermanS.J.MaruthachalamK.SubbaraoK.V.2011cThe inheritance of resistance to verticillium wilt caused by race 1 isolates of Verticillium dahliae in the lettuce cultivar La BrillanteTheor. Appl. Genet.123509517
HayesR.J.SandoyaG.SimkoI.LuoY.G.SubbaraoK.V.2015Notice of release of PI 673090 673091 673092 673093 673094 673095 673096 673097 lettuce. U.S. Department of Agriculture Agricultural Research Service Washington DC 2008
HayesR.J.ValladG.E.QinQ.-M.GrubeR.C.SubbaraoK.V.2007Variation for resistance to verticillium wilt in lettuce (Lactuca sativa L.)Plant Dis.91439445
MaruthachalamK.AtallahZ.K.ValladG.E.KlostermanS.J.HayesR.J.DavisR.M.SubbaraoK.V.2010Molecular variation among isolates of Verticillium dahliae and polymerase chain reaction-based differentiation of racesPhytopathology10012221230
Monterey County Crop Report2012. County of Monterey Agricultural Commissioner. 4 Apr. 2017. <http://www.co.monterey.ca.us/Home/ShowDocument?id=1483>.
RyderE.J.1999Lettuce endive and chicory. Crop production science in horticulture series. CABI Publishing New York NY
SaxtonA.M.1998A macro for converting mean separation output to letter groupings in Proc Mixed. 23rd SAS Users Group Intl. p. 1243–1246. SAS Institute Cary NC
SimkoI.HayesR.J.MouB.McCreightJ.D.2014Lettuce and spinach p. 53–86. In: S. Smith B. Diers J. Specht and B. Carver (eds.). Yield gains in major U.S. field crops. Series. CSSA Special Publications. American Society of Agronomy Inc. Crop Science Society of America Inc. and Soil Science Society of America Inc. Madison WI
StillD.2007Lettuce p. 380. In: C. Kole (ed.). Genome mapping and molecular breeding in plants. Springer-Verlag Berlin Heidelberg Germany
TanG.LiuK.KangJ.XuK.ZhangY.HuL.ZhangJ.LiC.2015Transcriptome analysis of the compatible interaction of tomato with Verticillium dahliae using RNA-sequencingFront. Plant Sci.648
ValladG.E.QinQ.M.GrubeR.HayesR.J.SubbaraoK.V.2006Characterization of race-specific interactions among isolates of Verticillium dahliae pathogenic on lettucePhytopathology9613801387
ValladG.E.SubbaraoK.V.2008Colonization of resistant and susceptible lettuce cultivars by a green fluorescent protein-tagged isolate of Verticillium dahliaePhytopathology98871885
VeroneseP.NarasimhanM.L.StevensonR.A.ZhuJ.-K.WellerS.C.SubbaraoK.V.BressanR.A.2003Identification of a locus controlling verticillium disease symptom response in Arabidopsis thalianaPlant J.35574587