First Report of Verticillium Wilt on Lettuce (Lactuca sativa) in Washington Caused by Verticillium tricorpus

Symptoms of Verticillium wilt were observed on lettuce (Lactuca sativa L.) harvested from high tunnel and open field experimental plots in annual, consecutive spring plantings in western Washington from 2010 to 2012. Leaves had v-shaped, chlorotic lesions, and yellow or brown vascular tissue was noted in the crowns. Total disease incidence increased from 0.2% in 2010 to 1.9% in 2011 and to 14.4% in 2012. Verticillium spp. obtained from infected crown tissues and cultured on half-strength potato dextrose agar medium produced yellow pigment, black microsclerotia, white mycelia, tan chlamydospores, and uniseptate conidia averaging 10.6 × 3.7 μm. Isolates were identified tentatively as Verticillium tricorpus I. (3). Three isolates, Vt.Ls.2010, Vt.Ls.2011-1, and Vt.Ls.2011-2, were evaluated for pathogenicity on 4-week-old 'Coastal Star' seedlings in two greenhouse trials. In Trial I, four replicates of two duplicate plants per each isolate, and in Trial II, five replicates of one plant per each isolate were inoculated with conidial suspensions adjusted to 2.0 × 10 and 5.0 × 10 conidia/ml, respectively. Additionally, in each trial, two sets of control treatments of five plants each were inoculated with either an isolate of V. dahliae at the same conidial concentration or with sterile water. Root tips were cut and exposed to the suspensions for 5 s, then seedlings were transplanted into Sunshine Mix #1 (SunGro Horticulture Distribution Inc., Bellevue, WA), and kept in a greenhouse at 17.7 ± 3.4°C. Plants were harvested 8 to 9 weeks post-inoculation, and symptoms were rated visually. Vt.Ls.2010, Vt.Ls.2011-1, and Vt.Ls.2011-2 caused chlorosis and vascular discoloration on 25, 13, and 13% of the plants in Trial I; and 40, 60, and 20% of plants in Trial II, respectively. V. dahliae caused similar symptoms on 25 and 40% of the plants in the two trials, respectively, but these plants had greater intensity and length of vascular discoloration compared with the three test isolates. None of the water control plants were symptomatic. All V. tricorpus isolates were recovered from inoculated plants, and colony morphologies were similar to the original isolates. The internal transcribed spacer (ITS) rDNA of isolate Vt.Ls.2010 was amplified with ITS4 and ITS6 primer sets. ITS rDNA sequences between Vt.Ls.2010 and two isolates of V. tricorpus in GenBank (Accession Nos. FJ900211 and AB353343) were 100% identical. V. tricorpus is considered a weak pathogen of lettuce crops in Ca