Location: Molecular Plant Pathology LaboratoryTitle: Aster yellows group (16SrI), subgroups 16SrI-A and 16SrI-B, phytoplasmas associated with lettuce yellows in Texas) Author
Submitted to: New Disease Reports
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/20/2014
Publication Date: 1/31/2014
Citation: Lee, I., Bottner-Parker, K.D., Zhao, Y., Davis, R.E., Black, M.C. 2014. Aster yellows group (16SrI), subgroups 16SrI-A and 16SrI-B, phytoplasmas associated with lettuce yellows in Texas. New Disease Reports. 29:5. Interpretive Summary: Lettuce is an important vegetable crop grown in the US. Lettuce yellows (previously termed lettuce aster yellows) is a devastating disease commonly found in lettuce fields and causes significant economic loss to growers, damaging 40-100% of the crop in some seasons. The identity of the causal pathogen was not verified until recently. The presence of aster yellows phytoplasma (insect transmitted small bacteria) strains in diseased lettuce from several states in the eastern and central regions of the US was confirmed by molecular means. In 2013, an epidemic of a new disease with symptoms similar to lettuce yellows occurred in the Winter Garden region of Texas. The infected plants were stunted with blanching and chlorosis in young heart leaves. Molecular-based identification indicated that the lettuce yellows occurring on Romaine and leaf lettuce cultivars in Texas were associated with two distinct aster yellows phytoplasma strains. This is the first report confirming that lettuce yellows in Texas is associated with aster yellows phytoplasmas. This information will aid extension workers and plant diagnostician in the development of an effective control measure for this newly emerging disease and will aid implementation of quarantine regulation.
Technical Abstract: In 2013, an epidemic of lettuce yellows occurred in the Winter Garden region of Texas. The infected plants were stunted with blanching and chlorosis in young heart leaves. A total of thirteen samples, including three apparently asymptomatic, from Romaine and leaf lettuce cultivars, on two different farms were collected and tested for phytoplasmas. DNA was extracted from leaf veinal tissue and nested PCRs using universal primer pair P1/16S-SR followed by R16F2n/R16R2n and P1A/16S-SR were performed to detect the presence of phytoplasmas in the samples. The taxonomic affiliation of the putative phytoplasmas detected was determined by RFLP analysis using nested PCR products. A nested PCR using primer pair rpF1/rpR1 followed by rp(I)F1/rp(I)R1A was used to amplify a phytoplasma DNA segment (about 1.2 kb) of the ribosomal protein (rp) operon that encompassed genes rplV (rpl22) and rpsC (rps3) and a nested PCR using primer pair AYsecYF1/secYR1(I) followed by AYsecYF1/AYsecYR1was used to amplify a phytoplasma DNA segment (about 1.3 kb) of the partial spc operon that includes the complete secY gene. Four PCR-amplified products from each of 16S rDNA, rp, and secY genes were cloned and sequenced. The results indicated all 13 lettuce samples were infected with phytoplasmas. Both lettuce cultivars were infected with 16SrI-A and 16SrI-B phytoplasmas. Phylogenetic analysis with 16S rRNA, rp and secY gene sequences confirmed the presence of two distinct lineages. This is the first report confirming that lettuce yellows in Texas is associated with two distinct phytoplasma strains.