Agronomy and Crop Science

Greenhouse tomato growers from Fort Lupton, CO contacted the USDA-ARS-USHRL in 2002 regarding plant symptoms resembling “psyllid yellows” associated with Bactericera cockerelli (Sulc) infestations that initially begin as retarded growth, erectness of new growth, chlorosis, and purpling of leaves followed by widespread chlorosis and production of many small, poor-quality fruit (1). Symptoms appeared ≈6 weeks after psyllids were observed and were generally restricted to the top half of the plant. Leaf cuttings from beefsteak tomatoes cv. Quest were immediately placed in RNAlater (Applied Biosystems, Austin, TX). Samples from symptomatic and asymptomatic plants were collected in September and December of 2002. At each date, leaves were sampled from multiple plants and placed in separate RNAlater bottles. September samples exhibited initial “psyllid yellows” symptoms and December samples exhibited severe symptoms. Samples remained at 4°C in RNAlater for 6 years until recent findings suggested that a new species of bacteria, named either “Candidatus Liberibacter psyllaurous” (2) or “Ca. L. solanacearum” (3), may be the causal agent of “psyllid yellows”. The Qiagen (Valencia, CA) DNeasy Plant Kit and recommended protocols were used for four separate DNA isolations from each of the four tomato samples that had previously remained unopened. Five PCR primer pairs designed to amplify three distinct genetic regions within the “Ca L. psyllaurous” rrn operon (16S rRNA, 16S-23S rRNA intergenic region, and 23S rRNA) were used and one primer pair specific to the tomato DNA (18S rRNA gene) that successfully amplified from all samples was used as a positive control. Bacterial primers included one pair designed specifically for 16S rRNA sequences of ‘Ca. L. asiaticus’, ‘americanus’, and ‘africanus’ species (USHRL-CL1) and four sets, Lp-1 through Lp-4, previously described (2) that amplify nonoverlapping regions of the 16S-23S rRNA operon. The USHRL-CL1 primers (USHRL-CL1f: 5′-CTTACCAGCCCTTGACATGTATAGGA-3′, and USHRL-CL1r: 5′-TCCCTATAAAGTACCCAACATCTAGGTAAA-3′) amplify a 195-bp fragment from bp 895 to 1,089 of the ‘Ca. Liberibacter’ sp. 16S rRNA Genbank Accession No. L22532. Only samples from severe symptomatic plants collected in December 2002 yielded amplicons that were purified and sequenced (Genbank: USHRL-CL1, FJ871062; Lp-1, FJ871058; Lp-2, FJ871059; Lp-3, FJ871060; Lp-4, FJ871061). For each bacterial primer pair, the fragment amplified was highly homologous (98 to 100% identity) to “Ca. L. psyllaurous” rRNA gene/intergenic space sequences. The 16S rRNA coding region was identical to two GenBank ‘Ca. Liberibacter’ sp. entries: EU921627 and EU921626 from B. cockerelli samples collected in Dalhart, TX and zebra chip potato samples from Garden City, KS, respectively; however, the whole 2,500 bp amplified and sequenced from our sample contained 11 to 14 polymorphisms when compared to nine “Ca. L. psyllaurous” sequences. Our results clearly indicate that “Ca. L. psyllaurous” isolates were associated with tomato “psyllid yellows” symptoms in Colorado as early as 2002 and significant sequence variation exists within the 16S/23S rRNA intergenic region and 23S rRNA coding region to allow analysis of genetic diversity among “Ca. L. psyllaurous” isolates. References: (1) L. B. Daniels. Ph.D. diss. University of Minnesota, St. Paul, 1954. (2) A. K. Hansen et al. Appl. Environ. Microbiol. 74:5862, 2008. (3) L. W. Liefting et al. Plant Dis. 93:208, 2009.

Tomato (Solanum lycopersicum) plants exhibiting symptoms resembling those of permanent yellowing disease (known in Mexico as “permanente del tomate”) that is commonly associated with phytoplasmas (1) were observed in tomato fields in Sinaloa, México in March 2009. Plant symptoms also resembled those caused by “Candidatus Liberibacter solanacearum” infection (2). Affected plants showed an overall chlorosis, severe stunting, leaf cupping, purple discoloration of veins, excessive branching of axillary shoots, and leaf scorching (1,2). Symptom incidence ranged from 18 to 40%. To investigate whether liberibacter is associated with permanent yellowing disease of tomato in México, eight symptomatic and five asymptomatic tomato plants were collected from two fields in La Cruz de Elota and Culiacán, Sinaloa. Total DNA was extracted from the top whole leaf tissue of symptomatic and asymptomatic plants with cetyltrimethylammoniumbromide (CTAB) buffer (3,4). DNA samples were tested by PCR using primer pairs OA2/OI2c and CL514F/CL514R, which amplify a sequence from the 16S rDNA and rplJ and rplL ribosomal protein genes, respectively, of “Ca. L. solanacearum” (2,4). The DNA samples were also tested for phytoplasmas with nested PCR using universal primer pairs P1/P7 and fU5/rU3 (3). DNA from five and four symptomatic plants yielded the expected 1,168-bp 16S rDNA and 669-bp rplJ/rplL amplicons, respectively, indicating the presence of liberibacter. Extracts from asymptomatic plants yielded no products with these primers. Amplicons generated from three symptomatic plants with each primer pair were cloned into pCRII-TOPO plasmid vectors (Invitrogen, Carlsbad, CA) and three clones of each of these amplicons were subsequently sequenced in both directions (ACGT, Inc., Wheeling, IL). BLAST analysis of the 16S rDNA consensus sequence (GenBank Accession No. FJ957897) showed 100% identity to 16S rDNA sequences of “Ca. L. solanacearum” amplified from S. betaceum (EU935004) and S. lycopersicum (EU834130) from New Zealand (2), and “Ca. L. psyllaurous” from potato psyllids (EU812559). The rplJ/rplL consensus sequence (GenBank Accession No. FJ957895) 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 ‘Ca. Liberibacter’ sp. sequence amplified from zebra chip-infected potatoes from Lancaster, CA (FJ498803). No phytoplasmas were detected in the symptomatic tomato plants. To our knowledge, this is the first report of “Ca. L. solanacearum” associated with tomatoes in México. In 2008, this bacterium was detected in glasshouse tomatoes in New Zealand and caused millions of dollars in losses to the commercial glasshouse tomato industry (2). References: (1) R. L. Holguín-Peña et al. Plant Dis. 91:328, 2007. (2) L. W. Liefting et al. Plant Dis. 93:208, 2009. (3) J. E. Munyaneza et al. J. Econ. Entomol. 100:656, 2007. (4) J. E. Munyaneza et al. Plant Dis. 93:552, 2009.

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). References: (1) A. K. Hansen et al. Appl. Environ. Microbiol. 74:5862, 2008. (2) L. W. Liefting et al. Plant Dis. 93:208, 2009. (3) J. E. Munyaneza et al. J. Econ. Entomol. 100:656, 2007. (4) J. E. Munyaneza et al. Plant Dis. 93:552, 2009.

In January of 2009, potato plants (Solanum tuberosum) from a commercial crop in the Waikato Region, New Zealand were observed to have symptoms of upward rolling and purpling of the leaves. The symptoms appeared similar to those of “zebra chip”, a disorder of potato recently found to be associated with ‘Candidatus Liberibacter solanacearum’ in New Zealand and the United States (4). Total DNA from the leaf midveins and tubers from one of the symptomatic plants was separately extracted with an InviMag Plant DNA Mini Kit (Invitek GmbH, Berlin, Germany) and a KingFisher mL workstation (Thermo Scientific, Waltham, MA). DNA extracted from leaf midveins and tubers tested negative for ‘Ca. L. solanacearum’ by nested-PCR using primer pair OA2/OI2c (4) followed by Lib16S01F/Lib16S01R (5′-TTCTACGGGATAACGCACGG-3′ and 5′-CGTCAGTATCAGGCCAGTGAG-3′), which amplifies a 580-bp region of the 16S rRNA gene. However, DNA extracted from the tuber tissue tested positive for phytoplasma by TaqMan real-time PCR (3). No phytoplasma was detected in the DNA extracted from leaf tissue. The 16S rRNA gene, 16S-23S rRNA intergenic spacer region, and part of the 23S rRNA gene of the phytoplasma were amplified with primers P1/P7 (1). The PCR product was cloned into the pCR 4-TOPO vector (Invitrogen, Carlsbad, CA) and sequenced (GenBank Accession No. FJ943262). BLAST analysis showed 100% identity to ‘Ca. Phytoplasma australiense’ (16SrXII, Stolbur group). A fragment of approximately 850-bp of the Tuf gene was also amplified (2) and sequenced directly (GenBank Accession No. FJ943263). BLAST analysis showed 100% identity to Tuf gene variant IX of ‘Ca. P. australiense’ (2). An additional 14 plants showing similar leaf symptoms and also production of aerial tubers were collected from seven different potato fields from the Auckland and Waikato regions. Total DNA from the leaf midveins, stem, and tubers were separately extracted from each of the plants. The samples were tested for phytoplasma by nested-PCR using primer pair R16F2/R16R2, followed by NGF/NGR (1), and tested for ‘Ca. L. solanacearum’ by nested-PCR as described above. Seven plants tested positive only for phytoplasma, three tested positive for only ‘Ca. L. solanacearum’, and four plants tested positive for both pathogens. The pathogens were most commonly detected in samples extracted from the stem with 9 and 5 of the 14 samples testing positive for phytoplasma and liberibacter, respectively. Six of each of the leaf and tuber samples tested positive for phytoplasma. Liberibacter was detected in one of the leaf samples and in four of the tuber samples. ‘Ca. P. australiense’ has only been reported from New Zealand and Australia. The only other known hosts of ‘Ca. P. australiense’ in New Zealand are strawberry and native plants belonging to the genera Cordyline, Coprosma, and Phormium (2). In Australia, ‘Ca. P. australiense’ is associated with Australian grapevine yellows and Papaya dieback (2). To our knowledge, this is the first report of ‘Ca. P. australiense’ infecting potato as well as the first report of phytoplasma and ‘Ca. L. solanacearum’ mixed infections in potato. References: (1) M. T. Andersen et al. Plant Pathol. 47:188, 1998. (2) M. T. Andersen et al. Phytopathology 96:838, 2006. (3) N. M. Christensen et al. Mol. Plant Microbe Interact. 17:1175, 2004. (4) L. W. Liefting et al. Plant Dis. 93:208, 2009.

In August 2008, unusual symptoms were observed in Mexican lime trees (Citrus aurantifolia (Christm.) Swing) in the municipality of Luperón, province of Puerto Plata on the north coast of the Dominican Republic. Symptoms observed in young and old trees included blotchy mottle on leaves, healthy-appearing larger branches with smaller side branches that displayed chlorotic leaves, abscised and lopsided fruit, and branch dieback, all symptoms similar to those of citrus huanglongbing associated with ‘Candidatus Liberibacter’ spp. (1). Symptoms were observed in an area of ~100 ha surrounding Luperón, where Mexican lime trees were grown as seedlings and no commercial plantings of other citrus were present. Symptomatic leaves were collected from 16 trees in September 2008, and DNA was extracted from petioles and midveins with a DNeasy kit (Qiagen, Gaithersburg, MD) or with chloroform/isoamyl alcohol (24:1). Real-time PCR with the16S rDNA primer/probe set specific to ‘Ca. Liberibacter asiaticus’ and performed as described (2) gave Ct values comparable with the positive control for five samples. Conventional PCR with the forward (5′-tcgagcgcgtatgcaatacg-3′) and reverse (5′-ctacctttttctacgggataacgc-3′) primers used in real-time PCR (2) amplified a 75-bp product from these five samples. Eleven to twelve clones were sequenced from each sample and BLAST analysis of a consensus sequence for each sample (GenBank Accession Nos. FJ489643–FJ489647) showed 98% (e.g., EU921622, EU921618) to 100% identity (e.g., FJ236554, FJ263702) with 16S rDNA sequences of ‘Ca. Liberibacter asiaticus’. A larger 912-bp portion of the 16S rDNA gene was amplified from each sample by conventional PCR with sense (5′-gagcctaccaaggctacgat-3′) and antisense (5′-gcgttatcccgtagaaaaaggtag-3′) primers designed from the sequence of ‘Ca. Liberibacter asiaticus’ strain LJZ-4730 (GenBank Accession No. FJ263700). Nine to twelve clones were sequenced for each sample and BLAST analysis of a consensus sequence for each sample (GenBank Accession Nos. FJ811891–FJ811895) showed 100% identity to ‘Ca. Liberibacter asiaticus’ (DQ471900, DQ4719010) from Florida and Brazil, respectively. To confirm this identification, 693 bp of the outer membrane protein gene (omp) were amplified from each of the five positive samples by conventional PCR using sense (5′-gtgattctgagggtgagcg-3′) and antisense (5′-cgaactcactgagaactgatc-3′) primers designed from nucleotides 15 to 33 and 687 to 707, respectively, of the omp gene from ‘Ca. Liberibacter asiaticus’ strain MZ (GenBank Accession No. EF580135). The consensus sequences from 12 clones from each sample (GenBank Accession Nos. FJ489638–FJ489642) showed 99% (e.g., FJ236564, AY842429) to 100% (e.g., FJ236566, EF580135) nucleotide identity and the predicted translation product showed 100% amino acid identity (e.g., AAX47433, AAX47431) with the omp gene from ‘Ca. Liberibacter asiaticus’. These results confirm the presence of and to our knowledge, this is the first report of ‘Ca. Liberibacter asiaticus’ in the Dominican Republic. Huanglongbing is a destructive disease of citrus (1) and its spread is expected to adversely affect citrus production in the Dominican Republic with subsequent negative effects predicted for the employment of thousands of people. References: (1) J. M. Bové, J. Plant Pathol. 88:7, 2006. (2) W. Li et al. J. Microbiol. Methods 66:104, 2006.

A new disease of potatoes, tentatively named zebra chip (ZC) because of the intermittent dark and light symptom pattern in affected tubers which is enhanced by frying, was first found in Mexico in 1994 and in the southwestern United States in 2000. The disease can cause severe economic losses in all market classes of potatoes. The cause of ZC has been elusive, and only recently has been associated with ‘Candidatus Liberibacter’ sp. Field samples of potato plants were collected from several locations in the United States, Mexico, and Guatemala to determine transmission to potato and tomato by grafting of ZC-infected scions and psyllid feeding. The disease was successfully transmitted, through up to three generations, by sequential top- and side-grafting ZC-infection scions to several potato cultivars and to tomato. The disease was also successfully transmitted to potato and tomato plants in greenhouse experiments by potato psyllids collected from potato plants naturally affected with ZC. Transmission electron microscopic observation of ZC-affected tissues revealed the presence of bacteria-like organisms (BLOs) in the phloem of potato and tomato plants inoculated by grafting and psyllid feeding. The BLOs were morphologically similar in appearance to BLOs associated with other plant diseases. Polymerase chain reaction (PCR) amplified 16S rDNA sequences from samples representing different geographic areas, including the United States, Mexico, and Guatemala, were almost identical to the 16S rDNA of ‘Ca. L. solanacearum’ previously reported from solanaceous plants in New Zealand and the United States. Two subclades were identified that differed in two single base-pair substitutions. New specific primers along with an innovative rapid PCR were developed. This test allows the detection of the bacteria in less than 90 min. These data confirm the association of ‘Ca. L. solanacearum’ with potatoes affected by ZC in the United States, Mexico, and Guatemala.

In North America, elm yellows, aster yellows (AY), and X-disease phytoplasmas have been detected in American grapevines (1), and recently, Bois noir was detected in Canadian vineyards from British Columbia (BC) and Ontario (ON) (2). Typical symptoms of grapevine yellows (GY) include leaf rolling and chlorosis, uneven or total lack of lignification of canes, flower abortion or berry withering, and stunting. In 2006 and 2007, independent surveys were conducted by the Canadian Food Inspection Agency (CFIA) and Agriculture and Agri-Food Canada (AAFC) to detect phytoplasmas in Canadian vineyards containing different cultivars in BC, ON, Québec (QC), Nova Scotia, New Brunswick, and Prince Edward Island. The CFIA collected and tested 651 fresh leaf samples from recently imported grapevines and older grapevines in the same or neighboring blocks displaying symptoms typical of those associated with disease caused by phytoplasmas. Many vineyards were surveyed only once. AAFC collected and tested 3,485 samples from symptomatic and asymptomatic grapevines from established vineyards in ON, BC, and QC. The same vineyards were sampled in ON and BC both years; QC vineyards were only sampled in 2007. AAFC-collected leaf samples were freeze dried and stored at –20°C before processing. CFIA samples were tested by a modified real-time PCR assay and TaqMan probe targeting the 16S ribosomal RNA gene that detects a wide range of known phytoplasmas (2). Positive samples were confirmed by conventional PCR using the phytoplasma-specific primers P1/P7 (3) and the resulting ~1,800-bp fragment was cloned and sequenced as previously described (2). DNA extracted by AAFC was amplified by nested PCR technology with universal phytoplasma specific primer pairs P1/P6 and R16R2/R16F2 (3) and the resulting 1,200-bp fragment was cloned and sequenced. Two plants, one located in ON in 2006 and the other in BC in 2007, were found to be infected with an AY-like phytoplasma by the CFIA. The phytoplasmas detected in both infected plants had a 99.9% nt sequence identity with AY phytoplasma sequences from GenBank (Accession Nos. AF222063 and AY665676, respectively), with the BC isolate also showing 100% identity to a strain of AY, ash witches'-broom phytoplasma (GenBank Accession No. AY566302). AAFC detected phytoplasma DNA in both years in a total of 17 symptomatic plants and 21 asymptomatic plants from different vine varieties in ON, BC, and QC. Positive samples were found to have a 99.0% nt sequence identity to AY subgroup 16SrI-A (GenBank Accession No. AY180956). Sequences were exchanged for confirmation of phytoplasma identity and were deposited in Genbank under Accession Nos. FJ659844 and FJ824597. Phytoplasma strains were identified for all plants in which phytoplasmas were detected. Results show that AY is present in vineyards in the provinces of ON, BC, and QC. To our knowledge, this is the first report of AY being detected in grapevines in Canada. References: (1) E. Boudon-Padieu. Bull. O I V, 79:299, 2003. (2) M. Rott et al. Plant Dis. 91:1682, 2007. (3) E. Tanne et al. Phytopathology 91:741, 2001.

A disease that severely affects processing potatoes (Solanum tuberosum L.), termed zebra chip (ZC), has been identified in several locations in the United States (Texas, Nebraska, Colorado, Kansas, New Mexico, Arizona, and Nevada), Mexico, and Central America (4). The disease name comes from the rapid oxidative darkening of freshly cut tubers and the dark stripes and blotches that occur in chips processed from infected tubers. Recently, the disorder has been associated with a new ‘Candidatus Liberibacter’ species in New Zealand (3). Also, a bacterium designated ‘Candidatus Liberibacter psyllaurous’ has been identified recently in potato plants with “psyllid yellows” symptoms that resemble foliar symptoms of ZC (2). In the fall of 2008, 10 tubers were received at the Prosser laboratory from a commercial potato grower and five had symptoms characteristic of ZC. The tubers, cv. Dakota Pearl, were grown near Lancaster in southern California. The tubers showed rapid oxidation upon slicing and the sunken stolon attachment characteristic of ZC (4). Nucleic acid was extracted from symptomatic tubers (1) and tested by PCR for ‘Ca. Liberibacter’ species with primer pairs OA2/OI2c (5′-GCGCTTATTTTTAATAGGAGCGGCA-3′ and 5′-GCCTCGCGACTTCGCAACCCAT-3′) and CL514F/R (5′-CTCTAAGATTTCGGTTGGTT-3′ and 5′-TATATCTATCGTTGCACCAG-3′), which amplify from the 16S rDNA and rplJ and rplL ribosomal protein genes, respectively (3). Four of the five tubers with distinct ZC symptoms yielded the expected amplicons with both primer pairs. Two tubers with mild internal discoloration yielded correctly sized amplicons but in lesser amounts than from the severely affected tubers. Nucleic acid from healthy potato tubers yielded no product with these primers. One clone of the 1,168-bp OA2/OI2c amplicon and two clones of the 669-bp CL514F/R amplicon from a strongly positive sample were sequenced in both directions (ACGT, Inc., Wheeling, IL). BLAST alignments of the consensus sequences of the OA2/OI2c and CL514F/R amplicons (GenBank Accessions Nos. FJ498802 and FJ498803, respectively) revealed 100% identity with analogous ‘Ca. Liberibacter’ sequences reported from ZC-symptomatic potatoes in New Zealand (GenBank Accession Nos. EU849020 and EU919514). The OA2/OI2c amplicon was also identical to a sequence of ‘Ca. Liberibacter psyllaurous’ (GenBank Accession No. EU812559) from psyllid yellows-affected potatoes in the United States (2) and also showed a 99% identity with sequences from a ‘Ca. Liberibacter’ species reported in ZC tubers from Kansas (GenBank Accession No. EU921626). Potato crops with symptoms of ZC have been observed previously in California (4), but this is the first identification of ‘Ca. Liberibacter psyllaurous’ from diseased potatoes grown in California. Since ZC was first reported in the mid- to late-1990s, information from potato growers and processors suggests that ZC is becoming more important. The disease has caused millions of dollars in losses, particularly in south Texas (4). The identification of ‘Ca. Liberibacter psyllaurous’ in California provides additional evidence that the disease is increasing in importance in other potato-growing regions. References: (1) J. M. Crosslin et al. Plant Dis. 90:663, 2006. (2) A. K. Hansen et al. Appl. Environ. Microbiol. 74:5862, 2008. (3) L. W. Liefting et al. Plant Dis. 92:1474, 2008. (4) J. E. Munyaneza et al. Subtrop. Plant Sci. 59:30, 2007.