First Report of Late Blight Caused by Phytophthora infestans Clonal Lineage US-23 on Tomato and Potato in Atlantic Canada

Phytophthora infestans (Mont.) de Bary has produced significant losses in potato and tomato yield and quality during recent late blight epidemics in North America. During the 1990s, more aggressive and genetically diverse P. infestans genotypes migrated to Canada and the United States (2). For example, US-8 became predominant and was found to be more aggressive in potato than previous clonal lineages of P. infestans. Recent P. infestans genotypes in potato and tomato plants from the United States and Canada include US-22, US-23, and US-24 representing clonal lineages with unique epidemiological characteristics (2,3,4). Characteristic phenotypic traits have been described for P. infestans clonal lineages US-8, US-22, US-23, and US-24 based on the mating type, mefenoxam sensitivity, pathogenicity, and rate of germination suggesting an association between phenotypic variations and the genotype (1,4). Analysis of P. infestans isolates collected in Canada during 2010 revealed the presence of the US-23 clonal lineage in four different areas of western Canada but not in eastern Canada (4). Isolates of P. infestans collected from eastern Canada for several years prior to 2011 were all US-8 A2 mating type. Isolation and analysis of 98 P. infestans isolates in 2011 from New Brunswick and Prince Edward Island followed standard procedures (2,3,4). Results confirmed the presence of the US-23 clonal lineage in Atlantic Canada on potato and tomato leaves with late blight symptoms, increasing the genetic complexity of P. infestans in eastern Canada. Allozyme banding patterns at the glucose-6-phosphate isomerase (Gpi) locus showed a 100/100 profile in 10 P. infestans isolates, consistent with the US-23 clonal lineage (2,3,4). Furthermore, in vitro mefenoxam sensitivity was observed in all 10 P. infestans US-23 isolates from New Brunswick and Prince Edward Island. Mating type assays confirmed the isolates were of the A1 mating type. RFLP analysis of EcoR1-digested genomic DNA using the multilocus RG57 sequence as a probe produced the DNA pattern 1, 2, 5, 6, 10, 13, 14, 17, 20, 21, 24, 24a, 25, indicative of US-23 (2,4). Microsatellite analysis using polymorphic markers on New Brunswick and Prince Edward Island P. infestans isolates produced the Pi4B 213/217 bp, D13 134 bp, and PiG11 140/155 bp profile of P. infestans US-23 (1). These results show the presence of the P. infestans A1 and A2 mating types in New Brunswick and Prince Edward Island, which increases the probability of sexual recombination. To our knowledge, this is the first report of P. infestans clonal lineage US-23 causing late blight in New Brunswick and Prince Edward Island, increasing the genetic diversity from previous years in eastern Canada and underscoring the annual fluctuation occurring in the population composition. References: (1) G. Danies et al. Plant Dis. 97:873, 2013. (2) S. B. Goodwin et al. Phytopathology 84:553, 1994. (3) C. H. Hu et al. Plant Dis. 96:1323, 2012. (4) M. L. Kalischuk et al. Plant Dis. 96:1729, 2012.

Proteomics analysis suggests broad functional changes in potato leaves triggered by phosphites and a complex indirect mode of action against Phytophthora infestans

Phosphite (salts of phosphorous acid; Phi)-based fungicides are increasingly used in controlling oomycete pathogens, such as the late blight agent Phytophthora infestans. In plants, low amounts of Phi induce pathogen resistance through an indirect mode of action. We used iTRAQ-based quantitative proteomics to investigate the effects of phosphite on potato plants before and after infection with P. infestans. Ninety-three (62 up-regulated and 31 down-regulated) differentially regulated proteins, from a total of 1172 reproducibly identified proteins, were identified in the leaf proteome of Phi-treated potato plants. Four days post-inoculation with P. infestans, 16 of the 31 down-regulated proteins remained down-regulated and 42 of the 62 up-regulated proteins remained up-regulated, including 90% of the defense proteins. This group includes pathogenesis-related, stress-responsive, and detoxification-related proteins. Callose deposition and ultrastructural analyses of leaf tissues after infection were used to complement the proteomics approach. This study represents the first comprehensive proteomics analysis of the indirect mode of action of Phi, demonstrating broad effects on plant defense and plant metabolism. The proteomics data and the microscopy study suggest that Phi triggers a hypersensitive response that is responsible for induced resistance of potato leaves against P. infestans.

BIOLOGICAL SIGNIFICANCE

Phosphie triggers complex functional changes in potato leaves that are responsible for the induced resistance against Phytophthora infestans. This article is part of a Special Issue entitled: Translational Plant Proteomics.

Genetic Composition of Phytophthora infestans in Canada Reveals Migration and Increased Diversity

A dramatic increase in the incidence of late blight and changes within populations of Phytophthora infestans have been observed in various regions of Canada. In this study, the occurrence of several new genotypes of the pathogen was documented with associated phenotypes that dominated pathogen populations. Genotype US-23, previously detected only among isolates from the United States, dominated in the western Canadian provinces of British Columbia, Alberta (AB), Saskatchewan, and Manitoba (MB). Although isolates of US-23 infect both potato and tomato, these isolates were the only genotype recovered from commercial garden centers in Canada. Isolates of genotype US-8, previously dominant throughout Canada, represented the only genotype detected from the eastern Canadian provinces of New Brunswick and Prince Edward Island. Isolates of other genotypes detected in Canada included US-11 in AB, US-24 in MB, and US-22 in Ontario (ON). An additional genotype was detected in ON which appears to be a derivative of US-22 that may have arisen through sexual reproduction. However, evidence of clonal reproduction dominated among the isolates collected, and opportunities for sexual reproduction were probably limited because of a surprising geographic separation of the A1 and A2 mating types in Canada. Sensitivity of the US-22, US-23, and US-24 isolates to the fungicide metalaxyl, movement of potato seed and transplants, and weather conditions may have contributed to reduced opportunities for contact between the mating types in fields in Canada. All P. infestans isolates were readily distinguished from other related oomycetes with RG57 restriction fragment length polymorphism analysis. Long-distance movement in seed tubers and garden center transplants may have contributed to the rapid spread of the P. infestans genotypes across Canada. Tracking pathogen movement and population composition should improve the ability to predict the genotypes expected each year in different regions of Canada.

First Report of Phytophthora infestans Genotype US23 Causing Late Blight in Canada

Late blight is caused by the oomycete Phytophthora infestans (Mont.) de Bary and is one of the most devastating diseases of potato and tomato. Late blight occurs in all major potato- and tomato-growing regions of Canada. Its incidence in North America increased during 2009 and 2010 (2). Foliar disease symptoms appeared earlier than usual (June rather than July) and coincided with the identification of several new P. infestans genotypes in the United States, each with unique characteristics. Prior to 2007, isolates collected from potato and tomato crops were mainly US8 or US11 genotypes (1). However, P. infestans populations in the United States have recently experienced a major genetic evolution, producing isolates with unique genotypes and epidemiological characteristics in Florida and throughout the northeastern states (2). Recent discoveries of tomato transplants with late blight for sale at Canadian retail outlets prompted an examination of the genotypes inadvertently being distributed and causing disease in commercial production areas in Canada. Analysis of isolates of P. infestans from across Canada in 2010 identified the US23 genotype for the first time from each of the four western provinces (Manitoba, Saskatchewan, Alberta, and British Columbia) but not from eastern Canada. Allozyme banding patterns at the glucose phosphate isomerase (Gpi) locus indicated a 100/100 profile consistent with US6 and US23 genotypes (4). Mating type assays confirmed the isolates to be A1 and in vivo metalaxyl sensitivity was observed. Restriction fragment length polymorphic analysis of 50 isolates from western Canada with the multilocus RG57 sequence and EcoRI produced the DNA pattern 1, 2, 5, 6, 10, 13, 14, 17, 20, 21, 24, 24a, 25 that was indicative of US23 (3). The recently described P. infestans genotype US23 appears to be more aggressive on tomato, and although isolates were recovered from both tomato and potato, disease symptoms were often more severe on tomato. Results indicate that movement and evolution of new P. infestans genotypes have contributed to the increased incidence of late blight and that movement of the pathogen on retail plantlets nationally and internationally may provide an additional early season source of inoculum. A major concern is that the introduced new A1 populations in western Canada have established a dichotomy with the endogenous A2 populations in eastern Canada, increasing the potential for sexual recombination producing oospores and additional genotypes should these populations merge. References: (1) Q. Chen et al. Am. J. Potato Res. 80:9, 2003. (2) K. Deahl. (Abstr.) Phytopathology 100(suppl.):S161, 2010. (3) S. B. Goodwin et al. Curr. Genet. 22:107, 1992. (4) S. B. Goodwin et al. Phytopathology 88:939, 2004.

First Report of Fludioxonil-Resistant Isolates of Fusarium spp. Causing Potato Seed-Piece Decay

Potato (Solanum tuberosum L.) diseases incited by Fusarium spp. include postharvest dry rot and seed-piece decay. Fusarium seed-piece decay is commonly controlled by preplant applications of chemical seed treatments. However, isolates of Fusarium spp. resistant to benzimidazole fungicides have been reported (2,4). In the spring of 2007, samples of cut seed tubers (cvs. Shepody and Russet Burbank) showing extensive symptoms of decay were received from three seedlots in Prince Edward Island (PE) and one seedlot in Saskatchewan (SK), Canada. All seed tubers had been treated with fludioxonil (Maxim Potato Seed Protectant [PSP], 0.5% fludioxonil) following cutting and then stored for 10 to 14 days prior to planting. Using standard isolation protocols (4), the 19 potato tuber pieces examined from PE and 2 from SK yielded 21 Fusarium isolates for further study. Five isolates (including both isolates from SK) were identified as Fusarium sambucinum Fuckel and the remaining 16 isolates were identified as F. coeruleum (Libert) Sacc. (3). To confirm identifications, isolates were compared with two known standards of each of F. sambucinum and F. coeruleum identified by K. Seifert (Agriculture and Agri-Food Canada, Ottawa, ON) by DNA sequencing of the partial β-tubulin gene or the translation elongation factor 1-α ( http://fusarium.cbio.psu.edu ; [1]). These standard isolates were also used as fludioxonil-sensitive controls in amended agar assays for chemical sensitivity. Agar plugs (5 mm in diameter) taken from the margins of 7-day-old cultures of the Fusarium isolates were transferred to petri dishes containing ½-strength potato dextrose agar amended with 0, 0.1, 1.0, 10.0, or 100.0 mg/liter of fludioxonil. Fludioxonil (Maxim PSP, 0.5% a.i.) was prepared as a stock solution in sterile distilled water and added to the molten agar after autoclaving. Culture incubation and mycelial growth measurements were performed as described previously (4). Measurements from four replicate petri dishes per concentration of fludioxonil were taken. Calculated EC₅₀ values (fludioxonil concentration inhibiting pathogen growth by 50%) were obtained. The trial was repeated three times. The two standard isolates of F. sambucinum were sensitive to fludioxonil, with mean EC₅₀ values of 0.002 (±0.002 standard error [SE]) and 0.005 (±0.002 SE) mg/liter. The two standard isolates of F. coeruleum were also sensitive to fludioxonil, with mean EC₅₀ values of 0.17 (±0.005 SE) and 0.19 (± 0.005 SE) mg/liter. All other tested isolates of F. sambucinum and F. coeruleum were resistant to fludioxonil and showed no growth inhibition even at 100 mg of fludioxonil per liter. To our knowledge, this is the first report of resistance to fludioxonil in isolates of Fusarium spp. causing potato seed-piece decay. Since the isolates of F. sambucinum were also resistant to thiophanate-methyl and thiabendazole (data not shown), multiclass (benzimidazole and pyrrole) resistance was also documented. References: (1) D. M. Geiser et al. Eur. J. Plant Pathol. 110:473, 2004. (2) L. M. Kawchuk et al. Am. Potato J. 71:185, 1994. (3) P. E. Nelson et al. Fusarium Species: An Illustrated Manual for Identification. Pennsylvania State University Press, 1983. (4) R. D. Peters et al. Plant Dis. 85:1030, 2001.

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.