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Published Date: 2004-04-15 23:50:00
Subject: PRO/PL> Potato purple top disease - USA (WA, OR)
Archive Number: 20040415.1041

POTATO PURPLE TOP DISEASE - USA (WASHINGTON, OREGON)
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Date: 14 Apr 2004
From: ProMED-mail<promed@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 Potato Purple Top Disease in Washington
and Oregon.
I.-M. Lee and K. D. Bottner, Molecular Plant Pathology Laboratory,
USDA, ARS, Beltsville, MD 20705; J. E. Munyaneza, USDA, ARS, YARL,
Wapato, WA 98951; and G. A. Secor and N. C. Gudmestad, North Dakota
State University, Department of Plant Pathology, Fargo 58105. Plant
Dis. 88:429, 2004; published on-line as D-2004-0204-02N, 2004.
Accepted for publication 14 Jan 2004.
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
(1). Symptoms of potato purple top (PPT) were upright terminal
shoots, upward leaf rolling, chlorosis, red or purplish discoloration
of new leaves, proliferation of axillary shoots with basal swelling,
and the formation of aerial tubers. Preliminary studies on PPT
disease suggested phytoplasma as a possible cause (1).
In this study, 78 potato samples (including 5 asymptomatic) were
collected from 5 fields throughout the region. A nested polymerase
chain reaction (PCR) with primer pair P1/P7 in the first
amplification followed with primer pair R16F2n/R16R2 was performed to
detect the presence of phytoplasmas in infected plants (2).
Restriction fragment length polymorphism (RFLP) and phylogenetic
analyses of amplified 16S rDNA sequences were used for phytoplasma
identification.
84 percent (63 percent in the first amplification) of the symptomatic
samples and 60 percent (0 in the first amplification) of the
asymptomatic samples tested positive. Low phytoplasma titers and the
presence of PCR inhibitors account for the low detection rate in the
1st PCR amplifications.
RFLP analyses of 16S rDNA with enzymes MseI, AluI, HhaI, RsaI, and
HpaII indicated that the phytoplasma associated with PPT belonged to
the clover proliferation (CP) group (16SrVI) subgroup A (16SrVI-A)
(2). 16SrVI-A currently consists of 3 members, CP (GenBank Accession
No. AY500130), potato witches'-broom (GenBank Accession No.
AY500818), and vinca virescence (VR) (GenBank Accession No.
AY500817), a strain of beet leafhopper-transmitted virescence agent
(BLTVA) phytoplasma (2).
The taxonomic affiliation of PPT phytoplasma was confirmed by
phylogenetic analysis of cloned 16S rDNA (GenBank Accession Nos.
PPT4, AY496004; PPT8, AY496005). The 16S rDNA sequences of the PPT
strains were closely related to VR with 99.7 percent sequence
homology compared with 99.2 percent with CP. A high correlation
between the symptoms and the presence of 16SrVI-A phytoplasmas in the
potato plants suggests that these phytoplasmas play an etiological
role in PPT disease.
To gain further evidence, a modified test of Koch's postulates was
conducted. Infected tissues from four phytoplasma-positive potato
samples (including PPT4 and PPT8) were grafted onto healthy potato
seedlings. Within 60 days after grafting, the potato seedlings
developed symptoms similar to those in the original diseased samples.
The newly infected plants were maintained through cuttings. RFLP
analysis of 16S rDNA indicated that the phytoplasmas detected in each
of the seedlings and cuttings were identical to those in the scions.
These results confirmed the probable etiological role of CP group,
subgroup 16SrVI-A phytoplasma strains in PPT disease in Washington
and Oregon. There are 2 other confirmed cases of phytoplasmas (BLTVA
and aster yellows phytoplasma) associated with PPT disease in Utah
(4) and Mexico (3).
References:
(1) P. B. Hamm et al. Potato Prog. Vol. 3, No. 1, 2003.
(2) I.-M. Lee et al. Int. J. Syst. Bacteriol. 48:1153, 1998.
(3) N. E. Leyva-Lopez et al. Can. J. Microbiol. 48:1062, 2002.
(4) C. D. Smart et al. Phytopathology 83:1399, 1993.
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[Phytoplasma-induced diseases are being recognized as significant
pathogens of food crops. PPT (a.k.a. aster yellows [haywire, purple
dwarf and Purple-top wilt]) disease in Washington is somewhat
ephemeral. It has been a factor in disease losses for many years in
Washington state but is seldom a major factor affecting potato crops
in the Columbia Basin. However, PPT is endemic in potato crops in
Mexico, where it ranks 2nd to late blight caused by _Phytophthora
infestans_. 2 phytoplasma diseases have been recognized in Mexico;
PPT and potato hair sprouts (PHS). PHS has the greater impact, since
infected, but symptomless, tubers generally fail to sprout or may
sprout poorly. Moreover, PHS-infected stems are weakened because they
are deficient in chlorophyll (etiolated). Moreover, psyllid nymphs
inject a toxin into potato tissue, causing PPT-like symptoms, which
confounds diagnosis.
Disease management basically depends upon planting certified seed in
areas free of phytoplasmas or, in the case of areas infested with
infected weeds and leafhoppers, use of insecticides to reduce vector
numbers. Development of resistance to phytoplasmas may offer a
measure of control.
Useful references:
<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]

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