Aster yellows is a disease that sporadically damages leafy green vegetables, especially lettuce. The disease is caused by a phytoplasma and vectored by phloem-feeding leafhoppers. Aster yellows induces various symptoms in lettuce including yellowing and witches’-broom of the leaves, and phyllody and virescence of the flowers, which can lead to severe economic losses. Previously, four aster yellows phytoplasma strains were identified and characterized based on their symptoms in lettuce fields in Ohio. In our study, a thorough assessment of the role that these different strains play in aster yellows epidemics in Ohio was made over time during two growing seasons (2008 and 2009). In addition, new aster yellows isolates that may contribute to the aster yellows phytoplasma strain complex were identified based on their 16SrRNA sequences. The role of herbaceous plants such as cilantro and parsley, which are commonly produced in lettuce production areas, was studied as well. Surveys of aster leafhoppers and host plants (lettuce, cilantro and parsley) were conducted to assess the distribution of these known strains (AY-WB, AY-S, AY-DB2 and AY-BW) of aster yellows phytoplasma in two major vegetable production areas (Celeryville and Hartville) in northern Ohio. Strains in two 16SrRNA group I subgroups (16SrI-A and 16SrI-B) were detected in both locations and years. Strain AY-WB belonged to subgroup 16SrI-A while strains AY-S, AY-BD2 and AY-BW belonged to 16SrI-B. Multiplex PCR assays for aster yellows phytoplasma were developed and used to screen leafhopper and plant samples to detect known phytoplasma strains. In both locations during both years, populations of leafhoppers collected were higher in romaine lettuce during August. There was a noticeable increase in the proportion of inoculative leafhoppers collected early in the season compared to that observed 3-8 years previously. Aster yellows infection of surveyed leafhoppers and plants was the highest early in the growing season, declining late in the season, in both years.
All lettuce types (romaine, red leaf and green leaf), cilantro and parsley plants were infected with both 16SrI-A and 16SrI-B strains. Aster yellows strains were distributed evenly across host plants and there was no association between aster yellows phytoplasma strain and plant host. Strain AY-S appeared to be relatively rare in leafhoppers and plants in both years at both locations; however, there were no significant differences in strain distribution in either location. Other strains in subgroup 16SrI-B (AY-BD2 and AY-BW) were numerically more abundant in leafhoppers and plants in this study.
Phylogenetic analysis of 16SrRNA gene sequences of sixteen aster yellows isolates that were detected amongst host plants and leafhoppers associated with these plants indicated that these isolates belonged to subgroups 16SrI-A and 16SrI-B.
In the last five years, cilantro and parsley production increased dramatically in intensive lettuce production areas in Ohio. While these plants remained symptomless in the field, the proportions of aster yellows-infected plants were no different than the proportions of aster yellows-infected lettuce plants. Therefore, it is very likely that parsley and cilantro play an important role in aster yellows epidemiology, providing a natural reservoir of aster yellows phytoplasmas.