April 14, 2004
From: 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.

[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]