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Clover Proliferation Group (16SrVI) Subgroup A (16SrVI-A) Phytoplasma is a Probable Causal Agent of Dry Bean Phyllody Disease in Washington

During 2003, a new disease, dry bean phyllody (DBPh), was observed in the Columbia Basin of Washington in dry bean (Phaseolus vulgaris L.) cultivars of Andean origin grown in Mattawa and Paterson, WA that caused great reduction in dry bean production. Symptoms of DBPh became apparent during mid-to-late pod development and were characterized by leafy petals (phyllody) and aborted seed pods resembling thin, twisted, and corrugated leaf-like structures. Deformed sterile pods that were small, sickle-shaped, upright, and leathery were also observed. The infected plants generally exhibited chlorosis, stunting, or bud proliferation from leaf axils. Symptoms of DBPh were indicative of possible infection by phytoplasmas. Restriction fragment length polymorphism (RFLP) and phylogenetic analyses of amplified 16S rDNA sequences were used for phytoplasma identification. Four symptomatic bean plants were analyzed and tested positive for phytoplasma infection on the basis of results of initial polymerase chain reaction (PCR) and subsequent nested-PCR amplifications (2). RFLP analyses of 16S rDNA sequences with restriction enzymes, MseI, AluI, HhaI, RsaI, and HpaII indicated that the phytoplasma strains associated with DBPh belonged to the clover proliferation group (16SrVI) subgroup A (16SrVI-A) (2). This subgroup currently consists of three members, clover proliferation (CP; GenBank Accession No. AY500130), potato witches'-broom (PWB; GenBank Accession No. AY500818), and vinca virescence (VR; GenBank Accession No. AY500817), a strain of beet leafhopper-transmitted virescence agent (BLTVA) phytoplasmas (1,2). The taxonomic affiliations of the DBPh phytoplasma strains were confirmed by phylogenetic analysis of cloned 16S rRNA gene sequences (GenBank Accession Nos. DBPh2, AY496002; DBPh3, AY496003). Among the existing members of subgroup 16SrVI-A, the four DBPh strains were closely related to the VR strain with 99.7% 16S rDNA sequence homology and to the CP strain with 99.2% sequence homology. To gain further evidence on the role of 16SrVI-A phytoplasma strains in DBPh disease, a modified test of Koch's postulates was conducted. Infected tissue from one phytoplasma-positive dry bean sample was grafted onto three Pinto UI-114 bean seedlings in the greenhouse. Within 60 days, the bean seedlings exhibited corrugated leaf-like structures from aborted seedpods, a lack of flower formation, general chlorosis, and stunting similar to the original diseased plants. The lower leaves of the inoculated bean plants became epinastic and leathery. The transmitted phytoplasma was detected in each of the grafted symptomatic seedlings, and the RFLP patterns of its 16S rRNA gene sequences were identical to those of the phytoplasmas in the scions. A high correlation between the presence of disease symptoms and the presence of subgroup 16SrVI-A phytoplasmas in the bean plants suggests that these phytoplasmas play an etiological role in DBPh disease. To our knowledge, these findings provide the first confirmed case of phytoplasma-associated DBPh in the United States.

References: (1) D. A. Golino et al. Plant Dis. 73:850, 1989. (2) I.-M. Lee et al. Int. J. Syst. Bacteriol. 48:1153, 1998.