Molecular Plant Pathology Laboratory, USDA, ARS, Beltsville, MD 20705
Texas A&M University Research and Extension Center, Uvalde 78802-1849
In early spring 2000 carrot crops in southwestern Texas were severely infected by an outbreak of phyllody associated with aster yellows phytoplasma. Cabbage crops that had been planted adjacent to these carrot fields began to display previously unobserved symptoms characteristic of phytoplasma infection. Symptoms included purple discoloration in leaf veins and at the outer edges of leaves on cabbage heads. Proliferation of sprouts also occurred at the base of the stem and between leaf layers of some plants, and sprouts sometimes continued to proliferate on extended stems. About 5% of cabbage plants in the field exhibited these symptoms. Two symptomless and four symptomatic cabbage heads were collected in early April from one cabbage field. Veinal tissues were stripped from each sample and used for total nucleic acid extraction. To obtain specific and sufficient amount of PCR products for analysis, nested PCR was performed by using primer pairs (first with P1/P7 followed by R16F2n/R16R2) (1,2) universal for phytoplasma detection. A specific 16S rDNA fragment (about 1.2 kb) was strongly amplified from the four symptomatic but not from the two asymptomatic samples. The nested PCR products obtained from the four symptomatic samples were then analyzed by restriction fragment length polymorphism (RFLP) using the restriction enzymes MseI, HhaI, and HpaII, and the RFLP patterns were compared to the published patterns of known phytoplasmas (1). The resulting RFLP patterns were identical to those of a phytoplasma belonging to subgroup B of the aster yellows phytoplasma group (16SrI). These RFLP patterns were also evident in putative restriction sites observed in a 1.5 kbp nucleotide sequence of the 16S rDNA. This is the first report of aster yellows phytoplasma associated disease symptoms in cabbage in Texas. The occurrence of cabbage proliferation coincided with the presence of high populations of the insect vector, aster leafhopper.
References: (1) I.-M. Lee et al. Int. J. Syst. Bacteriol. 48:1153, 1998. (2) B. Schneider et al. 1995. Molecular and Diagnostic Procedures in Mycoplasmology, Vol. I. Academic Press, San Diego, CA.