Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/14/2004
Publication Date: 4/4/2004
Citation: Lee, I., Bottner, K.D., Munyaneza, J.E., Secor, G.A., Gudmestad, N.C. 2004. Clover proliferation group (16srvi), subgroup a (16srvi-a) phytoplasma is probable causal agent of potato purple top disease in washington and oregon. Plant Disease. 88:429. Interpretive Summary: Phytoplasmas are very small bacteria that lack a cell wall and that cause several hundred economically important diseases in plants worldwide. Potato purple top wilt disease has caused tremendous damage to potato tuber production in Mexico and South America. In the past, this disease occurred only sporadically in potato growing regions of the United States. In 2002 and 2003, a major epidemic of potato purple top occurred in the Columbia Basin of Washington and Oregon, causing great economic damage to the potato industry. We have made an extensive survey of the affected plants and identified a phytoplasma belonging to clover proliferation group as the causal agent. This phytoplasma is similar to the one causing tomato big bud disease in California. This is the first confirmed case of a clover proliferation group phytoplasma strain as the causal agent of widespread potato purple top disease in the Columbia Basin. The information in this paper will aid implementation of quarantine regulation and it will help extension workers and plant diagnosticians to determine how to combat the disease.
Technical Abstract: An epidemic of purple top disease of potato (Solanum tuberosum) occurred in the Columbia Basin region of Washington and Oregon in 2002 and 2003, causing great economic loss in the potato industry. Symptoms of potato purple top (PPT) were characterized by upright growth of terminal shoots, upward leaf rolling, chlorosis, red or purplish discoloration of new leaves, proliferation of axillary shoots with swelling at the base, and formation of aerial tubers. In the present work, we have applied PCR-based methods to detect phytoplasmas present in the diseased potato plants. RFLP and phylogenetic analyses of amplified 16S rDNA sequences (nested-PCR products) were employed for phytoplasma identification. Seventy-eight potato samples (including five asymptomatic ones) were collected from five fields in various locations in the region. Eighty-four percent (63% by first PCR amplification) of the symptomatic samples tested positive for phytoplasma. None of the asymptomatic samples tested positive in the first PCR amplification, while 60% tested positive by nested-PCR. RFLP analyses of 16S rDNA sequences with restriction enzymes, MseI, AluI, HhaI, RsaI, and HpaII indicated that the phytoplasma strain associated with potato purple top belonged to the clover proliferation group (16SrVI), subgroup A (16SrVI-A). The taxonomic affiliation of PPT phytoplasma was confirmed by phylogenetic analysis of cloned 16S rRNA gene sequences (GenBank accession nos.: PPT4, AY496004; PPT8, AY496005). The 16S rDNA sequences of the PPT strains were most closely related to subgroup 16SrVI-A phytoplasma strains, VR (with 99.7% sequence homology) and CP (with 99.2% homology). The etiological role of 16SrVI-A phytoplasma strains in PPT disease was confirmed by a modified test of Koch's postulates. Infected tissues from four phytoplasma-positive potato samples were grafted onto four healthy potato seedlings in the greenhouse. The phytoplasma was detected in each of the grafted symptomatic seedlings, and the RFLP patterns of their 16S rRNA gene sequences were identical to those of the phytoplasmas in the scions. These results confirmed that CP group, subgroup 16SrVI-A phytoplasma strains are the probable cause of PPT in the U.S.