Populations of P. infestans are dynamic, with mutation, migration, sexual reproduction, and host having contributed to the evolution of new clonal lineages or genotypes with different epidemiological, genotypic, and phenotypic characteristics (Goodwin, 1997; Hu et al., 2012). Globalization has increased the trade in agricultural commodities such as potato between countries. Potato tubers intended for processing or for use as seed are imported or exported from one country to another based on supply and demand. P. infestans remains dormant (latent) in seed tubers at low temperatures (4°C). As such, symptomless seed tubers are easily overlooked in intercontinental shipments (Johnson and Cummings, 2009). The introduction of new genotypes in seed tubers has resulted in changes in local population compositions of
Figure 1Figure 2 Pectolytic Dickeya spp. and Pectobacterium spp. are commercially important seed-borne bacteria of potato (Solanum tuberosum) that cause blackleg, soft rot and aerial stem rot (Potrykus et al., 2014;Stevenson et al., 2001). Dickeya and Pectobacterium spp. have been recovered from potato production fields in parts of the USA (Dickey, 1979;Ma et al., 2007) Five seven-week-old cv. 'Russet Norkotah' potato plants were woundinoculated by inserting a sterile 18-gauge needle just above a central leaf axil at a depth of 1 mm. A 100 μl drop of inoculum (10 6 cfu/ml) was placed on the wound. Plants were exposed to a 24 h leaf wetness period (90 to 100% relative humidity in a mist chamber) until symptom expression (Figs. 1, 2), and lesions were measured. All three inoculated plants exhibited blackening of the stem and in advanced stages, drying and cracking (Figs. 1, 2). Upon desiccation, the lesions became shriveled and turned dark brown to black. Water-inoculated controls were nonsymptomatic (Figs. 1, 2). The reisolated bacteria caused pitting on crystal violet pectate agar (Hélias et al., 2012) and exhibited the same morphology as original cultures on NBY, and were confirmed as D. dianthicola using 16S rRNA and acnA coding sequences, and P. wasabiae using rRNA, acnA and mdh coding sequences, fulfilling Koch's postulates. To our knowledge, this is the first report of D. dianthicola and P. wasabiae causing aerial stem rot of potato in Michigan. References
Late blight, caused by Phytophthora infestans (Mont.) de Bary, is a destructive disease of potato (Solanum tuberosum) and tomato (S. lycopersicum) in the United States. Prior to 2007, the US-8 clonal lineage was the predominant genotype in the United States (4). Since 2007, a significant genetic change in the population of P. infestans occurred in the eastern United States with the appearance of new isolates with unique genotypes and epidemiological characteristics (3). These new genotypes US-22, US-23, and US-24 are sensitive to metalaxyl and represent mating types A2, A1, and A1, respectively (1,2). Prior to 2012, only US-8 had been documented in Idaho (5). In 2013, late blight was discovered in late August on potato crops (cv. Russet Norkotah) in Bingham and Madison counties, ID. Infected foliage (four samples from Bingham County and five from Madison) was sent to Michigan State University and the University of Wisconsin for confirmation of P. infestans and characterization of the isolates. Five sections from the leading edge of lesions were excised with a sterilized scalpel and placed on potato tuber slices (‘Dark Red Norkotah’). Pathogen sporulation on the excised lesions was enhanced by incubation in plastic boxes lined with moistened paper towels for 5 days at 18°C in the dark. The sporulating lesions were transferred onto pea agar medium (160 g peas, 5 g sucrose, 15 g agar, 700 ml distilled water) amended with 50 mg/ml vancomycin. Ten pure cultures were obtained for each of 4 isolates per county by hyphal tipping. Cellulose acetate electrophoresis was conducted to determine Gpi allozyme genotype of the 4 isolates (4). The allozyme banding patterns were 100/100 at the Gpi locus, consistent with previously reported analyses of the US-23 genotype (1,2). Genomic DNA was extracted from 10 pure cultures using the DNeasy Plant Mini Kit (Qiagen, Germantown, MD), and SSR analyses were performed. Microsatellite markers Pi02, Pi4B, Pi63, PiG11, and D13 were used in SSR analyses. Pi02, Pi4B, and Pi63 had alleles of 162/164, 213/217, and 270/279 bp in size, respectively which is consistent with the reference US-23 genotype (1). However, heterozygosity was detected at locus D13 in the Idaho genotype with allele size of 134/210 bp and an additional allele of 140/155/176 bp at locus PiG11. This is different from the standard US-23 genotype (homozygous alleles 134/134 at locus D13 and two alleles 140/155 at locus PiG11). These allele changes indicate the isolates may be variants of US-23 isolates as all other phenotypic characteristics were similar to those of reference US-23 isolates. The Idaho genotypes were sensitive to metalaxyl both in vitro on rye A agar medium amended with metalaxyl at <0.1 ppm, and in vivo on Ridomil treated foliage tests at <0.1 ppm (1,2). Mating type assays confirmed the pathogen to be the A1 mating type. In the 2009 and 2010 late blight epidemics in the eastern United States, US-23 was the predominant genotype, but to our knowledge this genotype has never been reported previously in Idaho. Thus, this is the first known report of P. infestans genotype US-23 causing late blight on potato in Idaho, indicating a change in the population of P. infestans. In Idaho, the source of this genotype remains unknown, although infected tomatoes have been implicated in the widespread dissemination of this genotype of P. infestans in the eastern United States. References: (1) G. Danies et al. Plant Dis. 97:873, 2013. (2) C. Hu et al. Plant Dis. 96:1323, 2012. (3) K. Deahl. (Abstr.) Phytopathology 100:S161, 2010. (4) S. B. Goodwin et al. Plant Dis. 79:1181, 1995. (5) USAblight. Recent US Genotypes. Online: www.usablight.org/node/52 , retrieved 3 January 2014.
Late blight, caused by Phytophthora infestans, is an economically important disease of potato that causes significant yield losses, with severe outbreaks regularly occurring in Bangladesh. The objective of this study was to do a large‐scale survey of potato fields in the main potato‐growing divisions of Bangladesh examining genotypic diversity of P. infestans populations. A total of 160 samples were collected in 2018 from both potato (n = 140) and tomato (n = 20). Isolates were mainly collected on FTA cards (n = 143), but 17 were also collected and isolated into pure culture. Microsatellite analysis revealed high levels of subclonal diversity in P. infestans populations with 116 multilocus genotypes recorded from 160 samples. Comparisons with standards of European and US isolates showed that 74% of samples could be categorized as genotype EU_13_A2, 7% clustered near EU_6_A1 and EU_1_A1, and 19% were unique. Discriminant analysis of principal components showed that the P. infestans population clustered into four distinct groups: a main group that contained most of the samples from potato, two distinct tomato groups and one group of samples originating from the division of Mymensingh. Of 17 isolates from cultures, 16 were EU_13_A2 and one was EU_6_A1; 15 were insensitive to metalaxyl‐M. Out of 160 samples, 158 were categorized as mating type A2 and two as mating type A1. These results indicate that Bangladesh populations of P. infestans from potato, like those from neighbouring countries, are dominated by genotype EU_13_A2. However, populations from tomato were distinct and appear to be specific to tomato.
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