PRO/PL> Dry bean phyllody disease - USA (WA): 1st report

[Brian Edmonds]

DRY BEAN PHYLLODY DISEASE - USA (WASHINGTON): 1ST REPORT
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Date: 15 Apr 2004
From: ProMED-mail<promed at promedmail.org>
Source: American Phytopathological Society, Plant Disease Notes [edited]

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

I.-M. Lee and K. D. Bottner, Molecular Plant Pathology Laboratory, 
USDA, ARS, Beltsville, MD 20705; P. N. Miklas, Vegetable and Forage 
Crop Research, USDA, ARS, Prosser, WA 99350; and M. A. 
Pastor-Corrales, Vegetable Laboratory, USDA, ARS, Beltsville, MD 
20705. Plant Dis. 88:429, 2004; published on-line as D-2004-0128-01N, 
2004. Accepted for publication 6 Jan 2004.

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. 4 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 3 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 4 DBPh strains were 
closely related to the VR strain with 99.7 percent 16S rDNA sequence 
homology and to the CP strain with 99.2 percent 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 3 Pinto UI-114 bean seedlings in the greenhouse. Within 
60 days, the bean seedlings exhibited corrugated leaflike 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. 1153, 1998.

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[Phyllody describes a retrograde metamorphosis of the floral organs 
to the condition of leaves. Because floral tissues in 
phytoplasma-infected plants revert to vegetative tissues, no seeds 
are produced, and crop losses due to phyllody can be very high. 
Phyllody-affected crop plants can be cured of phytoplasmas by heat 
treatment or by subjecting cuttings to tissue culture.

The beet leafhopper, _Circulifer tenellus_ (Ct), is the only known vector of 
BLTVA, although not much is known about other vectors. Phytoplasmas infect 
and multiply in their leafhopper vectors. The BLTVA is vectored in the same 
manner as other phytoplasmas are by other  leafhoppers. Ct acquires BLTVA 
only by feeding on infected plants.

Prophylactic use of insecticides is the only known means to prevent 
transmission of BLTVA. The Washington vegetable seed industry has 
been plagued with BLTVA for years, and even the prophylactic use of 
insecticides has not always successfully managed it. Some vegetable 
seed crops are no longer grown in the Columbia basin due to 
inadequate control of BLTVA.

Useful references:
<http://www.wsu.edu/~potatoes/leafhopper.htm>
<http://www.wsu.edu/~potatoes/BLTVA.htm>
<http://www.redepapa.org/nava.pdf>
<http://www.oardc.ohio-state.edu/spiroplasma/what.htm>
<http://www.uniud.it/phytoplasma/pap/flet2450.Html>
- Mod.DH]
.......................................dh/pg/jw
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