Molecular mechanisms of plant pathogenicity: the dark side of Streptomyces. D. R. D. BIGNELL. Department of Biology, Memorial University of Newfoundland, 232 Elizabeth Avenue, St. John’s, NL A1B 3X9, Canada
Bacteria from the genus Streptomyces are best known for their complex developmental life cycle and for their ability to synthesize a wealth of bioactive specialized metabolites with useful applications in medicine and in agriculture. However, there is a dark side to this genus in that certain species can additionally function as aggressive plant pathogens and cause economically important crop diseases, the most notable of which is potato common scab (CS). CS affects the quality and market value of seed, processing and table stock potatoes due to the formation of superficial, erumpent or pitted lesions on the tuber surface. Current management strategies are insufficient or unreliable in controlling the disease, and there are no commercial potato cultivars that display complete disease resistance. Among the factors that have hindered the development of effective CS control strategies is a limited understanding of the mechanisms by which phytopathogenic Streptomyces spp. form parasitic interactions with plants. My lab is interested in elucidating the microbial virulence determinants that contribute to host colonization and disease development by the best characterized CS pathogen, Streptomyces scabies Lambert and Loria. In this presentation, an overview of our work on specialized metabolites that are essential for or contribute to the pathogenicity of this organism was provided, as well as recent work on an uncharacterized (cryptic) specialized metabolite that might play a role in plant–microbe interactions. In addition, some new work on a potential virulence determinant that resembles toxins produced by human pathogenic bacteria was discussed.
Forage diseases and silage spoilage on the Avalon: a poorly characterized problem. K. COMPTON, W. MOLLOY AND L. E. JEWELL. St. John’s Research and Development Centre, Agriculture and Agri-Food Canada, 308 Brookfield Road, St. John’s, NL A1E 0B2, Canada
Economical dairy and beef farming is dependent upon the availability of locally produced forages, which are fed to animals fresh or after they have been converted to silage through a fermentation process. When insufficient forage or silage is available, farmers must import feed which, in the province of Newfoundland and Labrador (NL), is particularly undesirable due to high transportation costs. An exploration of the forage diseases present on the island of NL was undertaken during the growing seasons of 2016, 2017 and 2018, and a study of silage quality was undertaken during the early winter months of 2016. The most frequently observed diseases included purple eyespot (Cladosporium phlei (Greg.) de Vries) of timothy (Phleum pretense L.), snow moulds (Microdochium nivale (Fr.) Samuels & Hallett, Typhula spp.) on both grasses and legumes, and legume leaf spots (including Pseudopeziza trifolii (Biv.) Fuckel, Phoma medicaginis Malbr. & Roum., and Cercospora medicaginis Ellis & Everh.). Corn was affected by eyespot (Kabatiella zeae Narita & Y. Hirats.) and northern corn leaf blight (Exserohilum turcicum (Pass.) Leonard & Suggs). A variety of abiotic stresses caused by inappropriate nutrient application or climatic factors were also observed. Furthermore, forages stored as silage were found to be contaminated by yeasts on six/nine farms tested, and by filamentous fungi on four/nine farms tested, suggesting that post-ensiling spoilage is occurring, even on materials that are free from obvious signs of spoilage or damage. This preliminary study provides the first baseline information about forage and silage microbial contamination in the province of NL.
Microorganisms associated with lowbush blueberry (Vaccinium angustifolium) on the island of Newfoundland. K. COMPTON, D. WISEMAN, S. LEONARD, D. B. MCKENZIE, T. FORGE AND L. E. JEWELL. St. John’s Research and Development Centre, Agriculture and Agri-Food Canada (AAFC), 308 Brookfield Road, St. John’s, NL A1E 0B2, Canada; and (T.F.) Summerland Research and Development Centre, AAFC, 4200 Highway #97, South, Summerland, BC V0H 1Z0, Canada
Lowbush blueberry (Vaccinium angustifolium Ait.) is a small shrub found throughout eastern North America, prized for its nutritious berries which contain high levels of antioxidants. The host of microorganisms living in association with blueberries in the province of Newfoundland and Labrador (NL) have not been well characterized. For this reason, a study was undertaken to collect and identify the microorganisms found in association with both apparently healthy and unhealthy lowbush blueberry plants found in the wild and on farms in NL. Root and soil samples were collected during the summer of 2016 from eight sites on the island of NL. Soils were inspected for the presence of nematodes, whereas root samples were surface sterilized and plated onto potato dextrose agar and cornmeal agar in order to isolate associated fungi. In addition, samples of stems, fruits and leaves displaying disease symptoms were collected during the summers of 2016, 2017 and 2018. Over 100 different types of fungi were cultured from the root samples alone. The most common isolates included Trichoderma spp., Cladosporium spp. and Pezicula ericae (Sigler) Johnst. Foliar diseases observed included Exobasidium fruit and leaf spot, red leaf, Valdensinia leaf spot and Septoria leaf spot. Regional differences were also observed, with diseases caused by Exobasidium spp. observed much more frequently in the Avalon than in the western or central regions of the province. In addition, high levels of yet-unidentified ring nematodes were observed. Reinfection studies to validate and characterize the interactions between the root-associated fungi and plant health are currently underway.
‘Designer Genes’? Resistance labels as a new management tool to growers to combat blackleg in canola. D. FERNANDO. Department of Plant Science, 222 Agriculture Building, 66 Dafoe Road, University of Manitoba, Winnipeg, MB R3T 2N2, Canada
Leptosphaeria maculans (Sowerby) Karst. is a hemi-biotroph that causes blackleg disease and remains a significant threat to canola (Brassica napus L.) cultivation. Qualitative resistance has been utilized in many breeding programmes around the world with some adult plant resistance in their background to mitigate this disease. Although blackleg was well managed for over three decades with good genetics in the Canadian canola industry, there has been a steady increase of resistance being eroded due to the presence of new races of the pathogen in grower fields. Rlm3 is the predominant gene found in Canadian canola cultivars. AvrLm3 gene has been disappearing rapidly in many fields enabling the pathogen to cause moderate to severe disease in grower fields. Canada has also had a trade embargo with China due to blackleg presence in Canada. The successful implementation of research over 5 years by the government and grower groups has helped Canada to adopt new strategies in the mitigation of blackleg disease. The understanding of the R-genes in Canadian canola germplasm and the blackleg-pathogen races across the canola growing regions has helped the industry to introduce a new R-gene rotation strategy. The development of a diagnostic tool, Kompetitive Allele Specific PCR (KASP), and genotype markers has helped the producers to identify the predominant races present in their fields. This would help them to strategically select the R-genes and their combinations found in the variety they select to grow in the R-gene rotations. The seed companies are starting to label their varieties with the known R-genes and their combinations so growers can make a good selection of varieties that suits their needs. The adoption of major-gene resistance groups and the L. maculans race diagnostics test will provide producers with new tools to help manage and mitigate blackleg on their farms. The presentation took you through the past, present and future of canola, a 26 billion dollar crop, and the mitigation of blackleg disease in Canada.
Characterization of a cryptic secondary metabolite in the potato common scab pathogen Streptomyces scabies. J. LIU AND D. R. D. BIGNELL. Department of Biology, Memorial University of Newfoundland, 232 Elizabeth Avenue, St. John’s, NL A1B 3X9, Canada
Streptomyces scabies Lambert and Loria is one of the main causative agents of potato common scab (CS) disease, which is characterized by the formation of superficial, raised or pitted lesions on the tuber surface. CS affects fresh market, processing and seed potatoes, and it reduces the quality and market value of potato crops worldwide. The disease is difficult to manage, and there are currently no control strategies that work consistently and reliably. The ability of S. scabies to cause CS is primarily based on the production of a phytotoxic secondary metabolite (SM) called thaxtomin A, which is an essential pathogenicity factor for the organism. In addition, S. scabies has the genetic potential to produce other SMs that might contribute to CS disease development. However, most of the genes predicted to be involved in production of these SMs are silent under laboratory conditions, and thus there is little known regarding the nature of these metabolites and their role in CS disease. The aim of this research is to characterize one cryptic (unknown) SM that is conserved in other pathogenic Streptomyces spp. The putative biosynthetic genes that produce this cryptic metabolite are expressed at low level or not at all under laboratory conditions, and therefore different strategies are being employed in order to activate or enhance expression of these genes in S. scabies. This presentation discussed the strategies being used as well as our preliminary results of the bioassays being utilized to detect various bioactivities that might be associated with the cryptic metabolite.
Fluctuating pathogen populations predicate the need for adjustment to potato late blight management strategies in Canada. R. D. PETERS, K. I. AL-MUGHRABI, F. DAAYF, A. MACPHAIL AND L. M. KAWCHUK. Charlottetown Research and Development Centre, Agriculture and Agri-Food Canada (AAFC), 440 University Avenue, Charlottetown, PE C1A 4N6, Canada; (K.A.-M.) Department of Agriculture, Aquaculture and Fisheries, 39 Barker Lane, Wicklow, NB E7L 3S4, Canada; (F.D.) University of Manitoba, Winnipeg, MB R3T 2N2, Canada; and (L.K.) Lethbridge Research and Development Centre, AAFC, 5403–1 Avenue South, Lethbridge, AB T1J 4B1, Canada
Late blight has been an annual disease issue for both potato and tomato crops in Canada in recent years. Plant tissue samples infected with the late blight pathogen collected in Canada from 2015–2017 have yielded multiple clonal genotypes of Phytophthora infestans (Mont.) de Bary that varied by production region. Pathogen isolates collected from British Columbia potatoes were either US-8 (A2; metalaxyl-insensitive) or US-11 (A1; metalaxyl-insensitive) genotypes. Samples of potato or tomato received from most other production regions, including Alberta, Saskatchewan, Manitoba, Ontario, New Brunswick and Prince Edward Island yielded isolates of the US-23 genotype, which is an A1 mating type and largely sensitive to metalaxyl. Samples of potatoes from Quebec yielded US-23 or US-24 (A1; metalaxyl-insensitive) genotypes. Although isolates of US-23 were often sensitive to metalaxyl early in the season, increased resistance to this chemical pesticide was documented as the season progressed. Greenhouse/storage trials revealed that US-23 was less aggressive on potato foliage than US-8 or US-24, but equally aggressive as the other genotypes on potato tubers. Conversely, US-8 and US-24 were less aggressive on tomato foliage than US-23. The US-23 genotype has become the predominant genotype in most growing regions across Canada in recent years. Identifying and managing disease in tomatoes (transplants and home gardens) have become critical components of successful late blight management strategies for potato crops with the spread of US-23 across Canada.