Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/23/2003
Publication Date: 11/1/2003
Citation: Lee, I., Martini, M., Bottner, K.D., Dane, R.A., Black, M.C., Troxclair, N. 2003. Ecological aspects of phytoplasmas involved in an aster yellows epidemic in various crops in texas. Phytopathology. 99(11):1368-1377. Interpretive Summary: In 2000, a disease epidemic seriously damaged vegetable and herb crops in southwestern Texas. We investigated the cause of the epidemic by using molecular methods. The results of our research showed that the epidemic was caused by a very small bacterium called phytoplasma. Phytoplasmas are unusual bacteria that lack a cell wall, cause diseases only in plants, and are carried from plant to plant by insects. We found that the disease epidemic affected crops including carrot, cabbage, onion, dill, and parsley, and that it was caused by two different types of phytoplasma. We also found that three different species of insects were spreading the epidemic. The evidence that we gained in this research indicated that the insects had come from other geographical areas located south of the epidemic area and had carried the phytoplasmas from distant regions. This information will aid diagnosticians to diagnose future epidemics and will help scientists, extension agents, and growers to design and implement measures to control this devastating disease problem.
Technical Abstract: In the spring of 2000, an aster yellows (AY) epdiemic occurred in carrots in the Winter Garden region of southwestern Texas. A survey revealed that several vegetable crops including cabbage, onion, parsley and dill, and some weeds were also infected by AY phytoplasmas. Nested PCR and EFLP analysis of PCR-amplified phytoplasma 16S rDNA were employed for the detection and identification of phytoplasmas associated with these crops and weeds. Phytoplasmas belonging to two subgroups, (16SrI-A) and B (16SrI-B), in the aster yellows group (16SrI), were predominantly detected in infected plans. Carrot, parsley, and dill were infected by both subgroups. Onion and three species of weeds (prickly lettuce, lazy daisy, and false ragweed) were predominantly or exclusively infected by subgroup A phytoplasma strains, while cabbage was infected by subgroup B phytoplasmas. Both types of phytoplasmas were detected in the three leafhopper species (Macrosteles fascifrons, Scaphytopius irroratus and Ceratagallia abrupta) commonly present in this region during the period of the epidemic. Mixed infections were very common in individual carrot, parsley, and dill plants and in individual insects of the three leafhopper species. Sequence and phylogenetic analysis of 16S rDNA and ribosomal protein (rp) gene sequences indicated that phytoplasma strains within subgroup A or subgroup B, which were associated with various plant species and putative insect vectors, were highly homogeneous. This suggested that the phytoplasmas involved in the epidemic came from the same pool and that the three leafhoper species may be the vectors that transmitted and spread the disease. Based on rp sequences, phytoplasmas belonging to a new rp subgroup, I-N, was detected in one of the infect onions, which were infected predominantly by subgroup rpI-A, and in two leafhoppers (M. fascifrons and C. abrupta).