Gibberellic acid mediated disease susceptibility to Sclerotinia sclerotiorum in Arabidopsis thaliana . N. S. BAHIA, H. LI, S. K. ASIEDU, A. B. GRAY AND B. PRITHIVIRAJ. Department of Plant and Animal Sciences, Nova Scotia Agricultural College, P.O. Box 550, Truro, NS B2N 5E3, Canada
Plant growth hormones are known to be involved in the complex signalling processes associated with plant defence response. The effect of exogenous application of indole-3-acetic acid, gibberellic acid (GA3) and kinetin on the susceptibility of Arabidopsis thaliana (L.) Heynh. Col-0 to Sclerotinia sclerotiorum (Lib.) de Bary was investigated. Among the applied growth regulators, GA3 (20 μM) showed an increased susceptibility of A. thaliana to S. sclerotiorum. Arabidopsis thaliana wild-type Col-0 plants pretreated with GA3 and inoculated with S. sclerotiorum showed 13.16% increase in lesion size over control water-treated plants. A similar reaction was observed when pretreated plants were challenged with oxalic acid, the major pathogenicity factor of S. sclerotiorum. Defence response mutants of A. thaliana npr-1, cpr-5 and jar-1 were also assayed for their disease susceptibility to S. sclerotiorum after GA3 treatment. Mutants npr1 and jar-1 also exhibited enhanced disease susceptibility to S. sclerotiorum after pretreatment with GA3. Pretreatment of leaves with GA3 reduced the activity of phenylalanine ammonia lyase and a concomitant reduction in the total phenolics in the leaf tissue of wild-type Col-0 and defence response mutants. Quantitative real time polymerase chain reaction assay results also showed reduced expression of glucosinolates biosynthetic gene, CYP 83 B1, after GA3 application and S. sclerotiorum inoculation on A. thaliana leaves. Taken together, the results suggest that GA3 mediated susceptibility of A. thaliana to S. sclerotiorum at least in part was mediated by reduced antimicrobial phenolics and glucosinolates.
Microbial biodiversity of organic and conventional apple orchard soils. P. G. BRAUN, E. W. E. BEVIS AND S. A. E. FILLMORE. Atlantic Food and Horticulture Research Centre, Agriculture and Agri-Food Canada, 32 Main Street, Kentville, NS B4N 1J5, Canada
Interest in healthy soils defined as those with the greatest biodiversity has been increasing in the literature. It is commonly believed that synthetic pesticides and fertilizers used in conventional agriculture have a negative effect on soil microbial populations and in turn soil health. A 2005 study on pesticide use found that 9.7 kg of pesticide active ingredients ha−1 are used annually in Canadian orchards. Soil samples were collected from adjacent conventional and organic apple (Malus domestica Borkh.) orchards as well as forests and meadows in four locations along the Annapolis Valley, NS to compare soil microbial populations. Microbial population diversity was assessed by community level physiological profiles (CLPP) using GN2 96-well plates, DNA fingerprints generated by universal bacterial, fungal and oomycete polymerase chain reaction primers, and denaturing gradient gel electrophoresis (DGGE). Bacterial diversity was statistically similar for conventional and organic orchards and organic orchards were similar to forest soils. Meadow soils had the lowest bacterial diversity. Bacterial and oomycete populations were not significantly different between any of the soil samples based on DNA fingerprints. However, fungal population fingerprints were significantly more diverse for mixed forest and meadow soils than organic and conventional apple orchards which were not different from each other. In conclusion, the population diversity of conventional and organic apple orchards is not significantly different but they are less diverse than forest soils.
Prince Edward Island Plant Disease Diagnostic Service. M. M. CLARK AND B. BEATON. Prince Edward Island Department of Agriculture, Kensington Potato Services, P.O. Box 306, Kensington, PE C0B 1M0, Canada
The Plant Disease Diagnostic Service provides farmers, agri-businesses, and agricultural extension staff with a disease identification and control advisory service. Diagnoses are based on visual examination of symptoms, microscopic observation, and explicit laboratory techniques. The objective of this programme is to provide clients with a fundamental service, identifying the problem and providing information to control the problem if possible, and to prevent any reoccurrence. All sample results are followed up with a written disease diagnostic report. The collection of data is recorded using the Plant Health Reporting System (PHRS). This computer program can be utilized to query information on specific diseases and to monitor disease status for new and quarantine diseases. Deregistration of chemicals, catastrophic new diseases, and pesticide-resistant pathogen strains are all challenges faced by the PEI potato industry. In 1994, two confirmed cases of potato late blight were identified. By comparison, in 2008, 147 cases were recorded. Through the collection of data (PHRS), all pest levels are recorded and passed on to the potato industry through programmes such as the Potato Pest Hotline. Timely information on pest levels help potato producers make informed decisions on the utilization of appropriate chemical control measures and management practices.
Symptoms and epidemiology of a Septoria spp. causing leaf and stem canker of lowbush blueberry. P. D. HILDEBRAND, W. E. RENDEROS, S. A. E. FILLMORE AND B. WALKER. Atlantic Food and Horticulture Research Centre, Agriculture and Agri-Food Canada, 32 Main Street, Kentville, NS B4N 1J5, Canada
Lowbush blueberry (Vaccinium angustifolium Aiton) is often affected by premature leaf and fruit drop which is associated with leaf spots. Although leaves frequently can be infected with several minor leaf spotting pathogens, a Septoria spp. has been found to cause substantial leaf drop. In sprout fields, newly emerging leaves and stem tissues become infected during June from conidia produced in pycnidia in overwintered leaves. Conidia are hyaline, filiform, curved or straight and variable in size measuring 50–100 × 1.5–2.0 μm. Conidial release occurs over a four to five-week period beginning in late May in response to rain. Initial leaf symptoms appear as minute, water-soaked spots on the leaf undersurface that increase in number, enlarge and coalesce to produce irregularly shaped lesions that penetrate to the upper surface where they appear as red/purple spots that eventually turn various shades of red/brown. Infected leaves begin to drop in mid to late July. Because sprout stems grow from an apical meristem beginning in early June and continue to elongate into late July and early August, leaves primarily on the lower half of stems are exposed to inoculum and show symptoms. Infections on stems also occur and are similarly confined to the lower half, but remain latent until the following year of fruit production when they become visible in April. Inoculum from these stem lesions and overwintered leaves follows the same release pattern in June when leaves of the current season fruiting stems become infected. Severe leaf drop can lead to reduced yields. Clones show variable susceptibility to this disease.
Development of Arabidopsis thaliana – Pseudomonas marginalis flower pathosystem. D. HOBSON, M. HODGES, G. WANG-PRUSKI AND B. PRITHIVIRAJ. Department of Plant and Animal Sciences, Nova Scotia Agricultural College, P.O. Box 550, Truro, NS B2N 5E3, Canada
Flower-infecting pathogenic microbes cause significant economic damage on a wide variety of field and horticultural crops. Plant defence mechanisms against microbial pathogens in the leaf, stem, and root tissues have undergone substantial advances. However, genetic and biochemical resistance to pathogens in the flower is poorly understood. This is, at least in part, due to lack of a tractable flower pathosystem. Here we report a genetically tractable flower pathogen model Arabidopsis thaliana (L.) Heynh. – Pseudomonas marginalis (Brown) Stevens (causal agent of broccoli head rot). A reproducible infection was obtained by dip-inoculating A. thaliana inflorescence in a P. marginalis cell suspension. Among the concentrations tested, OD600 – 0.1 produced the maximum infection. Open flowers were the most susceptible. Infection appeared on the exposed and sticky surfaces of the flower, the anther and stigma, shown through light and fluorescence microscopy. While infection of the pistil alone caused flower death, this pathogen would not spread to other plant tissues. Pseudomonas marginalis did not infect other parts of the inflorescence, like bracts, pedicle, and peduncle. Further, pressure inoculation of A. thaliana leaf with P. marginalis did not elicit discernable symptoms, suggesting the flower-tissue specificity of the pathogen.
Evaluation of downy mildew resistance of 11 Camelina sativa genotypes. J. LI, C. D. CALDWELL, A. B. GRAY AND K. C. FALK. Department of Plant and Animal Sciences, Nova Scotia Agricultural College, P.O. Box 550, Truro, NS B2N 5E3, Canada
False flax (Camelina sativa (L.) Crantz) is an oilseed plant which has a long history in European agriculture. It has attracted a lot of interest in North America due to its high omega-3 polyunsaturated fatty acid content and good disease resistance. Although C. sativa possesses a good disease resistance, there are still some diseases observed in the field. Among these diseases, downy mildew caused by Peronospora parasitica (Pers.) Fr. has been reported the most frequently. In order to find resistant genotypes, 11 C. sativa genotypes were tested in environment controlled growth chambers with three downy mildew isolates. Disease severity was evaluated on the 12th day after inoculation. Camelina sativa genotype CN30478 displayed the best resistance to downy mildew and CN30475 was found to be the second best. In contrast, Calena and CN101985 were observed to be the most susceptible genotypes. Among these 11 genotypes, CN30476, CN101982, and SRS933 were special genotypes because their resistance varies among the three isolates. This indicates that the three isolates were acting differently from one another. This conclusion is also supported by the greater aggressiveness showed by the PEI isolate. A high spore suspension concentration of PEI isolate killed inoculated C. sativa seedlings in a preliminary experiment. Therefore, a low spore suspension concentration of PEI isolate was applied in this series of experiments.
Development and evaluation of loop-mediated isothermal amplification protocols for rapid and sensitive detection of plant pathogenic bacteria. X. LI, J. B. NIE, L. WARD AND S. H. DE BOER. Canadian Food Inspection Agency, Charlottetown Laboratory, 93 Mount Edward Road, PE C1A 5T1, Canada
Loop-mediated isothermal amplification (LAMP) is a particularly useful technology for diagnostic work because of its unique approach to specific nucleic acid amplification requiring only a single temperature for amplification and thereby obviating the need for expensive thermal cyclers as required for polymerase chain reaction (PCR). Amplified products can be visualized by gel electrophoresis as DNA ladders, by photometry as turbidity caused by accummulated magnesium pyrophosphate in the reaction mix, and by UV illumination after addition of SYBR green, a DNA intercalating dye. A rapid LAMP protocol for detecting and identifying Pectobacterium atrosepticum (van Hall) Gardan et al., causal agent of potato blackleg, was developed by selecting the genomic target through a comparative genomic approach utilizing available whole genome sequences of plant pathogenic and related bacteria. This approach was similar to the pathovar-specific LAMP assay that we previously developed for Pseudomonas syringae pv. phaseolicola (Burkholder) Young et al., and also subsequently used for developing another pathovar-specific LAMP assay for Pseudomonas syringae pv. pisi (Sackett) Young et al. The P. atrosepticum LAMP assay targeted the genomic region encoding the pathogenicity-related polyketide synthase gene, which shares homology with the coronafacic acid synthase gene of P. syringae. The assay was specific for eight strains of the species tested and showed no cross-amplification of related pectobacteria, dickeya strains, or other selected plant pathogenic bacteria. The LAMP assay was successful in detecting the presence of P. atrosepticum directly in diseased potato stem tissue generated from inoculated greenhouse plants. Sensitivity of the LAMP assays we developed to the various bacterial plant pathogens was in the range of 1 × 103 to 7 × 103 colony forming units mL−1, comparable to that of multiplex PCR and real-time PCR-based assays. Because a high concentration of amplified DNA is generated in the LAMP assay without the need for temperature cycling, this technology may be particularly useful for integration into hand-held devices for rapid detection of plant pathogens directly in the field.
Quantitative proteomic analysis for identification of phosphorous acid-responsive proteins. S. LIM, G. WANG-PRUSKI, D. PINTO, R. H. COFFIN, R. D. PETERS, K. I. AL-MUGHRABI, H. W. PLATT, S. VEENHUIS-MACNEILL, I. MACDONALD, K. DRAKE, W. HARDY AND T. HAMILL. Department of Environmental Sciences, Nova Scotia Agricultural College, P.O. Box 550, Truro, NS B2N 5E3, Canada; (D.P.) National Research Council, Institute for Marine Biosciences, Halifax, NS B3H 3Z1, Canada; (R.H.C., S.V.-M., W.H., T.H.) Cavendish Farms, P.O. Box 3500, Summerside, PE C1N 5J5, Canada; (R.D.P., H.W.P., I.M., K.D.) Crops and Livestock Research Centre, Agriculture and Agri-Food Canada, 440 University Avenue, Charlottetown, PE C1A 4N6, Canada; and (K.I.A.-M.) Potato Development Centre, New Brunswick Department of Agriculture and Aquaculture, 39 Barker Lane, Wicklow, NB E7L 3S4, Canada
Late blight is one of the most devastating diseases of potatoes (Solanum tuberosum L.). Phosphorous acid (PA; ConfineTM) was recently registered in Canada for late blight suppression in leaves and tubers. Field trials were conducted in Prince Edward Island between 2007 and 2009 to evaluate the efficacy of PA in foliar late blight control. Phosphorous acid is a relatively harmless chemical and the mechanism of increased resistance in plants treated with PA remains enigmatic. Evaluation of the infection rate revealed that plants treated with PA suppressed late blight infection. Proteomics is a powerful tool for investigating complex biological processes at the molecular level. To understand the plant response to the PA treatment, proteomic profiles of leaves from treated and untreated plants were collected. Since the cell wall and the cytosol are the battlefields for plants against the pathogen, identification of proteins in these subcellular locations is essential. Our earlier studies have confirmed that more than 60% of identical proteins could be obtained in replicated samples. In this study, 591 reproducible proteins in cytosolic fraction and 589 proteins in cell wall fraction were identified and their abundance was statistically calculated. Results showed that 38 proteins from the cytosolic fraction and 33 proteins from the cell wall fraction were up-regulated (mean fold change > 1.4). Biological processes for up-regulated proteins by PA are classified into response to stress, proteolysis, cell wall degradation, oxidative stress and signalling. Eighteen and 13 proteins in cytosolic and wall fractions, respectively, were down-regulated (mean fold change < 0.75). Approximately 60% of down-regulated proteins play roles in metabolic processes. These preliminary results provide us with some insight into the functions of PA for late blight resistance in potatoes.
Field efficacy of diversified groups of fungicides on suppression of monilinia blight of wild blueberry. D. PERCIVAL, R. R. BURLAKOTI AND H. H. HINES. Department of Environmental Sciences, Nova Scotia Agricultural College, P.O. Box 550, Truro, NS B2N 5E3, Canada
Nine fungicides with different modes of action were evaluated in wild blueberry (Vaccinium angustifolium Aiton) fields in Nova Scotia and Prince Edward Island during 2009 growing seasons with the following objectives: (i) to assess the effectiveness of these fungicides to suppress the monilinia blight in wild blueberry; (ii) to find out the possible tolerance of Monilinia vaccinii-corymbosi (Reade) Honey causing monilinia blight or mummy berry to presently registered fungicides; and (iii) to increase scrutiny of specific fungicides by end users. Active ingredients of propiconazole (Topas® and Orbit®), prothioconazole (Proline®) and penthiopyrad (LEM17) suppressed the monilinia blight effectively. No tolerance to propiconazole was observed in fields. The results from mixing multiple active ingredients with different modes of action and their levels of persistence need to be further verified. In addition, some fungicides had a profound influence on berry yield as indicated by 73.6, 111 and 112% greater berry yields from plots treated with propiconazole, prothioconazole and penthiopyrad than the untreated control, respectively, at the Nova Scotia site. However, these berry yield results need to be viewed with caution given the known suppressive attributes of some of these fungicides to other diseases present during treatment application (e.g. Septoria leaf spot).
Survey and management of Fusarium spp. causing potato seed-piece decay in Canada. R. D. PETERS AND T. BARASUBIYE. Crops and Livestock Research Centre, Agriculture and Agri-Food Canada (AAFC), 440 University Avenue, Charlottetown, PE C1A 4N6, Canada; and (T.B.) Eastern Cereal and Oilseed Research Centre, AAFC, K.W. Neatby Building, 960 Carling Avenue, Ottawa, ON K1A 0C6, Canada
Strains of Fusarium spp. with resistance to commonly used potato seed treatment fungicides have become commonplace in Canada. In particular, populations of F. sambucinum Fuckel and F. coeruleum Lib. ex Sacc. with resistance to thiabendazole, thiophanate-methyl, and/or fludioxonil have been frequently recovered. These pathogen populations have caused an increased incidence of seed-piece decay resulting in poor crop stands and reduced yields. Potato (Solanum tuberosum L.) seed tubers with symptoms of decay were sampled from across Canada in spring 2009 to obtain a collection of Fusarium isolates. Isolates were identified to species level using morphological and molecular methods and then tested for sensitivity to thiabendazole and fludioxonil using a fungicide-amended agar assay. The two major seed decay pathogens, F. sambucinum and F. coeruleum were frequently recovered. In addition, isolates of F. oxysporum Schlecht.:Fr., F. avenaceum (Fr.) Sacc., F. graminearum Schwabe, F. culmorum (Smith) Sacc., F. cerealis (Cooke) Sacc. and F. acuminatum Ellis & Everh. were also identified. Isolates of F. sambucinum varied in their sensitivity to fungicides, with resistance to one or both products found in strains obtained from most Canadian provinces. Isolates of F. coeruleum, F. oxysporum, F. avenaceum and F. acuminatum with fungicide resistance were also identified, although most minor seed decay species were sensitive to both products. Field and storage studies were conducted in Prince Edward Island, Canada to ascertain the impact of fungicide-resistant strains on crop loss and to define potential management strategies. In summary, inoculation of potato seed pieces with an isolate of F. sambucinum possessing multi-class fungicide resistance followed by application of thiophanate-methyl or fludioxonil as seed treatments, resulted in the complete loss of efficacy of both products. In all cases, treatment of potato seed pieces with mancozeb or difenoconazole completely controlled seed-piece decay caused by this isolate of F. sambucinum.
An Arabidopsis thaliana purple acid phosphatase 5 gene (At1g53940) mutant exhibit hypersusceptibility to Pseudomonas syringae pv . tomato DC3000. S. RAVICHANDRAN, A. B. GRAY, S. S. GNANAMANICKAM, J. ZHANG AND B. PRITHIVIRAJ. Department of Plant and Animal Sciences, Nova Scotia Agricultural College, P.O. Box 550, Truro, NS B2N 5E3, Canada; (S.S.G.) Texas AgriLife Research and Extension Urban Solutions Center, Dallas, Texas 75252, USA; and (J.Z.) National Research Council, 550 University Avenue, Charlottetown, PE C1A 4P3, Canada
Purple acid phosphatases (PAPs) are a class of binuclear metallo-phosphoesterase found in most microorganisms, plants and animals. PAPs hydrolyse a broad spectrum of phosphorylated substrates including ATP, PEP and sugar esters. PAPs in animals are known to reversibly phosphorylate serine/threonine residues modulating signalling events in response to biotic stresses. In Arabidopsis thaliana (L.) Heynh., there are over 29 predicted PAPs. A number of these PAPs are reported to hydrolyse inorganic phosphorus facilitating cellular uptake. Here we report on a T-DNA mutant of A. thaliana with an insertion event in purple acid phosphatase 5 gene (At1G52940) exhibiting hyper-susceptibility to Pseudomonas syringae pv. tomato (Okabe) Young et al. DC3000 (Pst) infection as compared with wild-type Colombia-0. There was a significant difference in Pst colonization of root surface between the PAP5 mutant and Col-0. Pst colonized PAP5 mutant extensively as compared to Col-0 root as evidenced by SYTO 9/propidium iodide fluorochrome. Reverse transcriptase–polymerase chain reaction experiment revealed a reduced transcript abundance of PAP5 in the leaf tissue of the mutant, however, the PAP5 transcripts were low in root tissues of both mutant and Col-0. Further experiments on the gene expression of defence genes in the PAP5 mutant will give an insight into the role of PAPs in plant defence.
Sulfated macroalgal polysaccharide λ-carrageenan induces resistance against Sclerotinia sclerotiorum in Arabidopsis thaliana whereas ι -carrageenan enhances susceptibility. J. S. SANGHA, S. RAVICHANDRAN, K. PRITHIVIRAJ, A. T. CRITCHLEY AND B. PRITHIVIRAJ. Department of Plant and Animal Sciences, Nova Scotia Agricultural College, P.O. Box 550, Truro, NS B2N 5E3, Canada; and (A.T.C.) Acadian Seaplants Limited, 30 Brown Avenue, Dartmouth, NS B3B 1X8, Canada
The effect of macroalgal polysaccharides ι- and λ-carrageenan, that differ in sulfation, on resistance of Arabidopsis thaliana (L.) Heynh. against a necrotropic pathogen, Sclerotinia sclerotiorum (Lib.) de Bary was studied. Arabidopsis thaliana leaves were sprayed with carrageenans and subsequently inoculated with S. sclerotiorum. Physiological, biochemical and genetic analyses was carried out to determine the effect on induced defence. Pretreatment of A. thaliana with highly sulfated λ-carrageenan induced resistance while the less sulfated ι-carrageenan enhanced susceptibility to S. sclerotiorum. λ-carrageenan induced resistance correlated with increased expression of jasmonic acid related genes AOS, PDF1.2, and PR3. Interestingly, it also lowered concentration of the fungal virulence factor, oxalic acid (OA), in planta and increased oxalase oxidase activity in leaves of treated plants. In contrast, ι-carrageenan did not affect the expression of these genes nor OA activity. λ-carrageenan treatment induced disease resistance in immune-compromised salicylic acid deficient mutant ics1, whereas it only partially rescued susceptibility of jar1 plant. Additionally, λ-carrageenan induced genes in the glucosinolate biosynthetic pathway. λ-carrageenan elicits resistance through salicylic acid independent, jasmonic acid and ethylene dependent pathways in concert with additional unknown mechanisms against S. sclerotiorum in A. thaliana. The difference in the bioactivity of carrageenans is, at least in part, due to the variation in the degree of sulfation.
λ-carrageenan reduces Tomato chlorotic dwarf viroid replication and symptom expression. J. S. SANGHA, R. P. SINGH, A. T. CRITCHLEY AND B. PRITHIVIRAJ. Department of Plant and Animal Sciences, Nova Scotia Agricultural College, P.O. Box 550, Truro, NS B2N 5E3, Canada; (R.P.S.) Department of Environmental Sciences, Nova Scotia Agricultural College, P.O. Box 550, Truro, NS B2N 5E3, Canada; and (A.T.C.) Acadian Seaplants Limited, 30 Brown Avenue, Dartmouth, NS B3B 1X8, Canada
Some members of the genus Pospiviroid have a wide host range that causes significant economic losses in crops like tomato (Solanum lycopersicum L.) and potato (Solanum tuberosum L.). We tested the effect of sulfated polysaccharides carrageenans isolated from red macroalgae on Tomato chlorotic dwarf viroid (TCDVd) replication and symptom expression in tomato variety ‘Sheyenne’. Three-week-old tomato seedlings were spray-treated with three carrageenans [iota (ι), lambda (λ), and kappa (k)] (1 g L−1) and inoculated with TCDVd 48 h after spray treatments. The test plants were monitored for symptom development at one, two, three, five and eight weeks after inoculation and leaf tissue was collected to detect the viroid by RT-PCR using viroid specific primers. Pretreatment of tomato plants with λ-carrageenan significantly reduced symptom expression on inoculated plants. The characteristic bunchy-top symptom was observed in 25% of the λ-carrageenan treated plants as compared to 83.33% in the untreated controls. RT-PCR confirmed the presence of TCDVd transcripts in diseased plants, which were absent in the healthy plants. A reduction in viroid concentrations was observed when infected shoots were incubated in a solution of λ-carrageenan while the viroid concentration slightly increased in the ι-carrageenan treatment during the same period. Taken together, the results suggest that λ-carrageenan is a potential inducer of resistance in tomato plants against TCDVd.
Foliar application of phosphorous acid for control of potato late blight. G. WANG-PRUSKI, S. LIM, R. H. COFFIN, R. D. PETERS, H. W. PLATT, K. I. AL-MUGHRABI, D. PINTO, S. VEENHUIS-MACNEILL, W. HARDY, I. MACDONALD, K. DRAKE AND T. HAMILL. Department of Plant and Animal Sciences, Nova Scotia Agricultural College, P.O. Box 550, Truro, NS B2N 5E3, Canada; (R.H.C., S.V.-M., W.H., T.H.) Cavendish Farms, P.O. Box 3500, Summerside, PE C1N 5J5, Canada; (R.D.P., H.W.P., I.M., K.D.) Crops and Livestock Research Centre, Agriculture and Agri-Food Canada, 440 University Avenue, Charlottetown, PE C1A 4N6, Canada; (K.I.A.-M.) Potato Development Centre, New Brunswick Department of Agriculture and Aquaculture, 39 Barker Lane, Wicklow, NB E7L 3S4, Canada; and (D.P.) National Research Council, Institute for Marine Biosciences, 1411 Oxford Street, Halifax, NS B3H 3Z1, Canada
Late blight, caused by Phytophthora infestans (Mont.) de Bary, is a devastating disease of potatoes (Solanum tuberosum L.). Current disease management is strongly dependent on extensive use of protectant fungicides. A three year field trial was conducted in Prince Edward Island to evaluate the efficacy of phosphorous acid (ConfineTM), applied alone or in combination with chlorothalonil (Bravo®500) during the growing season, for control of foliar late blight. The trial consisted of two French fry processing varieties (‘Russet Burbank’ and ‘Shepody’) and four treatments (check plots treated with water, phosphorous acid, chlorothalonil and a combined treatment of phosphorous acid + chlorothalonil). The experimental design was a randomized complete block with each of the four treatments occurring in each block. Fresh leaves from each plot were collected and challenge-inoculated with a sporangial suspension of P. infestans. The leaves were incubated and then assessed for disease severity at one, four, five, six and seven days after inoculation. In addition to a significant variety effect, all three treatments achieved significant reduction in late blight severity on detached leaves compared with the check plot. The combined treatment of phosphorous acid and chlorothalonil resulted in the highest reduction in disease development, followed by the treatment with chlorothalonil. The ‘phosphorous acid alone’ treatment provided substantial control up to four days post infection, but the effect diminished after seven days post infection. These results will help us develop management strategies for late blight in potatoes and other solanaceous crops.
Detection of potato viruses and viroids in quarantine programmes using molecular technologies – feasibility and challenges. H. XU. Canadian Food Inspection Agency, Charlottetown Laboratory, 93 Mount Edward Road, Charlottetown, PE C1A 5T1, Canada
To minimize the risk of introducing harmful and quarantine pests associated with the movement of potato (Solanum tuberosum L.) germplasm, potato post entry quarantine (PPEQ) programmes are implemented in many countries to screen potato materials for all pests including viruses and viroids. Methods used presently for detecting viruses and viroids in quarantine programmes are bioassay and ELISA. Electron microscopy is also used, but mainly as a supplementary test due to its technical nature and high cost. R-PAGE is sometimes employed in countries where Potato spindle tuber viroid is of concern. Microplants entered into a PPEQ programme are normally multiplied and grown out in a greenhouse before serial testing is initiated. This is a long and costly process. In this study, molecular technologies based on polymerase chain reaction (PCR) were evaluated for detecting viruses and viroids in potato microplants. An automated procedure utilizing magnetic beads was employed for extracting total RNA from potato microplants and compared with Tri-Reagent extraction method. Thirty-five isolates of 10 virus species, previously identified by ELISA and bioassay, were amplified by conventional and real-time RT-PCR. Identities of amplified products were verified by RFLP, sequencing and melting temperature analysis. These molecular methods were superior to the bioassay and ELISA in sensitivity, specificity and rapidity and have great potential for detecting potato viruses and viroids in microplants. The challenges, however, remain for using molecular technologies for a great number of potato viruses of which many have never been properly characterized or have had their genomes analyzed.