PRO/PL> Potato purple top disease - USA (WA, OR)

[Brian Edmonds]

POTATO PURPLE TOP DISEASE - USA (WASHINGTON, OREGON)
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Date: 14 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 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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ProMED-mail
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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]

[see also:
2001
----
Potato stolbur phytoplasma, potato - Austria 20011124.2877
2000
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Potato stolbur phytoplasma - Poland: EPPO report 20001127.2063]
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