First, second, third, and fourth authors: U.S. Department of Agriculture-Agricultural Research Service, Molecular Plant Pathology Laboratory, Beltsville, MD 20705; and fifth and sixth authors: Texas A&M University Research and Extension Center, Uvalde 78802
In the spring of 2000, an aster yellows (AY) epidemic occurred in carrot crops in the Winter Garden region of southwestern Texas. A survey revealed that vegetable crops, including cabbage, onion, parsley, and dill, and some weeds also were infected by AY phytoplasmas. Nested polymerase chain reaction (PCR) and restriction fragment length polymorphism 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 16SrI-B, in the AY group (16SrI), were predominantly detected in infected plants. Carrot, parsley, and dill were infected with both subgroups. Onion and three species of weeds (prickly lettuce, lazy daisy, and false ragweed) were predominantly or exclusively infected by subgroup 16SrI-A phytoplasma strains, while cabbage was infected by subgroup 16SrI-B phytoplasmas. Both types of phytoplasmas were detected in 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 leafhoppers. Sequence and phylogenetic analyses of 16S rDNA and ribosomal protein (rp) gene sequences indicated that phytoplasma strains within subgroup 16SrI-A or subgroup 16SrI-B, detected in various plant species and putative insect vectors, were highly homogeneous. However, based on rp sequences, two rpI subgroups were identified within the subgroup 16SrI-A strain cluster. The majority of subgroup 16SrI-A phytoplasma strains were classified as rp subgroup rpI-A, but phytoplasma strains detected in one onion sample and two leafhoppers (M. fascifrons and C. abrupta) were different and classified as a new rp subgroup, rpI-N. The degree of genetic homogeneity of the phytoplasmas involved in the epidemic suggested that the phytoplasmas came from the same pool and that all three leafhopper species may have been involved in the epidemic. The different phytoplasma population profiles present in various crops may be attributed to the ecological constraints as a result of the vector-phytoplasma-plant three-way interaction.