Pink Rot Research Articles

In-Furrow Applications of Metalaxyl and Phosphite for Control of Pink Rot (Phytophthora erythroseptica) of Potato in New Brunswick, Canada.

The efficacy of metalaxyl-m (Ridomil Gold 480EC) and phosphite (Phostrol) applied at planting in-furrow against pink rot (Phytophthora erythroseptica) of potato (Solanum tuberosum) 'Shepody' and 'Russet Burbank' was evaluated in field trials conducted in 2005 and 2006 in Florenceville, New Brunswick, Canada. Inoculum made from a metalaxyl-m-sensitive isolate of P. erythroseptica from New Brunswick was applied either in-furrow as a vermiculite slurry at planting or as a zoospore drench in soils adjacent to potato plants in late August. After harvest, the number and weight of tubers showing pink rot symptoms were assessed and expressed as percentages of the total tuber number and total weight of tubers. Metalaxyl-m applied in-furrow was significantly more effective against pink rot than phosphite. The mean percentage of diseased tubers as a percentage of total tuber weight was 1.5% (2005) and 1.2% (2006) for metalaxyl-m-treated plots and 9.6% (2005) and 2.8% (2006) for phosphite-treated plots, a percentage similar to that obtained in inoculated control plots with no fungicide treatment. The mean percentage of diseased tubers expressed as a percentage of the total number of tubers was 1.7% (2005) and 1.3% (2006) for metalaxyl-m-treated plots and 10.1% (2005) and 3.1% (2006) for phosphite-treated plots. Disease incidence was significantly higher using the late-season inoculation technique (respective means in 2005 and 2006 were 9.9 and 3.8% diseased tubers, by weight, and 10.6 and 3.9%, by number) than with the in-furrow inoculation method (respective means in 2005 and 2006 were 3.3 and 0.7% by weight, and 3.7 and 1.3%, by number). The potato cv. Shepody was significantly more susceptible to pink rot (9.9 and 3.3% diseased tubers, by weight, in 2005 and 2006, respectively, and 10.6 and 3.9%, by number) than Russet Burbank (respective means in 2005 and 2006 were 3.4,% and 1.2%, by weight, and 3.7,% and 1.2%, by number). Our findings indicate that metalaxyl applied in-furrow at planting is a viable option for control of pink rot caused by metalaxyl-sensitive strains of P. erythroseptica, whereas phosphite was ineffective.

Megachip — a new potato variety for chipping

‘MegaChip’ is a medium-late variety for chipping, with round-oval tubers of white tan skin and white flesh. MegaChip has consistently shown good to high yield potential. The tubers bulk early and produce a good size distribution for chipping. The specific gravity is consistently high and is close to the solid content of ‘Atlantic’. MegaChip has longer dormancy than ‘Snowden’ and Atlantic and stores well. It produces high quality chips that have an excellent color both from the field and after storage at 7.2 to 10 C. MegaChip showed a high level of resistance to common scab at most locations in North Central Regional trials and is moderately resistant to foliar early blight, powdery scab, pink rot, dry rot and soft rot. MegaChip is also fairly resistant to shatter bruise.

Use of silicon oxide and sodium silicate for controlling Trichothecium roseum postharvest rot in Chinese cantaloupe (Cucumis melo L.)

SummarySilicon oxide and sodium silicate were investigated as potential agents for the control of postharvest pink rot in Chinese cantaloupe (cultivar Yujingxing) caused by Trichothecium roseum. In vitro tests showed that sodium silicate, when added to potato dextrose agar, was effective in suppressing the radial growth of the pathogen on the medium, whereas silicon oxide was ineffective. The effectiveness of sodium silicate increased with concentration, and the growth of the fungus was completely inhibited at 100 mm. When melons were dipped in the solutions, both silicon oxide and sodium silicate significantly (P < 0.01) reduced the severity of pink rot of the cantaloupe with lesion diameters reduced by up to fivefold when compared with the controls. Scanning electron microscopy‐energy dispersive X‐ray analysis showed that silicon (Si)‐treated melons had a smoother surface feature and higher Si levels in the epidermis, especially at the stomata and along the junction between the exocarp and mesocarp. Enhanced peroxidase and phenylalanine ammonia lyase activities were observed in sodium‐silicate‐treated melons but not in those treated with silicon oxide. The results indicate that different mechanisms might be involved in sodium silicate and silicon‐oxide‐initiated reduction of postharvest pink rot in Chinese cantaloupe.

Development and Validation of Conventional and Quantitative Polymerase Chain Reaction Assays for the Detection of Storage Rot Potato Pathogens, Phytophthora erythroseptica, Pythium ultimum and Phoma foveata

AbstractThe diseases pink rot, watery wound rot and gangrene are important storage rot diseases of potato associated predominantly with Phytophthora erythroseptica (P.), Pythium ultimum (Py.) and Phoma exigua (Phoma) var. foveata respectively. Reliable molecular‐based diagnostic tests are required that will not only allow unequivocal identification of symptoms but will further advance epidemiological studies of these potato diseases to increase our understanding and contribute to more effective management and control strategies to the potato industry. Primers and probes were designed in specific regions of the internal transcribed spacer (ITS) regions to develop conventional and real‐time quantitative polymerase chain reaction (PCR) assays able to detect all possible fungal and oomycete pathogens causing pink rot, watery wound rot and gangrene. The specificity of each diagnostic assay was rigorously tested with over 500 fungal/oomycete plant pathogen isolates from potato and reference culture collections, and both conventional and real‐time PCR methods produced similar results. In terms of sensitivity, the detection limits for real‐time PCR went below ag DNA levels compared with pg DNA levels with conventional PCR. The real‐time PCR assays developed to detect Phoma foveata and Py. ultimum on tubers were suitable for the comparative Ct method (ΔΔCt) of quantification using the cytochrome oxidase gene of potato as a normalizer assay; an advantage as the need for a standard curve is eliminated. Each assay detected Phoma species (var. foveata or exigua) from naturally infected tubers showing symptoms of gangrene, and P. erythroseptica or Py. ultimum were also detected following inoculation of Russet Burbank tubers. Each diagnostic assay developed could reliably detect and distinguish between the pink rot, watery wound rot and gangrene‐causing potato pathogens.

Resistance to phytophthora erythroseptica and pythium ultimum in a potato clone derived from s. berthaultii and s. etuberosum

Tubers of several potato clones and cultivars were screened for susceptibility to infection by zoospores of Phytophthora erythroseptica (causal agent of pink rot) and mycelia of Pythium ultimum (causal agent of leak) over a three-year period, from 2003-2005. Incidence of infected tubers (%) and penetration of rot (mm) were the parameters used to determine the susceptibility of each potato clone. Responses of each potato clone were compared to cultivars with known resistance or susceptibility to these pathogens. Tubers of cultivars Atlantic and Snowden have moderate resistance to infection and colonization by P. erythroseptica and P. ultimum, respectively, and were used as the resistant checks. Cultivars Russet Norkotah and Red Norland are susceptible to infection by both pathogens. A number of potato clones demonstrated resistance to pink rot equal to or greater than the control cultivar Atlantic, including Etb 6-5-2, ND5822C-7, ND6956b-13, ND7443Ab-44, ND7443Ab-181, ND7818-1Y and J101K6A22. In addition to demonstrating the highest resistance to pink rot, Etb 6-5-2 was the only clone that demonstrated resistance to leak greater than or equivalent to the resistant cultivar Snowden. Etb 6-5-2 is a backcross derivative from a somatic hybrid of Solanum etuberosum and Solanum berthaultii and will be investigated further as a potential source of resistance to these two storage rot diseases.

Management of soilborne diseases of potato

A large number of soilborne diseases that affect potato are important in the United States. In the Midwestern USA, early dying, involving Verticillium dahliae and Colletotrichum coccodes and the storage rots, pink rot and leak tuber rot, caused by Phytophthora erythroseptica and Pythium ultimum are among the most serious. Various aspects of the biology and aetiology of these pathogens have been investigated to improve our understanding of the factors involved in disease development. These studies have addressed knowledge gaps and have assisted in the development of management strategies and tactics for each disease. Disease management integrates cultural practices, use of agrochemicals and to some degree, host resistance. Unfortunately, cultural management practices alone are currently inadequate to control these diseases, causing the potato industry to become over-reliant on the use of agrochemicals for effective management. Current research efforts are directed at the identification and incorporation of genetic resistance into cultivars with acceptable horticultural characteristics to provide more effective disease management.

In vitro somatic growth and reproduction of phenylamide‐resistant and ‐sensitive isolates of Phytophthora erythroseptica from infected potato tubers in Idaho

Pink rot of potato, most commonly caused by Phytophthora erythroseptica, is a major field and post‐harvest problem in southern Idaho, USA, particularly since 1998 when isolates resistant to the phenylamide fungicide metalaxyl‐M (mefenoxam) were detected. Isolates of P. erythroseptica were collected from infected tubers in 2001 and 2002 from six Idaho counties and tested for resistance to metalaxyl‐M on amended agar. Metalaxyl‐M resistant (MR) and metalaxyl‐M‐sensitive (MS) isolates were identified in six counties; 160 isolates were highly resistant, seven moderately resistant and 57 sensitive to metalaxyl‐M with mean EC50 values of 182, 23 and 0·5 mg L−1 ai metalaxyl‐M, respectively. Mycelial growth rates and oospore production in agar were assessed for 20 MS and 20 MR isolates at 10, 15, 20, 25 and 30°C. Growth rates of MR isolates were between 2·5 and 3·1 times greater (P < 0·05) than those of MS isolates at 10, 15, 20 and 25°C, and oospore production was between 6·8 and 20·5 times greater (P < 0·0001) for MR than for MS isolates at the same temperatures. Colony growth in V8 broth at 18°C was greater for MR than MS isolates (P < 0·0032). However, zoospore production at 18°C was greater for MS than for MR isolates (P < 0·0109), and zoospore production mm−1 of colony circumference was also greater for MS than for MR isolates, 14 191 and 9959, respectively (P = 0·0109). Sexual reproduction of MR isolates in nature may be greater than MS isolates, but MS isolates may be more asexually fit based on the fitness parameters studied.

Sensitivity to Hydrogen Peroxide in vitro of North American Isolates of Phytophthora erythroseptica, the Cause of Pink Rot of Potatoes

Sensitivity to Hydrogen Peroxide in vitro of North American Isolates of Phytophthora erythroseptica, the Cause of Pink Rot of Potatoes

Survey on the Occurrence of Apple Diseases in Korea from 1992 to 2000

In the survey from 1992 to 2000, twenty-eight parasitic diseases were observed in major apple producing areas in Korea. The predominant apple diseases were white rot (Botryosphaeria dothidea), Marssonina blotch (Marssonina mali), Valsa canker (Valsa ceratosperma), Alternaria leaf spot (Alternaria mali), and bitter rot (Collectotrichum gloeosporioides and C. acutatum). Apple scab that reappeared in 1990 after disappearance for 15 years was disappeared again since 1997. A viroid disease (caused by apple scar skin viroid) was newly found in this survey. The five diseases, fire blight (Erwinia amylovora), black rot (Botryosphaeria obtusa), scab (Cladosporium carpophilum), Monochaetia twig blight (Monochaetia sp.), and brown leaf spot (Hendersonia mali), which had once described in 1928 but no further reports on their occurrence, were not found in this survey. However, blossom blight (Monilinia mali), brown rot (Monilinia fructigena), and pink rot (Trichothecium roseum), which did not occur on apple after mid 1970s, were found in this survey.

A Methodology to Detect and Quantify Five Pathogens Causing Potato Tuber Decay Using Real-Time Quantitative Polymerase Chain Reaction

ABSTRACT Late blight (Phytophthora infestans), pink rot (Phytophthora erythroseptica), leak (Pythium ultimum), dry rot (Fusarium sambucinum), and soft rot (Erwinia carotovora subsp. carotovora and subsp. atroseptica) are particularly damaging diseases of stored potato tubers worldwide. In this study, we present a methodology to detect and quantify the causal agents of the five aforementioned diseases from whole potato tubers, using real-time quantitative-polymerase chain reaction. Six primer pairs were designed to amplify targets smaller than 150-bp DNA in single copy protein-coding gene targets of each of the pathogens and the potato host. Using a large collection of pure culture DNA samples, all primer pairs specifically detected the DNA target in the intended pathogenic species. Amplification efficiencies over a five-log dilution series ranged between 95 and 100% and were unaffected by the presence of large amounts of host DNA. The detection level of the primers reached 0.5 pg of target DNA. Pathogens were detected in 100 pg of total DNA extracted from 170 to 250 g of tubers, 4 days after inoculation, regardless of the presence of symptoms. The presence of P. erythroseptica, Pythium ultimum, or E. carotovora was also detected in 1 ng of DNA extracted from potato tubers collected from a commercial storage facility. This study provides the first step in a methodology to predict the storability of potato tubers following harvest.

Biological Significance of Mefenoxam Resistance in Phytophthora erythroseptica and Its Implications for the Management of Pink Rot of Potato.

Tubers from plants treated with in-furrow and foliar applications of mefenoxam were inoculated with eight isolates of Phytophthora erythroseptica having varying levels of sensitivity to the fungicide. Two isolates with effective concentration causing 50% reduction of mycelial growth (EC50) values of 0.02 and 0.04 μg ml-1 were categorized as being mefenoxam sensitive. Isolates with EC50 values >1.0 μg ml-1 were designated as insensitive to mefenoxam and were grouped two each into low intermediate (EC50 = 1.1 and 5.3 μg ml-1), high intermediate (EC50 = 26 and 74 μg ml-1), and resistant (EC50 ≥ 100 μg ml-1). The biological significance of these isolates was examined by quantifying disease control. P. erythroseptica isolates classified in the resistant group infected a significantly greater proportion of untreated tubers than isolates in any other group. Mefenoxam reduced infection frequency of sensitive isolates by as much as 37%. Mefenoxam did not provide disease control of any isolate possessing insensitivity to the fungicide, with the greatest decrease in control observed with the low intermediate group. Aggressiveness indices, representing tuber infection frequency and depth of penetration, were calculated for untreated and mefenoxam-treated tubers. According to these indices, both isolates classified in the resistant group and high intermediate isolate 252-4 were more aggressive than sensitive isolates in the absence of mefenoxam pressure, and significantly so in the presence of mefenoxam. These results suggest that pink rot may become more severe in fields known to contain P. erythroseptica populations with mefenoxam EC50 values >1.0 μg ml-1 if the fungicide is applied. These factors should be considered when developing strategies to manage pink rot and mefenoxam-resistant populations of P. erythroseptica.

Maritime Regional Meeting, 2005

Evaluation of at-planting in-furrow application of Phostrol™ and Ridomil Gold ® 480EC for control of pink rot in potato. K.I.AI-Mughrabi and R.D. Peters. Potato Development Centre, New Brunswick and Department of Agriculture, Fisheries and Aquaculture, 39 Barker Lance, Wicklow, NB E7L 3S4, Canada; and (R.D.P.) Crops and Livestock Research, Agriculture and Agri-food Canada, 440 University Avenue, Charlottetown, PE C1A 4N6, Canada. Biological control of apple replant disease P.G. Braun and K.K. Fuller. Atlantic Food and Horticulture Research Centre, Agriculture and Agri-Food Canada, 32 Main Street Kentville, NS B4N 1J5, Canada. Potato gene expression during infection of tubers by Streptomyces scabiei. C. Goyer and J. Zeng. Potato Research Centre, Agriculture and Agri-Food Canada, P.O. Box 20280, 850 Licoln Road, Fredericton, NB E3B 4Z7, Canada. Suppression of potato early blight (in vivo) and germination of Alternaria spp. conidia (in vitro) with strobilurin fungicides. W. MacDonald, R.D. Peters, R.H. Coffin, and C. Lacroix. Department of Biology, University of Prince Edward Island, 550 University Avenue, Charlotttetown, PE C1A 4P3, Canada; (R.D.P.) Crops and Livestock Research Centre, Agriculture and Agri-Food Canada, 440 University Avenue, Charlottetown, PE C1A 4N6, Canada; and (R.H.C.) Cavendish Farms, New Annan, Kensington, PE C0B 1M0, Canada. Plant pathology and potato breeding: interlaced disciplines. A. Murphy. Potato Research Centre, Agriculture and Agru-Food Canada, P.O. Box 20280, 850 Lincoln Road, Frederecton, NB E3B 4Z7, Canada. Systemic acquired resistance against potato virus Y in tobacco is salicylate mediated and ethylene independent. X. Nie. Potato Research Centre, Agriculture amd Agri-Food Canada, 850 Lincoln Road, P.O. Box 20280, Fredericton, NB E3B 4Z7, Canada. Increasing the sustainability of potato production, in Atlantic Canada, with crop rotation and conservation tillage. R.D. Peters, M.R. Carter, J.B. Sanderson, and A.V. Sturz. Crops and Livestock Research Centre, Agriculture and Agri-Food Canada, 440 University Avenue, Charlottetown, PE C1A 4N6, Canada; and (A.V.S.) Prince Edward Island Department of Agriculture and Forestry, Plant Health Research and Diagnostics, P.O. Box 1600, Charlottetown, PE C1A 7N3, Canada. Host-pthogen interactions of various combinations of verticillium spp. inoculum in potato. N. Robinson, H.W. (Bud) Platt, L. Hale, and V. MacLean. Crops and Livestock Research Center, Agriculture and Agri-Food Canada; and (L.H.) Department of Biology, University of Prince Edward Island, 550 University Avenue, Charlottetown, PE C1A 4P3, Canada. Real-time PCR assay for the detection of potato viruses in a triplex format. M. Singh, R.P. Singh, and R. Coffin. Agricultural Certification Services, 1030 Lincoln Road, Frederiction, NB E3B 8B7, Canada; (R.P.S.) Potato Research Centre, Agriculture and Agri-Food Canada; and (R.C.) Cavendish Farms, P.O. 3500, Summerside, PE C1N 5J5, Canada. Viroids from oranamental plants – a potential threat to tomato and potato crops. R.P. Singh. Potato Research Centre, Agriculture and Agri-Food Canada, 850 Lincold Road, Fredericton, NB E3B 4T7 canada, Reassessment of the presence of species of the genus Pospiviroid in floral parts, using reverse transcription - PCR and infectivity assays R.P. Singh. Potato Research Centre, Agriculture and Agri-Food Canada, 850 Lincoln Road, Fredericton, NB E3B 4Z7, Canada. Viroid RNAs, PCR fragments, and plasmid purification simplified by spotting an alkaline extract onto nitrocellulose membrance. R.P. Singh, A.D. Dilworth, M. Singh, and K.M. Babock. Potato Research Centre, Agriculture and Agri-Food Canada, 850 Lincoln Road, Fredericton, NB E3B 4Z7, Canada; and (M.S., K.M.B.) Agricultural Certification Services, 1030 Lincoln Road, Fredericton, NB E3B 8B7, Canada. Sensitivity to mefenoxam of Canadian strains of Phytophthora erythroseptica the causal agent of potato pink rot, and their interactions with Phytophthora infestans in tubers. P.D. Young, R.D. Peters, H.W. (Bud) Platt, and L.R. Hale. Department of Biologu, University of Prince Edward Island, 550 University Avenue, Charlottetown, PE C1A 4P3, Canada; and (R.D.P., H.W.P.) Crops and Livestock Research Centre, Agriculture and Agri-Food Canada, 440 University Avenue, Charlottetown, PE C1A 4N6, Canada.

Annual Meeting, Québec, Quebec, 2006, The Canadian Phytopathological Society / Réunion annuelle, Québec, Québec, 2006, La Société canadienne de phytopathologie

Annual Meeting, Québec, Quebec, 2006, The Canadian Phytopathological Society / Réunion annuelle, Québec, Québec, 2006, La Société canadienne de phytopathologie

Post-harvest applications of zoxamide and phosphite for control of potato tuber rots caused by oomycetes at harvest

Potato storage tuber rots caused by the late blight and pink rot pathogens at harvest can cause severe economic losses, warranting the need for effective post-harvest fungicide applications. The purpose of this study was to evaluate the efficacy of select post-harvest fungicides in reducing tuber infections by the late blight and pink rot pathogens when applied at various post-inoculation time intervals. ‘Russet Burbank’ potatoes were inoculated by submersion in an aqueous suspension ofPhytophthora infestans orPhytophthora erythroseptica zoospores at 0, 1, 2, 4, and 6 h prior to receiving a post-harvest treatment. Products evaluated were zoxamide (various rates and formulation), phosphite (335 g a.i./MT), and a hydrogen peroxide/peroxyacetic acid mixture (HPPA, 9 g a.i./MT), all applied at 2.08 L/MT of tubers as a low pressure spray prior to storage. Zoxamide and phosphite significantly reduced late blight and pink rot incidence and severity when applied immediately after inoculation. HPPA was less effective at controlling disease development. Phosphite was effective at reducing late blight development at all time intervals up to 6 h post-inoculation (7% vs 80% in untreated). Zoxamide appeared to have good post-harvest disease control if applied soon after inoculation. The maximum time intervals between inoculation and treatment where significant reductions in pink rot incidence were observed was 0 h for HPPA (28%), 2 h for zoxamide (55%; 64 g a.i./MT) and 6 h for phosphite (13%) compared to the untreated (73%). Phosphite provided consistent disease control even when applied several h after inoculation and has potential to be a reliable post-harvest fungicide for the potato industry.

White Pearl—A chipping potato variety with high level of resistance to cold sweetening

White Pearl is a chipping cultivar with high level of resistance to cold sweetening. The cold-chipping capacity of White Pearl is better than the current chipping cultivars, including Atlantic, Snowden and NorValley. White Pearl has low glucose content and good chip color when processed directly after a 7-month storage period at 3.3 to 5.5 C. White Pearl tubers are similar to Snowden with a round oval shape, light netting, shallow eyes, white flesh, small to medium size, uniform shape, and are practically free of external and internal defects. The yield and specific gravity of White Pearl are medium. White Pearl is similar to Snowden in susceptibility to early blight, pink rot and soft rot, but it is slightly more resistant to common scab. White Pearl vines are susceptible to late blight, but can be managed effectively with fungicides. White Pearl is more sensitive to blackspot bruise than Atlantic or Snowden at 7.2 C, but is more resistant at higher temperatures. White Pearl is sensitive to metribuzin (Sencor), but not rimsulfuron (Matrix) or sulfentrazone (Spartan).

Defender: A high-yielding, processing potato cultivar with foliar and tuber resistance to late blight

The potato cultivar Defender is high-yielding, white-skinned, and notable for having foliar and tuber resistance to late blight infection caused byPhytophthora infestans (Mont.) de Bary. It was released in 2004 by the USDA-ARS and the agricultural experiment stations of Idaho, Oregon, and Washington. Defender is suitable for processing into french fries and other frozen potato products directly from the field or from storage. Defender also may be used for fresh markets in regions such as California, where cultivars with long tubers and white skin are traditionally grown. Resistances to late blight and other potato diseases make Defender an ideal candidate for organic potato production. Defender consistently produced greater total and U.S. No. 1 yields than ‘Russet Burbank’ in Idaho trials. In early harvest trials conducted in the western U.S., average total yields of Defender were 17% and 23% greater than yields for ‘Ranger Russet’ and ‘Shepody’, respectively. In full-season trials conducted in the western U.S., Defender averaged 10% and 15% higher yields than Ranger Russet and Russet Burbank, respectively. Specific gravity of Defender is consistently high, with values comparable to those of Ranger Russet; tuber ascorbic acid (Vitamin C) levels are also high. In addition to late blight, Defender also is resistant to tuber early blight (Alternaria solani, (Ellis & G. Martin), L.R. Jones & Grout), potato virus X, and net necrosis; it has moderate levels of resistance to Verticillium wilt (Verticillium dahliae, Kleb), pink rot, foliar early blight, corky ringspot, and Erwinia soft rot. Defender is susceptible to scab (common and powdery) and potato leafroll virus; it has moderate susceptibility to dry rot (Fusarium spp.) and potato virus Y. Susceptibilities to internal necrosis, tuber greening, and blackspot bruise also have been noted, but may be minimized through cultural and harvest practices.

Variation in antibiosis ability, against potato pathogens, of bacterial communities recovered from the endo- and exoroots of potato crops produced under conventional versus minimum tillage systems

The culturable component of bacterial communities found in the endoroot and associated exoroot (root zone soil) was examined in potatoes (Solanum tuberosum L.) grown under either conventional or minimum tillage systems. Bacterial species--abundance relationships were determined and in vitro antibiosis ability investigated to discover whether tillage practice or bacteria source (endo- or exoroot) influenced bacterial community structure and functional versatility. Antibiosis abilities against Phytophthora erythroseptica Pethyb. (causal agent of pink rot of potatoes), Streptomyces scabies (Thaxt.) Waksm. and Henrici) (causal agent of potato common scab), and Fusarium oxysporum Schlecht. Emend. Snyder and Hansen (causal agent of fusarium potato wilt) were selected as indicators of functional versatility. Bacterial community species richness and diversity indices were significantly greater (P = 0.001) in the exoroot than in the endoroot. While both endo- and exoroot communities possessed antibiosis ability against the phytopathogens tested, a significantly greater proportion (P = 0.0001) of the endoroot population demonstrated antibiosis ability than its exoroot counterpart against P. erythroseptica and F. oxysporum. Tillage regime had no significant influence on species-abundance relationships in the endo- or exoroot but did influence the relative antibiosis ability of bacteria in in vitro challenges against S. scabies, where bacteria sourced from minimum tillage systems were more likely to have antibiosis ability (P = 0.0151). We postulate that the difference in the frequency of isolates with antibiosis ability among endoroot versus exoroot populations points to the adaptation of endophytic bacterial communities that favour plant host defence against pathogens that attack the host systemically.

Genetic modification of potato against microbial diseases: in vitro and in planta activity of a dermaseptin B1 derivative, MsrA2

Dermaseptin B1 is a potent cationic antimicrobial peptide found in skin secretions of the arboreal frog Phyllomedusa bicolor. A synthetic derivative of dermaseptin B1, MsrA2 (N-Met-dermaseptin B1), elicited strong antimicrobial activities against various phytopathogenic fungi and bacteria in vitro. To assess its potential for plant protection, MsrA2 was expressed at low levels (1-5 microg/g of fresh tissue) in the transgenic potato (Solanum tuberosum L.) cv. Desiree. Stringent challenges of these transgenic potato plants with a variety of highly virulent fungal phytopathogens--Alternaria, Cercospora, Fusarium, Phytophthora, Pythium, Rhizoctonia and Verticillium species--and with the bacterial pathogen Erwinia carotovora demonstrated that the plants had an unusually broad-spectrum and powerful resistance to infection. MsrA2 profoundly protected both plants and tubers from diseases such as late blight, dry rot and pink rot and markedly extended the storage life of tubers. Due to these properties in planta, MsrA2 is proposed as an ideal antimicrobial peptide candidate to significantly increase resistance to phytopathogens and improve quality in a variety of crops worldwide with the potential to obviate fungicides and facilitate storage under difficult conditions.

Salt compounds as control agents of late blight and pink rot of potatoes in storage

Each year, potato late blight [Phytophthora infestans] and pink rot [Phytophthora erythroseptica] are responsible for significant postharvest economic losses. Because of restrictions on the use of synthetic fungicides on food, there is currently a need for less toxic, disease-controlling compounds for postharvest application. To address this need, the preventative and curative effects, as well as seed piece phytotoxicity, of various salt compounds were tested using live tubers of the potato 'Superior'. Compounds were tested as dips at the concentrations 0.002, 0.02, 0.2 mol/L and compared with a distilled water dip as a control. Tubers were inoculated with sporangia of P. infestans or active zoospores of P. erythroseptica either before or after treatment with the various compounds. When concentration data were pooled and grouped means were tested for significant differences (P < 0.05), potassium sorbate, sodium metabisulfite, alum, sodium hypochlorite, copper sulfate pentahydrate, acetic acid, and Penncozeb® 75DF applied curatively provided disease inhibition that was significantly greater than that of the control for both late blight and pink rot. When applied as a preventative treatment, all compounds were significantly different (P < 0.05) from the untreated control in the inhibition of pink rot; however, only sodium metabisulfite, sodium hypochlorite, copper sulfate pentahydrate, and Penncozeb 75DF significantly reduced the severity of late blight. Phytotoxicity was observed when sodium hypochlorite, sodium metabisulfite, acetic acid, and copper sulfate pentahydrate were applied as seed piece treatments, at a concentration of 0.2 mol/L. Our studies suggest there is a potential for the use of selected salt compounds for control of postharvest diseases of potato.

Crop rotation can confer resistance to potatoes from Phytophthora erythroseptica attack

The influence of 2- and 3-yr crop rotations and conservation tillage practices on the severity of pink rot of potato (causal agent: Phytophthora erythroseptica) was examined in Prince Edward Island, Canada. The 2-yr rotation crops included spring barley and potato (cv. Russet Burbank), and the 3-yr rotation was comprised of barley (undersown with red clover), red clover and potato. In the potato year only, the main plots were split to examine conventional and minimum tillage (sub-plot) treatments. Soil samples and potato tubers were extracted from field plots 6-7 yr after the initiation of the field trial. Following post-harvest inoculation of field-grown tubers with P. erythroseptica, we determined that pink rot disease development was significantly (P = 0. 05) less pronounced in potatoes from 3-yr rotational than from 2-yr rotational soils. The effect of tillage management was not significant. Potato plants grown in greenhouse trials in 3-yr rotational soils (from field plots) were significantly (P = 0.05) less diseased than those grown in 2-yr rotational soils following inoculation with P. erythroseptica. These results suggest that potato plants grown in soils managed under a 3-yr rotation are intrinsically more resistant to pathogen attack than those managed under a 2-yr rotation. We confirmed that crop rotation provides disease control benefits beyond those normally associated with pathogen population decline in the absence of the susceptible host. The possibility that beneficial soil microflora and tuber endophytes were involved in disease suppression is discussed. Key words: Phytophthora erythroseptica, pink rot, potato, Solanum tuberosum, crop rotation, minimum tillage, beneficial endophytes, biocontrol