Potato Psyllid Bactericera Cockerelli Research Articles

Movement of Bactericera cockerelli (Heteroptera: Psyllidae) in Relation to Potato Canopy Structure, and Effects on Potato Tuber Weights

With the threat of new plant diseases on the increase, plant disease epidemiology requires research on pathogen vector movement. Here, releases were performed in planted potato fields of different ages and canopy structures, located in the Texas Panhandle, to evaluate the range of movement of the potato psyllid Bactericera cockerelli (Sulc.). This insect is a known causative agent of psyllid yellows disease, and is a vector of the reported etiological agents of zebra chip disease of potato, 'Candidatus Liberibacter solanacearum/psyllaurous'. Based on collections of B. cockerelli immatures along transects 9 m long radiating in four cardinal directions from release points, adult females dispersed considerable distances, regardless of plant age or canopy structure. Immature abundance declined along transects and were well described by linear and nonlinear models, but abundance patterns did not differ among the different planting dates and canopy structures. However, unequal immature abundance was detected among the four cardinal directions, with more immatures generally collected along transects to the north and west of release points, opposite of prevailing winds in the area at the time of release. Plots where B. cockerelli were released had significantly lower mean potato tuber weights than control plots with no B. cockerelli. However, few plots with B. cockerelli released in them had declining trends in tuber weights with increasing distance from release points.

First Report of "Candidatus Liberibacter solanacearum" on Field Tomatoes in the United States.

In August 2008, 30% of tomato (Solanum lycopersicum) plants in plots in Lubbock County, Texas showed yellowing, lateral stem dieback, upward leaf curling, enlargement of stems, adventitious roots, and swollen nodes. Yellowing in leaves was similar to that seen with zebra chip disease (ZC) of potato that was confirmed in a potato field 112 km away in July 2008 and was associated with a 'Candidatus Liberibacter' species (1), similar to findings earlier in 2008 in New Zealand and California (2,3). Tissue from four symptomatic plants of cv. Spitfire and two of cv. Celebrity were collected and DNA was extracted from midribs and petioles with a FastDNA Spin Kit (Qbiogene, Inc., Carlsbad, CA,). PCR amplification was done with 16S rRNA gene primers OA2 and OI2c, which are specific for "Ca. Liberibacter solanacearum" from potato and tomato and amplify a 1.1-kb fragment of the 16S rRNA gene of this new species (1,3). Amplicons of 1.1 kb were obtained from all samples and these were sequenced in both orientations (McLab, San Francisco, CA). Sequences of the 16S rRNA gene were identical for both Spitfire and Celebrity and were submitted to the NCBI as GenBank Accession Nos. FJ939136 and FJ939137, respectively. On the basis of a BLAST search, sequence alignments revealed 99.9% identity with a new species of 'Ca. Liberibacter' from potato (EU884128 and EU884129) in Texas (1); 99.7% identity with the new species "Ca. Liberibacter solanacearum" described from potato and tomato (3) in New Zealand (EU849020 and EU834130, respectively) and from the potato psyllid Bactericera cockerelli in California (2) (EU812559, EU812556); 97% identity with 'Ca L. asiaticus' from citrus in Malaysia (EU224393) and 94% identity with both 'Ca. L. africanus' and 'Ca. L. americanus' from citrus (EU921620 and AY742824, respectively). A neighbor-joining cladogram constructed using the 16S rRNA gene fragments delineated four clusters corresponding to each species, and these sequences clustered with "Ca. L. solanacearum". A second PCR analysis was conducted with the CL514F/CL514R primer pair, which amplifies a sequence from the rplJ and rplL ribosomal protein genes of "Ca. L. solanacearum". The resulting 669-bp products were 100% identical to a sequence reported from tomato in Mexico (FJ498807). This sequence was submitted to NCBI (GU169328). ZC, a disease causing losses to the potato industry, is associated with a 'Candidatus Liberibacter' species (1-3) and was reported in Central America and Mexico in the 1990s, in Texas in 2000, and more recently in other states in the United States (4). In 2008, a "Ca. Liberibacter solanacearum" was detected on Capsicum annuum, S. betaceum, and Physalis peruviana in New Zealand (3). Several studies have shown that the potato psyllid, B. cockerelli, is a potential vector for this pathogen (2,4). To our knowledge, this is the first report of "Ca. Liberibacter solanacearum" in field tomatoes showing ZC-like foliar disease symptoms in the United States.

First Report of "Candidatus Liberibacter psyllaurous" or "Ca. Liberibacter solanacearum" Associated with Severe Foliar Chlorosis, Curling, and Necrosis and Tuber Discoloration of Potato Plants in Honduras.

From 2006 to 2009, all commercial potato fields in Azacualpa F.M. Honduras were heavily infested with the potato psyllid Bactericera cockerelli (Sulc.). Plants exhibited interveinal chlorosis, vein-greening, downward curling, stunting, above ground tuber formation, and brownish flecks in some tubers. Disease incidence ranged from 50 to 95%. Leaf samples and psyllids were collected from seven fields in two potato-growing regions of Honduras. Total DNA was purified from the leaves of 30 symptomatic and three asymptomatic plants. DNA was extracted from 20 adult and 10 immature (4th to 5th instar) psyllids according to Frohlich et al (1). PCR primers, PSY680F 5'-GTTCGGAATAACTGGGCGTA-3' and PSY1R 5'-CCCATAAGGGCCATGAGGACT-3', were used to amplify a 680-bp fragment of the 16S rDNA for the recently described "Candidatus Liberibacter physallaurous" (2) and "Ca. L. solanacearum" (3). PSY1R/PSY680F primer design was based on the association of a previously undescribed liberibacter with vein-greening symptoms in greenhouse tomato plants in Arizona from 2006 to 2007 (GenBank Accession No. GQ926918) that lead to the hypothesis that a similar bacterium could be associated with symptomatic potato plants in Honduras. PCR amplification, cloning, and sequencing of the resultant 16S rDNA amplicons indicated that 17 of 30 potato plants, 8 of 20 adult and 7 of 10 third to fourth instar psyllids, respectively, were positive for liberibacter based on 99 to 100% shared nucleotide sequence (nt) identity with the analogous sequence from liberibacter (EU812558 [2]). To substantiate these results, a second molecular marker was targeted using the 1611F and 480R primers (~980 bp) that amplify the 16S-23S-ITSrDNA of liberibacter (2) for selected liberibacter-positive samples (above). Amplicons of the expected size were obtained from 12 of 17 potato and 7 of 10 immature psyllids. No PCR product of the expected size was obtained from asymptomatic potato samples or the PCR negative (water) control. The resultant PCR amplicons were cloned and 12 to 15 clones per amplicon were sequenced. The sequences were aligned and the percentage pair wise nt identity was calculated by Clustal W revealing that the 16S rDNA and 16S-23S-ITS sequences, respectively, shared 99 to 100% nt identity with each other. BLAST analysis against the NCBI database indicated that the 16S rRNA sequences from potato plants (GQ926922) and immature psyllids (GQ926923), and the 16S-ITS-23S sequence from potato plants (GQ926924) and immature psyllids (GQ926925), shared 98.5 to 100% nt identity with 'Ca. Liberibacter' reported from potato (EU812556; [2,4]) and tomato (EU812558, EU812559, EU935005; [2,3]). Evidence for the widespread presence of liberibacter and the potato psyllid in potato fields in Honduras, together with foliar and tuber symptoms that are reminiscent of those recently described in potato plants in the United States affected with 'zebra chip' disease (4), suggest that a similar or identical disease of the potato also occurs in Honduras. This emergent disease poses a serious threat to potato production in Honduras and elsewhere in Central America.

First Report of "Candidatus Liberibacter psyllaurous" (synonym "Ca. L. solanacearum") Associated with 'Tomato Vein-Greening' and 'Tomato Psyllid Yellows' Diseases in Commercial Greenhouses in Arizona.

During the winter of 2006-2007, plants in commercial tomato greenhouses (GH-1 and GH-2; total 320 acres [129.5 ha]) in Arizona were infested with the potato psyllid Bactericera cockerelli (Sulc) and more than 60% and ~20% of the plants, respectively, exhibited leaf curling, chlorosis, and shortened internodes. In addition, some plants in GH-1 developed an unusual 'vein-greening' phenotype. Nucleic acids were isolated from 10 symptomatic and three asymptomatic plants from each greenhouse. PCR primers designed to amplify a phytoplasma-like 16S rDNA (850 bp) yielded the expected size product from GH-1 samples, whereas samples from GH-2 and the asymptomatic samples from both greenhouses did not. Several 16S rDNA PCR products (3 of 60) when cloned and sequenced, surprisingly shared 97% homology with 'Candidatus Liberibacter asiaticus' (GenBank No. GQ926917). PCR primers PSY680F 5'-GTTCGGAATAACTGGGCGTA-3' and PSY1R 5'-CCCATAAGGGCCATGAGGACT-3', based on the resultant 16S rDNA sequences, were used to amplify a 680-bp fragment from plant DNA extracts and psyllid lysates (1). A robust PCR product (~680 bp) was obtained from 10 of 10 GH-1 plant extracts (GQ926918) and from a GH-1-derived psyllid colony (28 of 35 adults) (GQ926919) and the tomato plants on which they were reared. In contrast, no 680-bp product was obtained from GH-1 asymptomatic plants (0 of 3), GH-2 plants (0 of 10 symptomatic; 0 of 3 asymptomatic), GH-2-derived psyllid colonies (0 of 35 adults), or psyllid colony tomato plants (data not shown). At least three 680-bp amplicons for each sample type were cloned and 8 to 10 inserts were sequenced for each. BLAST analysis revealed that all 680-bp sequences shared 99 to 100% nt identity with the analogous 16SrDNA from "Ca. Liberibacter psyllaurous" (2) and synonym "Ca. L. solanacearum" (3). A second molecular marker was obtained with the 1611F and 480R primers (2) to amplify the 16SrDNA-23S-ITS (980 bp) from >3 plant extracts and psyllid lysates that tested positive for liberibacter. Clustal W alignment of the 16S-23S-ITS sequences from GH-1 original tomato plants and psyllid colony plants (GQ926920) and psyllids (GQ926921) indicated they were 100% identical to each other and BLAST analysis indicated 99 to 100% shared identity with "Ca. L. psyllaurous" (EU812558) (synonym "Ca. L. solanacearum"). Transmission electron microscopy examination of GH-1 and GH-2 psyllids revealed rod and pleomorphic-shaped bacteria (0.5 to 2.0+ μm) at the brain-salivary gland interface in psyllids from the GH-1 liberibacter-positive colony. No such bacteria were observed in GH-2 liberibacter-negative psyllids. These results support an etiological role of a new liberibacter spp. in the development of the 'vein-greening' symptom phenotype. In contrast, the GH-2 'yellows' phenotype is reminiscent of 'psyllid toxicity' in tomato colonized by B. cockerelli (4). To our knowledge, this is the first report of distinct psyllid-associated diseases in greenhouse tomato in Arizona, one associated with a new 'Ca. Liberibacter' spp., manifest as 'vein-greening' disease, and the other associated with psyllid feeding, in which liberibacter is undetectable in plants and psyllids, and is manifest as the 'tomato psyllid yellows' disease.

Potato Zebra Chip Disease: A Phytopathological Tale

Potato zebra chip (ZC) disease is a relative newcomer to the world of important potato diseases. First reported in Mexico in the 1990s, by 2004-2005 the disease was causing serious economic damage in parts of Texas. ZC is now widespread in the south-western and central United States, Mexico, Central America, and was recently reported in New Zealand. By 2006, there seemed to be an association between ZC and the potato psyllid (Bactericera cockerelli). The exact nature of the relationship, however, has only recently been identified by the discovery of a new Candidatus Liberibacter bacterium that is transmitted to potatoes, tomatoes, and other solanaceous hosts by the potato psyllid. This review examines the history of this disease, the association of ZC with the potato psyllid, the host range, and recent research into the bacterial pathogen. Accepted for publication 15 December 2009. Published 17 March 2010.

First Report of "Candidatus Liberibacter solanacearum" in Pepper Plants in México.

Bell pepper (Capsicum annuum) plants exhibiting symptoms that resembled those of potato psyllid (Bactericera cockerelli Sulc) damage and "Candidatus Liberibacter solanacearum" infection (2) were observed in a pepper field in La Cruz de Elota, Sinaloa, México in March 2009, with an infection rate of 1.5%. Plants exhibited chlorotic or pale green apical growth and leaf cupping, sharp tapering of the leaf apex, shortened internodes, and an overall stunting (2). Total DNA was extracted from the top whole leaf tissue of nine symptomatic and five asymptomatic pepper plants with cetyltrimethylammoniumbromide (CTAB) buffer (3,4). Seven and eight of the nine selected symptomatic pepper plants yielded the expected 1,168-bp 16S rDNA and the expected 669-bp rplJ/rplL ribosomal protein gene amplicons with the "Ca. L. solanacearum" specific OA2/OI2c and CL514F/CL514R primer pairs, respectively, indicating the presence of liberibacter (2,4). Nucleic acid from asymptomatic pepper plants yielded no products with these primers. Three amplicons generated from symptomatic pepper plants with each primer pair were cloned into pCRII-TOPO plasmid vectors (Invitrogen, Carlsbad, CA) and three clones of each amplicon were sequenced in both directions (ACGT, Inc., Wheeling, IL). BLAST analysis of the 16S rDNA consensus sequence (GenBank Accession No. FJ957896) showed 100% identity to 16S rDNA sequences of "Ca. L. solanacearum" amplified from Solanum betaceum (EU935004) and S. lycopersicum (EU834130) from New Zealand (2), and "Ca. L. psyllaurous" from potato psyllids (EU812559) (1). The ribosomal protein gene consensus sequence (GenBank Accession No. FJ957894) was 100% identical to the analogous rplJ and rplL "Ca. L. solanacearum" ribosomal protein gene sequence amplified from S. lycopersicum (EU834131) from New Zealand (2) and to 'Ca. Liberibacter' sp. sequence amplified from zebra chip-infected potato tubers from Lancaster, CA (FJ498803). To our knowledge, this is the first report of "Ca. L. solanacearum" associated with bell pepper in México. "Ca. L. solanacearum" was first reported in tomato and pepper plants in 2008 in New Zealand, where it has resulted in plant decline and significant yield loss, resulting in millions of dollars in losses to the commercial glasshouse tomato and pepper industry (2). Zebra chip, a new and emerging potato disease associated with 'Ca. Liberibacter' sp., was first identified in México in 1994, where it has caused significant economic damage, often leading to abandonment of entire potato fields (3,4).