Maritimes Regional Meeting, 2012/Réunion régionale des Maritimes, 2012

Managing potato common scab in the field using biopesticides, soil additives or soil fumigants. K. I. AL-MUGHRABI, A. VIKRAM, R. POIRIER AND K. E. JAYASURIYA. Potato Development Centre, New Brunswick Department of Agriculture, Aquaculture and Fisheries, 39 Barker Lane, Wicklow, NB E7L 3S4, Canada

Two field trials were conducted in 2008 and 2009 at McCain's Research Farm, Florenceville-Bristol, New Brunswick, Canada to assess the efficacy of Bacillus subtilis, Enterobacter cloacae, Chloropicrin, Pic-Plus, manganese sulphate and mustard meal against common scab of potato caused by Streptomyces scabiei. The trials consisted of the following treatments, each replicated four times: (1) control; (2) seed treated with a 2.8% solution of B. subtilis (7.3 × 10⁹ CFU g⁻¹) applied at 1 mL/seed; (3) seed treated with a solution of E. cloacae (10⁸ CFU mL⁻¹) in nutrient broth applied at 0.3 mL/seed; (4) seed dusted with fludioxonil applied at 500 g/100 kg of cut seed; (5) seed dusted with mancozeb applied at 882 g/100 kg of cut seed; (6) mustard meal applied to soil at 1065 kg ha⁻¹ 3 days prior to planting; (7) manganese sulphate applied to soil at 75 kg ha⁻¹ at planting; (8) Pic-Plus injected to soil at 64 L ha⁻¹; and (9) chloropicrin 100 injected to soil at 55 L ha⁻¹. Disease severity was significantly reduced due to seed treatments with B. subtilis or fludioxonil or due to the addition of mustard meal to the soil. The same three treatments increased marketable yield by 32.5%, 24.6% and 24.6%, respectively. These findings indicate the potential of biopesticides and soil additives in managing common scab of potatoes.

How fast, how many and for how long – deciphering the defence mechanisms induced by phosphite-based fungicides in potatoes. T. BORZA, Y. WU AND G. WANG-PRUSKI. Department of Plant and Animal Sciences, Faculty of Agriculture, Dalhousie University, 50 Pictou Road, Truro, NS B2N 5E3, Canada

Fungicides containing phosphorous acid derived salts (phosphites – Phi) are increasingly used in controlling the development of the oomycete Phytophthora infestans (Mont.) de Bary, which is responsible for the occurrence of late blight disease in potatoes. The fungistatic effects of Phi are exerted directly by inhibiting pathogen's development and indirectly by inducing plant defence mechanisms. The way Phi induces resistance in plants is still largely unknown. With the aim to determine the effects of Phi on potato plants’ signalling pathways and on defence mechanisms that are likely to be responsible for induced resistance against P. infestans we analysed, using quantitative RT-PCR, the expression pattern of several genes involved in the salicylate (SA), jasmonic (JA) and ethylene (ET) signalling pathways before and after treatment with the Phi-based fungicide Confine^TM (Winfield Solutions, LLC, St. Paul, MN). The results indicate that Phi induces rapid activation (30 min to 24 h) of positive transcriptional regulators in the SA pathway. ET seems to work in conjunction with SA to transiently silence (2 h to 48 h) the expression of genes involved in the JA signalling pathway. Notably, SA-related expression pattern overlaps with Phi translocation pattern determined by ion chromatography. Limited responses have been detected in Phi-treated and untreated plants infected with P. infestans. The finding that Phi triggers fast but short-lasting activation of the SA signalling pathway, and of other defence mechanisms, suggests that improved field practices, e.g. the timing of Phi application, should be considered in order to achieve maximum crop protection.

Mannitol's possible mode of action against angular leaf spot of strawberry. P. G. BRAUN AND P. D. HILDEBRAND. Atlantic Food and Horticulture Research Centre, Agriculture and Agri-Food Canada, 32 Main Street, Kentville, NS B4N 1J5, Canada

Bacterial angular leaf spot of strawberry (Fragariae × ananassa Duchesne), caused by Xanthomonas fragariae Kenn. & King is a widely distributed disease with few control options. Planting disease-free material and application of copper-based bactericides have been the primary management strategies but they have had limited success. In a search for new chemical controls we discovered that mannitol, a simple sugar alcohol, provided >90% disease control in controlled environment experiments. It has been reported that mannitol induces disease resistance in tobacco. In strawberry, mannitol provided excellent disease control when applied between 8 days pre-infection and 4 days post-infection. The best control was obtained when mannitol remained on the leaf surface for 24–48 h. These responses to mannitol could be consistent with induced resistance. However, treatment of the centre leaflets of trifoliate leaves with mannitol protected only the treated leaflet from infection. Thus the induced resistance does not appear to be systemic. In addition, in liquid shake cultures of X. fragariae in the presence of mannitol the exopolysaccharide normally produced is reduced by ∼50%. Exopolysaccharide is considered a virulence factor in some pathosystems. RT-PCR analysis of known resistance genes is currently being conducted to provide more specific evidence of induced resistance.

Assessment of in-furrow application of Vertisan^TM (penthiopyrad) fungicide for control of black scurf [ Rhizoctonia ] and common scab in three varieties of potatoes. R. COFFIN, J. COFFIN, R. PETERS, K. DRAKE, B. BEATON AND B. FRASER. Privar Farm Inc. 909 Eliot River Road, North Wiltshire, PE C0A 1Y0, Canada; (R.P., K.D.) Crops and Livestock Research Centre, Agriculture and Agri-Food Canada, 440 University Avenue, Charlottetown, PE C1A 4N6, Canada; (B.B.) PEI Department of Agriculture and Forestry, P.O. Box 2000, Charlottetown, PE C1A 7N8, Canada; and (B.F.) Dupont Canada Company, P.O. Box 2200, Streetville, Mississauga, ON L5M 2H3, Canada

Potato disease symptoms on potato skin can render tubers unmarketable for fancy tablestock markets and seed use. Vertisan^TM was applied in-furrow, over seed pieces at planting, at the rate of 30 mL of product per 100 metres of row in a band width of approximately 25 cm. Three potato varieties (‘Yukon Gold’, ‘Arbor Globe’, ‘Hot Pink’) were planted in the treated and non-treated blocks. Previous crops of potatoes in this land had extensive development of black scurf and common scab. Six tubers per plant were harvested from four plants in each variety from the treated and non-treated soil. Tuber samples were coded and rated without bias by six evaluators. Common scab was prevalent (10–15% of skin surface) on the three potato varieties and the extent of development was unaffected by the treatment. Daughter tubers from plants of all three potato varieties, in the Vertisan^TM treated rows, did not show any Rhizoctonia symptoms (black scurf stage). In non-treated samples, Rhizoctonia was most prevalent on ‘Yukon Gold’, low in ‘Hot Pink’ and non-detectable in ‘Arbour Globe’. The Vertisan^TM treatment suppressed the development of black scurf and improved acceptability of potato tubers to meet requirements for fancy tablestock and seed potatoes.

Evaluation of seven tomato varieties for field resistance to late blight [ Phytophthora infestans ] . R. COFFIN, J. COFFIN, R. PETERS, K. DRAKE, D. GREGORY, M. MACDONALD, A. MACPHAIL, G. WANG-PRUSKI, B. BEATON, C. BANKS, L. KAWCHUCK AND A. MELISH. Privar Farm Inc., 909 Eliot River Road, North Wiltshire, PE C0A 1Y0, Canada; (R.P., K.D., D.G., M.M., A.M.) Crops and Livestock Research Centre, Agriculture and Agri-Food Canada (AAFC), 440 University Avenue, Charlottetown, PE C1A 4N6, Canada; (G.W.-P.) Department of Plant and Animal Sciences, Faculty of Agriculture, Dalhousie University, 50 Pictou Road, Truro, NS B2N 5E3, Canada; (B.B., C.B.) PEI Department of Agriculture and Forestry, P.O. Box 2000, Charlottetown, PE C1A 7N8, Canada; (L.K.) Lethbridge Research Centre, AAFC, 5403 1^st Avenue South, Lethbridge, AB T1J 4B1, Canada; and (A.M.) Veseys Seeds Ltd., 411 York Road, Highway 25, York, PE C0A 1P0, Canada

Gardeners often encounter fruit rot in tomatoes as they have limited access to fungicides or resistant varieties. A breeding project in North Carolina has released new varieties (‘Mountain Magic’, ‘Defiant’, ‘Plum Regal’) with resistance to late blight. Field plots were established on Prince Edward Island in 2012 to assess resistance to late blight in these three resistant varieties in comparison to four susceptible varieties (‘Scotia’, ‘Brandywine’, ‘Oxheart’ and ‘Monster’). Two blocks were planted with seven varieties; one block sprayed weekly with the fungicide chlorothalonil rotated with manzate. The check block did not receive fungicides. Extensive development of late blight, initiated by naturally occurring inoculum, occurred in both foliage and fruit of the four susceptible varieties, in the unsprayed check block, compared with very limited disease development in ‘Mountain Magic’ and ‘Defiant’ (highly resistant, but not immune) and moderate infection in ‘Plum Regal’. Tomato plants managed with fungicides were free of disease. Sweet pepper plants (variety, ‘New Ace’), included in both blocks, did not develop any symptoms of late blight in foliage or fruit. Volunteer potato plants near the tomato trial were found to be infected with the late blight pathogen. Isolation of Phytophthora infestans (Mont.) de Bary from the infected potato and potato tissues and subsequent analysis of allozymes at the glucosephosphate isomerase (GPI) locus via cellulose acetate electrophoresis provided a preliminary identification of pathogen strain (US-23).The US-23 genotype of P. infestans has migrated from western to eastern Canada in recent years and colonizes potatoes and tomatoes in Atlantic Canada.

Plant pathogen detection. S. H. DE BOER, Emeritus. Canadian Food Inspection Agency, Charlottetown Laboratory, 93 Mount Edward Road, Charlottetown, PE C1A 5T1, Canada

There is a great need for accurate means to detect plant pathogens because such methods play a major role in bringing to market healthy seed, tubers, bulbs, rootstocks, cuttings, etc., mitigating pathogen spread in both domestic and international trade, and the implementation of disease-control strategies. Technologies used for detection of plant pathogens are under rapid development and coincide considerably with those that are also needed for diagnosing plant diseases and identifying pathogenic microorganisms. Over the last century, particularly over the last 30 or so years, detection methods have evolved from simple visual observation to sophisticated antibody- and nucleic acid-based tests. Methodologies such as those employing lateral flow devices have led the way in on-the-spot testing capabilities. While hand-held nucleic acid amplifying devices based on PCR technology are available, they are likely to be replaced by simplified devices based on one of several isothermal amplification technologies. Multiplex detection techniques including antibody and nucleic acid arrays on membrane platforms and ingeniously labelled beads in aqueous formats form the base of sophisticated methods suitable for screening large number of samples in a laboratory setting. Additionally bar-coding and other DNA sequencing strategies may yet also play a role in future detection methods. Notwithstanding the sophistication of new technologies, questions related to their appropriate usage, particularly for high-impact pathogens that cannot be isolated or visualized, remain a challenge with the need to anticipate when and where confirmatory and verification tests of primary test results will be required.

Phosphites affect mycelial growth and induce morphological changes in Phytophthora infestans . X. GAO, G. SAKTHIVEL, T. BORZA, Y. WU AND G. WANG-PRUSKI. Department of Plant and Animal Sciences, Faculty of Agriculture, Dalhousie University, 50 Pictou Road, Truro, NS B2N 5E3, Canada

Phosphite-based fungicides are increasingly used in controlling late blight development in potato crops. However, the effectiveness of various formulations of phosphites in limiting the development of Phytophthora infestans (Mont.) de Bary, is still unclear. This study investigated the effects of two different phosphite fungicides (Confine^TM and Phostrol^TM) on the growth and morphology of P. infestans A2 genotype US8 strain in vitro. Mycelium growth and sporangia production were estimated by culturing the pathogen on pea agar medium supplemented with different concentrations of Confine^TM and Phostrol^TM (0.01% to 0.3% phosphites), followed by measuring the diameter of colonies at 7 and 14 days post-inoculation, and counting sporangia at 14 days post-inoculation. To investigate whether phosphites induce morphological changes in P. infestans, the pathogen was cultivated in pea broth for 7 days at 18 °C, and then in soil-water extract containing different concentrations of Confine^TM and Phostrol^TM (0.005 to 0.1% phosphites) for 3 more days at 18 °C. Morphological changes of sporangia were observed using an inverted microscope. Our results indicated that both fungicides showed very strong inhibition on P. infestans mycelia growth, with an IC₅₀ of around 0.007% (920 μm) phosphites. Sporangia production was strongly inhibited by 0.05% Confine^TM and 0.1% Phostrol^TM. Concentrations of 0.1% Confine^TM and 0.3% Phostrol^TM completely inhibited the growth and sporangia production. Both Confine^TM and Phostrol^TM in the concentration range of 0.05% to 0.1% caused morphological changes of sporangia in P. infestans, distorted, abnormal-shaped and elongated sporangia were observed.

Seasonal release of conidia of Valdensia heterodoxa and disease progress in lowbush blueberry. P. D. HILDEBRAND AND W. E. RENDEROS. Atlantic Food and Horticulture Research Centre, Agriculture and Agri-Food Canada, 32 Main Street, Kentville, NS B4N 1J5, Canada

Valdensinia leaf spot of lowbush blueberry is caused by the anamorph Valdensia heterodoxa Peyronel of the ascomycetous fungus Valdensinia heterodoxa Peyronel. Initial inoculum originates as unusually large conidia (400–600 μm) formed on sclerotia in colonized leaves of the previous season. Conidia are forcibly propelled upward and infect leaves resulting in premature defoliation and subsequent production of new conidia. In 2010 and 2011, we monitored the seasonal release of conidia and disease progress in fruiting and sprout fields with the aim to improve timing of fungicide applications. Conidia were trapped on Petri dish lids suspended 15 cm above ground level. In early May, the ground beneath the traps was seeded once with leaves bearing sclerotia. Traps were changed at intervals of 5–10 days throughout the season and conidia were counted with a stereomicroscope. Disease progress was monitored at intervals in replicated plots. In fruiting fields, the incidence of affected leaves (dropped plus infected but attached) was assessed on lower, middle and upper leaf shoots while in sprout fields, the incidence of affected leaves was assessed on the entire stem. In fruiting fields, first conidia were trapped in late May and several days later in sprout fields in both years. In 2010, disease onset occurred a few days after initial conidia were trapped, but in 2011, disease onset occurred about 19 days later. Once disease was initiated, the incidence of affected leaves increased sharply. In fruiting fields, disease progressed most rapidly on lower followed by middle and upper leaves and then slowed by mid-July coinciding with a decrease in trapped conidia. In sprout fields, disease progress did not slow until mid-August. These results show that disease begins early in the season deep within the canopy and will require early monitoring of symptoms to effectively time fungicide applications.

Tomato as a latent carrier of ‘ Ca. Liberibacter solanacearum’, the causal agent of potato zebra chip disease. X. LI, J. NIE, H. ARSENAULT AND S. H. DE BOER. Canadian Food Inspection Agency, Charlottetown Laboratory, 93 Mount Edward Road, Charlottetown, PE C1A 5T1, Canada

‘Candidatus Liberibacter solanacearum’ (CLS) has been identified as the causal agent of potato zebra chip (ZC) disease that is characterized by severe internal necrosis of potato tuber tissue, and results in millions of dollars in losses to potato growers annually. The disease occurs in North America from Mexico to the northern USA, and in Guatemala, New Zealand and northern Europe on various crops. In order to prevent the spread of the pathogen, the European Plant Protection Organization (EPPO) recently added CLS to the EPPO A1 list of quarantine pests. Comparison and observation of both ZC-infected tomato and potato plants propagated in growth rooms indicated that tomato (varieties ‘Money Maker’ and ‘Roma’) can be a latent carrier of CLS. Tomato plants graft-inoculated with scions from latently infected tomato plants remained symptomless, but tested positive in a ZC-specific PCR assay. CLS was consistently detected in the top, medium and bottom portion of symptomless tomato plants, including petiole, midrib, flowers and fruit. This is the first report that CLS is present in tomato flowers and fruit, leading to speculation that the disease may be seedborne. Furthermore, plants of five potato cultivars showed typical symptoms of purple top and leaf scorch four weeks after being grafted with scions from the asymptomatic tomato plants. Tubers from the graft-inoculated potato plants also showed typical symptoms of brown discoloration in the vascular ring and medullary rays. While CLS could not be detected in the aerial tubers of graft-inoculated greenhouse-grown plants, it was readily detected in the stems and progeny tubers of the same plants.

Assessing pathogenicity and chemical sensitivity of Fusarium spp. infecting carrots. M. M. MACDONALD, R. D. PETERS, L. HALE, J. KEMP, J. DRISCOLL, G. DYKERMAN, S. ADAMS, A. RYAN, C. BANKS, A. MACPHAIL, D. GREGORY AND K. A. DRAKE. Department of Biology, University of Prince Edward Island, 550 University Avenue, Charlottetown, PE C1A 4P3, Canada; (R.D.P, S.A., A.M., D.G., K.A.D.) Crops and Livestock Research Centre, Agriculture and Agri-Food Canada, 440 University Avenue, Charlottetown, PE C1A 4N6, Canada; (J.D., A.R.) Prince Edward Island Horticultural Association, P.O. Box 2232, Charlottetown, PE C1A 8B9, Canada; (G.D.) Brookfield Gardens, 1067 Millboro Road, RR#4 North Wiltshire, PE C0A 1Y0, Canada; and (C.B.) Prince Edward Island Department of Agriculture and Forestry, P.O. Box 2000, Charlottetown, PE C1A 7N8, Canada

Fusarium crown rot, formerly a storage disease of carrots, was present in Prince Edward Island (PEI) fields in 2011 and 2012. Crown lesions on carrots resulted in rejection rates as high as 60–70% during grading for storage in 2011. Infecting organisms were identified as Fusarium avenaceum (Fr.) Sacc. and Fusarium oxysporum Schlecht. emend Snyder & Hans. in samples obtained from field and storage. Research was conducted to assess the pathogenicity and chemical sensitivity of isolates of each species. In a replicated trial, crown tissue was inoculated and carrots incubated for approximately 4 weeks to rate pathogenicity according to Koch's postulates. Measurements of area and depth of wound indicated that all isolates (10) of F. avenaceum were highly pathogenic to carrot tissue. All isolates (four) of F. oxysporum were found to be weakly pathogenic in comparison to F. avenaceum and treatment controls (uninoculated). Isolates of both species were tested for sensitivity to difenoconazole, fludioxonil, and thiabendazole in an amended agar assay. Chemicals were distributed in six concentrations in an attempt to determine the EC₅₀ (the concentration at which fungal growth is inhibited by 50%) for each isolate. Isolates of F. avenaceum had EC₅₀ values of >100 ppm, 0.1–1.0 ppm, and 1–10 ppm and isolates of F. oxysporum had EC₅₀ values of 1–10 ppm, > 100 ppm, and 1–10 ppm, for difenoconazole, fludioxonil, and thiabendazole, respectively. F. oxysporum was found to be resistant to fludioxonil and sensitive to thiabendazole, whereas F. avenaceum was sensitive to both chemicals. However, F. oxysporum was sensitive to difenoconazole, whereas F. avenaceum was found to be highly resistant. Field studies are underway to develop management options to combat this destructive disease.

Discovery of resistance to metalaxyl-m in populations of Phytophthora erythroseptica causing pink rot of potato in Prince Edward Island. R. D. PETERS, B. W. BEATON, M. M. CLARK, B. FORRESTER, A. MACPHAIL, K. A. DRAKE, D. GREGORY AND M. M. MACDONALD. Crops and Livestock Research Centre, Agriculture and Agri-Food Canada, 440 University Avenue, Charlottetown, PE C1A 4N6, Canada; (B.W.B., M.M.C.) Prince Edward Island Department of Agriculture and Forestry, P.O. Box 2000, Charlottetown, PE C1A 7N8, Canada; and (B.F.) Cavendish Agri Services, 4504 Alleys Mills Road, Pooles Corner, PE C0A 1G0, Canada

After a hot, dry summer, autumn rains delayed harvest and contributed to incidences of potato tuber rot in several regions of Prince Edward Island (PEI) in 2012. Disease diagnosis indicated that pink rot, caused by Phytophthora erythroseptica Pethybr., was prevalent in many affected fields. A collection of tubers sampled from 12 individual fields from various production regions in PEI yielded 59 isolates of P. erythroseptica. These isolates were tested for their sensitivity to metalaxyl-m in amended agar assays. All isolates (22 isolates) from three fields in eastern PEI were found to be highly resistant to metalaxyl-m, whereas isolates (37 isolates) from nine fields in central portions of the province were found to be metalaxyl-sensitive. This is the first report of metalaxyl-m resistance in populations of P. erythroseptica in PEI. Previously, isolates of the pathogen with resistance to metalaxyl-m had been recovered from surrounding potato production areas in New Brunswick and Maine, USA. The discovery of metalaxyl-m resistance raises concerns about the efficacy of applications of Ridomil Gold® for pink rot control and may add importance to the role played by phosphites in the management of this disease.

Phosphite for late blight control in potato production – update of the CHC project. G. WANG-PRUSKI, R. COFFIN, R. PETERS, Z. GANGA, K. AL-MUGHRABI, B. PRITHIVIRAJ AND D. PINTO. Department of Plant and Animal Sciences, Faculty of Agriculture, Dalhousie University, 50 Pictou Road, Truro, NS B2N 5E3 Canada; (R.C.) Privar Farm Inc., 909 Eliot River Road, North Wiltshire, PE C0A 1Y0, Canada; (R.P.) Crops and Livestock Research Centre, Agriculture and Agri-Food Canada, 440 University Avenue, Charlottetown, PE C1A 4N6, Canada; (Z.G.) Cavendish Farms, New Annan, PE C1N 4J9, Canada; (K.A.) New Brunswick Department of Agriculture, Aquaculture and Fisheries, 39 Barker Lane, Wicklow, NB E7L 3S4, Canada; and (D.P.) National Research Council Institute for Marine Biosciences, Halifax, NS B3H 3Z1, Canada

The CHC Agri-Science Cluster for Horticulture was initiated in 2010. We participated in this 3-year programme, focusing on studying the functions of phosphorous acid-related compounds on suppression of late blight in potatoes. The programme is aimed at four major objectives: (1) study of phosphorous acid-response proteins for their functions against late blight in foliage and tubers; (2) effect of phosphorous acid on disease development on foliage and tubers; (3) examination of translocation of phosphorous acid in potato plants and in potato tubers and subsequent functions against pink rot and other tuber diseases; and (4) effective use of phosphorous acid in potato production systems. To date, significant data from proteomic profiling, gene expression, field and greenhouse evaluations, translocation and residue analyses, and the behaviour of the pathogen, have been obtained. We now have better understanding of the molecular mechanisms of induced resistance by phosphite and, we are able to provide its better usage to manage late blight for potato production.

Standardized RT-PCR procedures for detecting common viruses in potatoes. H. XU, S. CODY AND R. THIBODEAU-DOYLE. Canadian Food Inspection Agency, Charlottetown Laboratory, 93 Mount Edward Road, Charlottetown, PE C1A 5T1, Canada

Over 50 viruses have been reported to infect potato naturally and 10 of them have been confirmed to present in commercial potato lots in Canada. They are potato viruses A, M, S, X, Y (PVA, PVM, PVS, PVX, PVY), Potato leafroll virus (PLRV), Potato mop-top virus (PMTV), Potato latent virus (PLV), Tobacco rattle virus (TRV) and Alfalfa mosaic virus (AMV). There is a requirement for detecting some or all these viruses in nuclear stock production in Canada and seed potato trade with some countries. Primers and probes specific to these viruses were produced based on sequence analysis in this study or according to the published data and evaluated for their sensitivity and specificity for detecting RNA sequences of these viruses in various potato tissues including leaves, sprouts, microplantlets and tubers. Three to ten isolates of each virus were used in the specificity tests. Serial dilutions of extracted RNA and composite samples (w/w or v/v) were tested by conventional and real-time RT-PCR for determining the sensitivity of the primers and probes in PCR. Procedures for RNA extraction were standardized based on cost-efficiency, quality of RNA extracts and quantity of RNA yield. Viral RNA in dormant potato tubers was readily detected without the need to break tuber dormancy and testing results of seed tubers and grown-out plants agreed perfectly. Virus was easily detected in composite samples of 200 (for PVY, PLRV, PVS, PMTV and TRV) to 400 (for PVA, PVX, PVM, PLV and AMV) dormant tubers. Standard protocols were developed, optimized and standardized followed by repeatability and reproducibility tests. The RT-PCR procedures (duplex/multiplex) developed have been employed successfully for screening field tuber samples for seed potato certification.

PVY strains detected between 2010 and 2012 on Prince Edward Island. H. XU, R. COFFIN, B. BEATON, R. THIBODEAU-DOYLE AND S. CODY. Canadian Food Inspection Agency, Charlottetown Laboratory, 93 Mount Edward Road, Charlottetown, PE C1A 5T1, Canada; (R.C.) Potato Consulting Services Inc., 909 Eliot River Road, North Wiltshire, PE C0A 1Y0, Canada; and (B.B.) PEI Department of Agriculture and Forestry, P.O. Box 2000, Charlottetown, PE C1A 7N8, Canada

Potato virus Y (PVY) has a single strand, plus sense RNA as its genome. Like many other RNA viruses, PVY displays a high degree of genetic variability mainly due to RNA recombination between different strains in mixed infections. To date, many distinct PVY strains and sub-strain variants have been detected and characterized and some of them, including PVY^O, PVY^N, PVY^NTN and PVY^N-Wi (also known as PVY^N:O) have been detected in Canada. Isolates of PVY^O have been the most prevalent in North America and accounted for approximately 70% of all PVY isolates. However, a survey conducted from 2010 to 2012 showed that isolates of the common strain in the province of Prince Edward Island (PEI) only accounted for 30–40% of over 1000 PVY isolates detected. The traditional PVY^N strain was not detected in this survey. Among the 1000 PVY isolates detected in PEI, the PVY^NTN strain accounted for 30% and the recombinant strain type, PVY^N-Wi accounted for about 20%. Most of the PVY^NTN isolates detected in this survey were identified as European (EU) type of PVY^NTN. Visual inspection revealed that plants of common potato cultivars infected with PVY^NTN and PVY^N-Wi isolates developed only mild symptoms (mosaic/mottling) and might produce tubers showing symptoms of potato tuber necrotic ringspot disease (PTNRD). Over 50% of the tubers harvested from Yukon Gold plants infected with PVY^NTN (both primary and secondary infection) showed PTNRD symptoms at harvest and a disease index (0 to 1) as high as 0.8 based on external necrosis of all symptomatic tubers. In the survey, all potato samples (tubers and leaves) were screened by RT-PCR for all PVY strain types followed by multiplex RT-PCR for strain typing. RFLP and sequence analysis was also performed for verifying PCR amplicons for proper identification.

The origins of Xylella fastidiosa subsp. fastidiosa . K. YUAN, L. NUNNEY AND R. STOUTHAMER. (K.Y.) University of California, Riverside (UCR), 900 University Avenue, Riverside, CA 92521, USA; (K.Y., L.N., R.S.) Canadian Food Inspection Agency, Charlottetown Laboratory, 93 Mount Edward Road, Charlottetown, PE C1A 5T1, Canada

Pierce's disease of grape is caused by Xylella fastidiosa subsp. fastidiosa Wells et al. (XF) that infects the xylem vessels of grapevine, and has long posed a serious threat to the wine industry in the USA. To elucidate the evolution and origin of the pathogen, the multi-locus sequence typing (MLST) method based on seven housekeeping gene sequences (4161 bp) was used to identify and group 86 isolates of XF from across the US. We also compared two whole genomic sequences of XF (isolates Temecula-1 and M23), and detected limited genetic variations between them. Although XF is generally assumed to be native to the USA, our data suggests that all XF strains in the USA are derived from elsewhere. This hypothesis is supported by the fact that the US genotypes nest together within the phylogenetic grouping of 24 Costa Rican isolates of XF, which have extensive genetic variation. The level of variation found among the US strains is consistent with derivation from a single common ancestor within the last 150 years. Rather than being native to the USA, a single introduction of the pathogen into the USA possibly occurred in the period of 1850–1870 when known hosts such as coffee plants were brought to the USA from Central America.