Potato Purple Top Disease Research Articles

Bacterial Endosymbionts Identified From Leafhopper (Hemiptera: Cicadellidae) Vectors of Phytoplasmas.

Insects often harbor bacterial endosymbionts that provide them with nutritional benefit or with protection against natural enemies, plant defenses, insecticides, and abiotic stresses. Certain endosymbionts may also alter acquisition and transmission of plant pathogens by insect vectors. We identified bacterial endosymbionts from four leafhopper vectors (Hemiptera: Cicadellidae) of 'Candidatus Phytoplasma' species by direct sequencing 16S rDNA and confirmed endosymbiont presence and identity by species-specific conventional PCR. We examined three vectors of Ca. Phytoplasma pruni, causal agent of cherry X-disease [Colladonus geminatus (Van Duzee), Colladonus montanus reductus (Van Duzee), Euscelidius variegatus (Kirschbaum)] - and a vector of Ca. Phytoplasma trifolii, the causal agent of potato purple top disease [Circulifer tenellus (Baker)]. Direct sequencing of 16S identified the two obligate endosymbionts of leafhoppers, 'Ca. Sulcia' and 'Ca. Nasuia', which are known to produce essential amino acids lacking in the leafhoppers' phloem sap diet. About 57% of C. geminatus also harbored endosymbiotic Rickettsia. We identified 'Ca. Yamatotoia cicadellidicola' in Euscelidius variegatus, providing just the second host record for this endosymbiont. Circulifer tenellus harbored the facultative endosymbiont Wolbachia, although the average infection rate was only 13% and all males were Wolbachia-uninfected. A significantly greater percentage of Wolbachia-infected Ci. tenellus adults than uninfected adults carried Ca. P. trifolii, suggesting that Wolbachia may increase this insect's ability to tolerate or acquire this pathogen. Results of our study provide a foundation for continued work on interactions between leafhoppers, bacterial endosymbionts, and phytoplasma.

Molecular detection of phytoplasmas in potato psyllids in ecuador

Since the potato purple top appeared and the potato psyllid was reported in Ecuador, crop losses have been significant. So far, phytoplasmas enclosed in two ribosomal groups (16SrI-F, 16SrII) and Candidatus Liberibacter solanacearum’ were molecularly identified in symptomatic potato plants. Later, Bactericera cockerelli, a potato psyllid, was also first reported and correlated to the potato purple top disease. Molecular identification of the pathogens involved in this disease has been quite elusive. Samples of potato psyllids were analyzed in this study resulting in the identification of phytoplasmas enclosed in group 16SrXI, and others related to Candidatus Phytoplasma prunorum’ and Ca. P. solani’. Potato purple top in Ecuador is a complex disease that constitutes a severe threat for the potato production in the region.

First Report of Strawberry (Fragaria × ananassa) as a Host of a ‘Candidatus Phytoplasma americanum’-Related Strain in the United States

In June 2018, while conducting a survey for possible presence of phytoplasma pathogens in Pennsylvania production fields, a strawberry plant (Fragaria × ananassa) was observed with symptoms of stunting and unseasonal reddening and distortion of leaves. Only one out of 500 strawberry plants expressed the described symptoms in a farm in Perry County, PA. Several leaves and stems were collected from the symptomatic plant as well as from a few asymptomatic ones for further work. Total DNA was extracted from 100 mg of leaf midveins with a DNeasy Plant Mini Kit (Qiagen, Valencia, CA). All DNA samples were initially tested using a phytoplasma universal real-time polymerase chain reaction (PCR) assay (Hodgetts et al. 2009). Positive amplification results were obtained only with samples derived from the symptomatic strawberry plant. To detect and identify the suspected phytoplasmal pathogen strain(s), the DNA samples were used as a template in conventional PCRs for amplification of phytoplasma 16S rRNA, secY, secA, and tuf gene sequences as previously described (Dickinson and Hodgetts 2013; Lee et al. 2004, 2010; Makarova et al. 2012). All of the obtained amplicons were of expected sizes, and no amplification was observed from the asymptomatic strawberry samples. PCR products were purified with a QIAquick PCR Purification Kit (Qiagen) and sequenced. Nucleotide sequences of the PCR products were deposited in the GenBank database under accession numbers MN227133 to MN227136. The nucleotide sequences were analyzed using Geneious software (Biomatters, Auckland, NZ) for sequence alignments and using iPhyClassifier (Zhao et al. 2009) for group-subgroup and ‘Candidatus Phytoplasma’ species assignments. The classification into 16Sr groups and subgroups was established by virtual restriction fragment length polymorphism (RFLP) analysis of PCR products compared with reference strains. The phytoplasma, termed strain SRL1-PA (Strawberry Red Leaf1-PA), was found to share 99.9% similarity with that of the ‘Candidatus Phytoplasma americanum’ reference strain (GenBank accession DQ174122). The virtual RFLP pattern derived from the query 16S rDNA F2nR2 fragment is most similar to the reference group XVIII, subgroup A (GenBank accession DQ174122), with a pattern similarity coefficient of 0.98. Another sample from the same strawberry plant was independently determined positive for ‘Ca. P. americanum’ by CPHST Beltsville Lab using nucleotide sequence analysis of 16S rRNA gene sequences amplified in end point PCR. Alignments and phylogenetic analyses of secY, secA, and tuf gene sequences confirmed the close relatedness of the Pennsylvania SRL1-PA strain with the reference strains of ‘Ca. P. americanum’. ‘Ca. P. americanum’ was previously reported (Lee et al. 2006) in Texas and Nebraska on potato and associated with potato purple top wilt disease. In 2019, the Pennsylvania Department of Agriculture team revisited the same farm and collected additional samples including some from potato but was not able to detect any phytoplasma. To our knowledge, this report provides the first evidence for the presence of a ‘Ca. P. americanum’-related strain in Pennsylvania and more importantly describes strawberry (Fragaria × ananassa) as a new host for this phytoplasma. With only one single positive strawberry plant in a single year, the true extent of disease spread on strawberries remains undetermined.

Potato purple top disease in Ecuador

Potato purple top disease in Ecuador

Potato purple top disease associated with the novel subgroup 16SrII-X phytoplasma.

Potato (Solanum tuberosum) is a very economically important perennial tuberous crop in Saudi Arabia. Potato plants displaying symptoms associated with potato purple top disease, such as aerial tubers and purple and small leaves, were observed in Al-Bukairiyah, Fowlq and Buraydah, Al-Tarafiyah, Qassim governorate, Saudi Arabia. In this study, we examined samples taken from 12 symptomatic potato plants and confirmed the presence of phytoplasma DNA. Analysis of the 16S rRNA-encoding sequences revealed that the symptomatic plants were infected with phytoplasma belonging to the peanut witches'-broom group (16SrII). Sequencing of the 16S rRNA- encoding gene, computer-simulated RFLP analysis and phylogenetic analysis revealed the presence of a novel representative of the 16SrII-X subgroup. The present study identified potato plants as a novel host for novel phytoplasma strains belonging to the pigeon pea witches'-broom group in Saudi Arabia.

Detection and molecular characterization of a 16SrI-F phytoplasma in potato showing purple top disease in Ecuador

Potato purple top (PPT) disease has been reported worldwide, including Canada, United States, Mexico, Russia and Pakistan. During surveys carried out in a recent epidemic in Ecuador of potato purple top disease phytoplasmas were identified in symptomatic potatoes as well as in some tissue cultured potato shoots by PCR/RFLP analyses and by sequencing and phylogenetic clustering. The majority of the samples resulted positive to phytoplasmas enclosed in the ribosomal subgroup 16SrI-F, for the first time detected in potato and in the American continent. Further studies will allow verification of the transmission and further presence in Ecuador of this phytoplasma associated with purple top disease of potatoes.

Monitoring and Controlling the Beet Leafhopper Circulifer tenellus in the Columbia Basin

The beet leafhopper (BLH, Circulifer tenellus Baker) is a frequent pest of potatoes in the Columbia Basin of Oregon and Washington. Beet leafhoppers vector the causal agent of potato Purple top disease, known as Beet Leafhopper Transmitted Virescence Agent (BLTVA). Over 70 % of BLH in the region can be infected with BLVTA. Current control methods include making regular insecticide applications beginning in May; however it is unclear if those applications are needed. Thus, are insecticide applications necessary to control BLH and reduce BLTVA through the entire season? To answer that question, a field study was conducted in 2008 and 2009 to assess season-long control of BLH. There were 14 treatments, including the control, that were based on gradually eliminating insecticide applications weekly. Plots were sampled weekly for BLH and surveyed for Purple Top disease. Beet leafhoppers were sampled using sweep nets and an inverted leaf blower. Natural BLH populations were low in 2008 and 2009, but differed significantly among treatments in 2009 for inverted leaf blower and sweep net samples. Yields were assessed at the end of each season and no significant differences were found. Trends in the yield, BLH populations and BLTVA incidence support early-season insecticide application when BLH populations are low. The practice of using a calendar spray regime was not supported as yields were similar regardless of insecticide applications. As BLH populations were low, the results emphasized the importance of monitoring before making insecticide applications.

Evaluation of Beet Leafhopper Transmitted Virescence Agent Damage in the Columbia Basin

Potato purple top disease is caused by a phytoplas- ma known as Beet Leafhopper Transmitted Virescence Agent (BLTVA), which is vectored by the beet leafhopper (BLH, Circulifer tenellus Baker). Previous studies determined that BLTVA can cause significant reductions in yield and tuber quality; however, quantifying the damage caused by BLTVA and the insect vector has been challenging. In 2009-2011, potato plants at different growth stages were exposed to vary- ing densities of BLH in a screen house located at the Hermiston Agricultural Research and Extension Center in Hermiston, OR. The densities of potentially infective BLH were one BLH per plant (low), two BLH per plant (medium), and five BLH per plant (high). Releases occurred at the following growth stages: vegetative, tuber initiation, tuber bulking, and maturation. The treatments were arranged in a randomized complete block design with three replications per treatment. Disease incidence was monitored weekly and yield was assessed. When all 3 years were combined, we found that increasing rates of disease incidence correlated with decreas- ing yields. We also found that greater yield losses were ob- served with later BLH release times. With both correlations, differences between years were a strong contributing factor. There was a mean decrease in yield of 0-12 % at a density of oneBLH per plant,6-19% attwoBLH per plant,and 6-20% for five BLH per plant. These general trends in yield loss suggest that economically relevant damage may occur at levelsas low as one or two potentially infective BLH per plant in the Columbia Basin.

Population Dynamics of the Beet Leafhopper in Northeastern Oregon and Incidence of the Beet Leafhopper-Transmitted Virescence Agent Phytoplasma

Beet leafhoppers were collected weekly on yellow sticky traps placed at 36 locations in Morrow and Umatilla Counties in northeastern Oregon in April through November 2007, 2008, and 2009. Insects were counted, collected, and a subset of the insects was tested for the presence of the beet leafhopper-transmitted virescence agent phytoplasma, the causal agent of potato purple top disease in this region. Beet leafhoppers were present throughout the sampling period and the number of insects collected peaked in June of each year with smaller peaks in July and October. Of the 804 insects tested for phytoplasma in 2007, 2008, and 2009, 21, 18, and 22% tested positive for phytoplasma, respectively. Most of the phytoplasma-positive insects were collected from mid-June through July. Positive insects, however, were collected as late as 13 November in 2007 and 2008. These data indicate that a relatively high proportion of the beet leafhoppers in this area are harboring the phytoplasma. Therefore, the potential for development of purple top disease of potatoes from migrating beet leafhoppers in this important potato producing region is quite high and measures to control this pest throughout the growing season are probably necessary in order to reduce disease pressure.

Molecular Characterization of a Phytoplasma Associated with Potato Witches’-broom Disease in Iran

Potato plants showing symptoms suggestive of potato witches’-broom disease including witches’-broom, little leaf, stunting, yellowing and swollen shoots formation in tubers were observed in the central Iran. For phytoplasma detection, Polymerase Chain Reaction (PCR) and nested PCR assays were performed using phytoplasma universal primer pair P1/P7, followed by primer pair R16F2n/R16R2. Random fragment length polymorphism analysis of potato phytoplasma isolates collected from different production areas using the CfoI restriction enzyme indicated that potato witches’-broom phytoplasma isolate (PoWB) is genetically different from phytoplasmas associated with potato purple top disease in Iran. Sequence analysis of the partial 16S rRNA gene amplified by nested PCR indicated that ‘Candidatus Phytoplasma trifolii’ is associated with potato witches’-broom disease in Iran. This is the first report of potato witches’-broom disease in Iran.

Molecular Characterization of Phytoplasmas Associated with Potato Purple Top Disease in Iran

Potato plants with symptoms suggestive of potato purple top disease (PPTD) occurred in the central, western and north-western regions of Iran. Polymerase chain reaction (PCR) and nested PCR assays were performed using phytoplasma universal primer pair P1/P7 followed by primer pairs R16F2n/R16R2 and fU5/rU3 for phytoplasma detection. Using primer pairs R16F2n/R16R2 and fU5/rU3 in nested PCR, the expected fragments were amplified from 53% of symptomatic potatoes. Restriction fragment length polymorphism (RFLP) analysis using AluI, CfoI, EcoRI, KpnI, HindIII, MseI, RsaI and TaqI restriction enzymes confirmed that different phytoplasma isolates caused PPTD in several Iranian potato-growing areas. Sequences analysis of partial 16S rRNA gene amplified by nested PCR indicated that ‘Candidatus Phytoplasma solani’, ‘Ca. Phytoplasma astris’ and ‘Ca. Phytoplasma trifolii’ are prevalent in potato plants showing PPTD symptoms in the production areas of central, western and north-western regions of Iran, although ‘Ca. Phytoplasma solani’ is more prevalent than other phytoplasmas. This is the first report of phytoplasmas related to ‘Ca. Phytoplasma astris’, ‘Ca. Phytoplasma solani’ and ‘Ca. Phytoplasma trifolii’ causing PPTD in Iran.

Genetic Diversity and Geographical Distribution of Phytoplasmas Associated with Potato Purple Top Disease in Mexico

Potato purple top (PPT) disease has caused severe economic losses in some potato (Solanum tuberosum) growing areas of Mexico. Two distinct phytoplasma strains belonging to the aster yellows and peanut witches'-broom groups (16SrI and 16SrII groups) have been associated with PPT disease in several regions of Mexico. However, there has been no previous large-scale survey in the main potato growing areas in Mexico to analyze the diversity and geographical distribution of phytoplasmas. Potato samples were collected between 2003 and 2006 and were analyzed by nested polymerase chain reaction assays. On the basis of results from nucleotide sequence comparisons and virtual restriction fragment length polymorphism analysis of 16S rDNA, four different phytoplasma groups were detected in potato growing areas in Mexico. The aster yellows group (16SrI) 'Candidatus Phytoplasma asteris' was distributed in all potato growing areas, whereas peanut witches'-broom group (16SrII) 'Candidatus Phytoplasma aurantifolia' was detected in Guanajuato and Sinaloa, X-disease group (16SrIII) was detected in Coahuila and Guanajuato, and the Mexican periwinkle virescence (16SrXIII) was only detected in Sinaloa. Phytoplasmas from X-disease and Mexican periwinkle virescence groups were detected in potato samples for the first time in Mexico.

Incidence of the beet leafhopper-transmitted virescence agent phytoplasma in local populations of the beet leafhopper, Circulifer tenellus, in Washington State.

Phytoplasma diseases are increasingly becoming important in vegetable crops in the Pacific Northwest. Recently, growers in the Columbia Basin and Yakima Valley experienced serious outbreaks of potato purple top disease that caused significant yield loss and a reduction in tuber processing quality. It was determined that the beet leafhopper-transmitted virescence agent (BLTVA) phytoplasma was the causal agent of the disease in the area and that this pathogen was transmitted by the beet leafhopper, Circulifer tenellus Baker (Hemiptera: Cicadellidae). To provide the most effective management of phytoplasmas, timing of insecticide applications targeted against insects vectoring these pathogens should be correlated with both insect abundance and infectivity. Beet leafhoppers were collected from a potato field and nearby weeds in Washington throughout the 2005, 2006, and 2007 growing seasons and tested for BLTVA by PCR to determine the incidence of this phytoplasma in the insects. In addition, overwintering beet leafhoppers were collected throughout Columbia Basin and Yakima Valley and tested for BLTVA to investigate if these insects might constitute a source of inoculum for this phytoplasma from one season to the next. Results showed that 29.6% of overwintering leafhoppers collected near potato fields carried the phytoplasma. BLTVA-infected leafhoppers were also found in both potatoes and nearby weedy habitats throughout the growing season. PCR testing indicated that a large proportion of beet leafhoppers invading potatoes were infected with the phytoplasma, with an average of 20.8, 34.8, and 9.2% in 2005, 2006, and 2007, respectively. Similarly, BLTVA infection rate in leafhoppers collected from weeds in the vicinity of potatoes averaged 28.3, 24.5, and 5.6% in 2005, 2006, and 2007, respectively. Information from this study will help develop action thresholds for beet leafhopper control to reduce incidence of purple top disease in potatoes.

ASOCIACIÓN MOLECULAR DE Xylella fastidiosa EN PLANTAS DE PAPA (Solanum tuberosum L.) CON SÍNTOMAS DE PUNTA MORADA, EN MÉXICO

Tuber necrosis is one of the symptoms associated with potato purple top (PT) disease. In 2005 in Texas, USA, this symptom was named “zebra chip” or striped potato and was associated with the presence of Xyllela fastidiosa. The aim of this research was to detect the bacterium in plants exhibiting symptoms of PT. The three major regions of potato production in the State of Mexico were selected (Toluca, Atlacomulco and Valle de Bravo). Directed sampling was carried out in the spring-summer season of 2006. Polymerase Chain Reaction (PCR) with the specific primers RST31/RST33 was used to detect Xyllela fastidiosa; 25 % of the plants tested positive for the pathogen. Sequences of amplified fragments compared with sequences registered in the GENEBANK were 97 and 99 % homologous. The results suggest that necrosis and the symptoms of PT expressed in the potato plants in the State of Mexico are associated with X. fastidiosa.

An Emerging Potato Purple Top Disease Associated with a New 16SrIII Group Phytoplasma in Montana.

Potato purple top (PPT) is a devastating disease that occurs in the United States, Canada, Mexico, Russia, and elsewhere causing great economic loss to the potato industry through substantially reduced tuber yield and quality. Chips and fries processed from infected tubers often develop brown discoloration, greatly reducing their marketability. At least seven distinct phytoplasma strains belonging to five different phytoplasma groups (16SrI, 16SrII, 16SrVI, 16SrXII, and 16SrXVIII) have been reported to cause purple top and related symptoms in potato (3). During an unusual drought in 2007, a newly emerging potato disease with extensive yellowish or reddish purple discoloration of terminal shoots and leaves, similar to PPT symptoms, was observed in isolated potato fields in Montana where over 50% of plants exhibited symptoms. Shoot tissues were collected from three symptomatic plants and 17 tubers randomly collected from 17 other symptomatic plants. The tubers were cold treated to induce sprouting and then planted in the greenhouse. All tubers produced plants of which seven exhibited PPT symptoms including severe stunting. Total nucleic acid was extracted from leaf veinal tissue, stolons, or tubers of 10 symptomatic and 10 asymptomatic plants (both field-collected and greenhouse samples) as previously described (3). A nested-PCR assay, using universal primer pair P1/16S-SR followed by R16F2n/R16R2n, was performed as previously described (2,3) to detect phytoplasmas in these samples. Phytoplasma strains were detected in all symptomatic plants. Restriction fragment length polymorphism (RFLP) analyses of nested-PCR products (approximately 1.2 kb) with seven key restriction enzymes (AluI, MseI, HhaI, Tsp509I, HpaII, RsaI, and BfaI) indicated that all samples contained a very similar or identical phytoplasma most closely related to reference strain MW1 (belonging to subgroup 16SrIII-F) (1). Analysis of cloned 16S rDNA sequences confirmed the identity of this new phytoplasma and sequences of three representative PPT-MT strains were deposited in GenBank with Accession Nos. FJ226074-FJ226076. Computer-simulated RFLP analyses of 1.2-kb 16S rDNA sequences of this new phytoplasma and representative members in the peach X-disease phytoplasma group (16SrIII) available in GenBank indicated the strain is distinct and represents a new subgroup, 16SrIII-M (4). This study also indicated that the phytoplasma is tuber transmissible since approximately 35% of plants produced from infected tubers collected in this study developed symptoms. Transmission via infected tubers may pose a potential threat for disease spread by planting uncertified seed potatoes. To our knowledge, this is the first report of 16SrIII group phytoplasmas-associated diseases in potato. A phytoplasma closely related to the PPT-MT strains has recently been detected in potato seedlings exhibiting purple top, rosette, and stunting in Alaska (GenBank Accession No. FJ376628).

‘Candidatus Phytoplasma americanum’, a phytoplasma associated with a potato purple top wilt disease complex

Potato purple top wilt (PPT) is a devastating disease that occurs in various regions of North America and Mexico. At least three distinct phytoplasma strains belonging to three different phytoplasma groups (16SrI, 16SrII and 16SrVI) have been associated with this disease. A new disease with symptoms similar to PPT was recently observed in Texas and Nebraska, USA. Two distinct phytoplasma strain clusters were identified. One belongs to the 16SrI phytoplasma group, subgroup A, and the other is a novel phytoplasma that is most closely related to, and shares 96.6 % 16S rRNA gene sequence similarity with, a member of group 16SrXII. Phylogenetic analysis of 16S rRNA gene sequences of the novel PPT-associated phytoplasma strains, previously described 'Candidatus Phytoplasma' organisms and other distinct unnamed phytoplasmas indicated that the novel phytoplasma, termed American potato purple top wilt (APPTW) phytoplasma, represents a distinct lineage and shares a common ancestor with stolbur phytoplasma, "Candidatus Phytoplasma australiense", "Candidatus Phytoplasma japonicum", "Candidatus Phytoplasma fragariae", bindweed yellows phytoplasma (IBS), "Candidatus Phytoplasma caricae" and "Candidatus Phytoplasma graminis". On the basis of unique 16S rRNA gene sequences and biological properties, it is proposed that the APPTW phytoplasma represents "Candidatus Phytoplasma americanum", with APPTW12-NE as the reference strain.

Beet Leafhopper (Hemiptera: Cicadellidae) Transmits the Columbia Basin Potato Purple Top Phytoplasma to Potatoes, Beets, and Weeds

Experiments were conducted to determine whether the beet leafhopper, Circulifer tenellus (Baker) (Hemiptera: Cicadellidae), transmits the purple top phytoplasma to potato, Solanum tuberosum L.; beets, Beta vulgaris L.; and selected weed hosts. The beet leafhopper-transmitted virescence agent (BLTVA) phytoplasma was identified as the causal agent of the potato purple top disease outbreaks that recently occurred in the Columbia Basin of Washington and Oregon. The phytoplasma previously was found to be associated almost exclusively with the beet leafhopper, suggesting that this insect is the probable vector of BLTVA in this important potato-growing region. Eight potato cultivars, including 'Russet Burbank', 'Ranger Russet', 'Shepody', 'Umatilla Russet', 'Atlantic', 'FL-1879', 'FL-1867', and 'FL-1833', were exposed for a week to BLTVA-infected beet leafhoppers. After exposure, the plants were maintained outdoors in large cages and then tested for BLTVA by using polymerase chain reaction after 6 to 7 wk. The leafhoppers transmitted BLTVA to seven of the eight exposed potato cultivars. Sixty-four percent of the exposed plants tested positive for the phytoplasma. In addition, 81% of the BLTVA-infected potato plants developed distinct potato purple top disease symptoms. Beet leafhoppers also transmitted BLTVA to beets and several weeds, including groundsel, Senecio vulgaris L.; shepherd's purse, Capsella bursa-pastoris (L.) Medik); kochia, Kochia scoparia (L.) Schrad; and Russian thistle, Salsola kali L. This is the first report of transmission of BLTVA to potatoes, beets, and the above-mentioned four weed species. Results of the current study prove that the beet leafhopper is a vector of the potato purple top disease.

Beet Leafhopper (Hemiptera: Cicadellidae) Transmits the Columbia Basin Potato Purple Top Phytoplasma to Potatoes, Beets, and Weeds

Experiments were conducted to determine whether the beet leafhopper, Circulifer tenellus (Baker) (Hemiptera: Cicadellidae), transmits the purple top phytoplasma to potato, Solanum tuberosum L.; beets, Beta vulgaris L.; and selected weed hosts. The beet leafhopper-transmitted virescence agent (BLTVA) phytoplasma was identified as the causal agent of the potato purple top disease outbreaks that recently occurred in the Columbia Basin of Washington and Oregon. The phytoplasma previously was found to be associated almost exclusively with the beet leafhopper, suggesting that this insect is the probable vector of BLTVA in this important potato-growing region. Eight potato cultivars, including ‘Russet Burbank’, ‘Ranger Russet’, ‘Shepody’, ‘Umatilla Russet’, ‘Atlantic’, ‘FL-1879’, ‘FL-1867’, and ‘FL-1833’, were exposed for a week to BLTVA-infected beet leafhoppers. After exposure, the plants were maintained outdoors in large cages and then tested for BLTVA by using polymerase chain reaction after 6 to 7 wk. The leafhoppers transmitted BLTVA to seven of the eight exposed potato cultivars. Sixty-four percent of the exposed plants tested positive for the phytoplasma. In addition, 81% of the BLTVA-infected potato plants developed distinct potato purple top disease symptoms. Beet leafhoppers also transmitted BLTVA to beets and several weeds, including groundsel, Senecio vulgaris L.; shepherd’s purse, Capsella bursa-pastoris (L.) Medik); kochia, Kochia scoparia (L.) Schrad; and Russian thistle, Salsola kali L. This is the first report of transmission of BLTVA to potatoes, beets, and the above-mentioned four weed species. Results of the current study prove that the beet leafhopper is a vector of the potato purple top disease.

Association of Beet Leafhopper (Hemiptera: Cicadellidae) with a Clover Proliferation Group Phytoplasma in Columbia Basin of Washington and Oregon

At least 16 taxa of cicadellids and delphacids were tested by polymerase chain reaction (PCR) for the presence of a phytoplasma in the clover proliferation group, designated 16SrVI. Nucleic acid extracts from individual insects or groups of 5-10 were tested using PCR primers designed from the DNA sequence of 16S-23S rRNA or ribosomal protein genes of the pathogen. The beet leafhopper, Circulifer tenellus (Baker), was most often associated with the phytoplasma, with approximately 16% of the insects testing positive. The phytoplasma was occasionally found associated with Ceratagallia spp. Leafhopper species that were not associated with the phytoplasma included Macrosteles spp., Dikraneura spp., Colladonus montanus (Van Duzee), Circulifer geminatus (Van Duzee), Ballana spp., Amplysellus spp., Paraphlepsius spp., Texananus spp., Balclutha spp., Latalus spp., Erythroneura spp., Exitianus exitiosus (Uhler), and unidentified delphacids. The detected phytoplasma was similar to, or synonymous with, the beet leafhopper-transmitted virescence agent that is associated with the potato purple top disease in the Columbia Basin region of Washington and Oregon. This is in contrast to the phytoplasma associated with potato purple top disease in Mexico that is related to aster yellows (group 16SrI). The association of the group 16SrVI phytoplasma almost exclusively with the beet leafhopper suggests that this insect is the major vector of the phytoplasma in this region.

Clover Proliferation Group (16SrVI) Subgroup A (16SrVI-A) Phytoplasma is a Probable Causal Agent of Potato Purple Top Disease in Washington and Oregon.

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 characterized by 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 five asymptomatic) were collected from five 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. Eighty-four percent (63% in the first amplification) of the symptomatic samples and 60% (0% in the first amplification) of the asymptomatic samples tested positive. Low phytoplasma titers and the presence of PCR inhibitors accounts for the low detection rate in the first 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 three 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% sequence homology compared with 99.2% 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 two other confirmed cases of phytoplasmas (BLTVA and aster yellows phytoplasma) associated with PPT disease in Utah (4) and Mexico (3).