Decreased defense gene expression in tolerance versus resistance to Verticillium dahliae in potato.

Verticillium dahliae Kleb., a soil-borne fungus that colonizes vascular tissues, induces wilting, chlorosis and early senescence in potato. Difference in senescence timing found in two diploid potato clones, 07506-01 and 12120-03, was studied and genetic variation in response to V. dahliae infection was identified as a causal factor. The clone, 07506-01, was infected with V. dahliae but did not develop symptoms, indicating tolerance to the pathogen. The other diploid clone, 12120-03 had low levels of pathogen with infection and moderate symptoms indicating partial resistance. 07506-01 was found to carry two susceptible alleles of the Ve2 gene and 12120-03 carried one Ve2 resistant and one susceptible allele. Infected leaves of the two clones were compared using gene expression profiling with the Potato Oligonucleotide Chip Initiative (POCI) microrarray. The results provide further evidence for differences in response of the two clones to infection with V. dahliae. Chlorophyll biosynthesis was higher in the tolerant 07506-01 compared to partially resistant 12120-03. On the other hand, expression of fungal defense genes, Ve resistance genes and defense phytohormone biosynthetic enzyme genes was decreased in 07506-01 compared to 12120-03 suggesting defense responses were suppressed in tolerance compared to resistance. Transcription factor gene expression differences pointed to the WRKY family as potential regulators of V. dahliae responses in potato.

Molecular phylogeny of Boliniales (Sordariomycetes) with an assessment of the systematics of Apiorhynchostoma, Endoxyla and Pseudovalsaria.

The systematics of the ascomycete genera Apiorhynchostoma, Endoxyla and Pseudovalsaria are reevaluated based on the comparison of cultural characteristics, teleomorph morphology and DNA sequence data. Analyses of sequences of the internal transcribed spacer (ITS) of the ribosomal DNA operon and the large subunit (LSU) of the nuclear ribosomal DNA gene resolve Boliniales as a robustly supported lineage comprising Apiorhynchostoma, Camarops, Camaropella, Cornipulvina, Endoxyla and Pseudovalsaria. Within Boliniales, species of Endoxyla form a strongly supported lineage. Apiorhynchostoma curreyi and Pseudovalsaria ferruginea group with Cornipulvina ellipsoides. Species of Camarops are paraphyletic and comprise two clades, one of which includes Camaropella. Boliniaceae is emended, Endoxyla mallochii is described as new and Apiorhynchostoma trabicola is considered a synonym of Apiorhynchostoma altipetum. We also propose the combinations Endoxyla occulta, Endoxylina luteobasis and Jobellisia peckii. Keys to genera included in the Boliniaceae and to species of Apiocamarops, Apiorhynchostoma and Endoxyla are provided.

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 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 ß-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.

Variation in Aggressiveness of Canadian Isolates of Phytophthora infestans as Indicated by Their Relative Abilities to Cause Potato Tuber Rot.

Studies were conducted in 1995, 1996, and 1997 to determine the ability of Canadian isolates of Phytophthora infestans to cause tuber disease in a variety of potato cultivars. Most isolates of recently introduced multilocus genotypes (US-7, US-8, g11, g26, g29, and g40) were more aggressive on tuber tissue than isolates of the traditional US-1 genotype, based on surface necrosis (SN), lesion depth (LD), and compound aggressiveness index (CAI = SN × LD) components. Other multilocus genotypes (g30, g41, g42, and UN-3) were similar in aggressiveness to US-1. The g11 (US-11) genotype consistently displayed aggressiveness that was intermediate to that of the US-8 and US-1 genotypes. Variation also occurred within a multilocus genotype, and an isolate of the US-8 genotype from New Brunswick was consistently less aggressive than other US-8 isolates. Cvs. Dorita and Island Sunshine were consistently the most resistant to infection, regardless of isolate, relative to the more susceptible responses of Green Mountain, Russet Bur-bank, Kennebec, Superior, Shepody, Red Pontiac, Sebago, and Bintje. Even so, isolates of the newly introduced US-8 genotype were able to cause significantly more disease on these resistant cultivars than isolates of the US-1 genotype. The predominant genotypes currently found in potato production areas in Canada (US-8 and g11) have higher fitness on tuber tissue than displaced, preexisting strains (US-1).

Plant-parasitic nematodes and fungi associated with root rot of peas on prince edward island.

Eight commercial pea fields on Prince Edward Island were sampled in June and July over a 2-year period (1986-87) to determine soil population densities and the incidence of nematodes and fungi associated with root rot of peas. Root lesion nematodes (Pratylenchus spp.) were the dominant endoparasitic nematodes recovered from roots and soil. Low populations of the northern root-knot nematode (Meloidogyne hapla) were also present. Tylenchorhynchus spp. and Paratylenchus spp. were recovered frequently from soil in the root zone, and Helicotylenchus spp. were also frequent, but in low numbers. Fusarium solani was the most common fungal species isolated from the epicotyl and hypocotyl tissues of pea. Fusarium oxysporum was also isolated frequently, and both Fusarium species were found in soil from all fields. Rhizoctonia solani and Verticillium albo-atrum were common in hypocotyl tissue, but V. dahliae was isolated infrequently. Root rot was rated as severe in all fields and was positively and significantly correlated (P

Molecular evidence that Verticillium ablo-atrum Grp 2 isolates are distinct from V. albo-atrum Grp 1 and V. tricorpus.

summary Verticillium species-specific primers were used in a polymerase chain reaction to differentiate between Verticillium albo-atrum groups and V. tricorpus. Amplification with species-specific primers identified 21 isolates from the 64 V. albo-atrum isolates tested as Grp2. Genome analysis using RAPDs and restriction fragment length polymorphism (RFLP) of the intergenic (IGS) region of the rDNA showed that V. albo-atrum Grp2 isolate were genetically distinct from either V. albo-atrum Grp1 or V. tricorpus, demonstrating a significant differentiation between these species. The sizes of the amplified IGS fragment were different, with Grp1 isolates having a smaller fragment ( approximately 2.1 kb) than either Grp2 ( approximately 2.3 kb) or V. tricorpus ( approximately 2.7 kb). Based on RAPD analysis, the average similarity coefficients between Grp1 and Grp2 were 35% and 34% between Grp2 and V. tricorpus. Multiple correspondence analysis separated the isolates into three major groups corresponding to Grp1, Grp2 and V. tricorpus. Surprisingly, isolates collected from Pisum sativa were distinct from other Grp1 V. albo-atrum isolates. The observed low levels of genetic similarity, the differences in sizes of IGS fragments, IGS-RFLP profiles and the RAPD patterns point to the possibility of Grp2 isolates comprising a different species of Verticillium than those occupied by either V. albo-atrum Grp1 isolates or V. tricorpus.

Formation of trichothecenes by Fusarium solani var. coeruleum and Fusarium sambucinum in potatoes.

Fusarium solani var. coeruleum can form deoxynivalenol in potato tubers and in liquid medium, although concentrations observed in the rot were highly variable; acetyldeoxynivalenol and HT-2 toxin were detected in 1 to 3 tubers only (of 57). Trichothecenes were also detected in a very few (3 of 20) cultures of Fusarium sambucinum in potato tubers.