•Cercospora spp. harbor MAT gene fragments at random locations in the genome.•MAT fragments reconstituted MAT exons with high identity when manually assembled.•MAT fragmentation pattern suggests ...fused MAT genes in homothallic ancestor.•Concerted evolution homogenized MAT fragments and MAT genes in each species.
Dothideomycetes is one of the most ecologically diverse and economically important classes of fungi. Sexual reproduction in this group is governed by mating type (MAT) genes at the MAT1 locus. Self-sterile (heterothallic) species contain one of two genes at MAT1 (MAT1-1-1 or MAT1-2-1) and only isolates of opposite mating type are sexually compatible. In contrast, self-fertile (homothallic) species contain both MAT genes at MAT1. Knowledge of the reproductive capacities of plant pathogens are of particular interest because recombining populations tend to be more difficult to manage in agricultural settings. In this study, we sequenced MAT1 in the heterothallic Dothideomycete fungus Cercospora beticola to gain insight into the reproductive capabilities of this important plant pathogen. In addition to the expected MAT gene at MAT1, each isolate contained fragments of both MAT1-1-1 and MAT1-2-1 at ostensibly random loci across the genome. When MAT fragments from each locus were manually assembled, they reconstituted MAT1-1-1 and MAT1-2-1 exons with high identity, suggesting a retroposition event occurred in a homothallic ancestor in which both MAT genes were fused. The genome sequences of related taxa revealed that MAT gene fragment pattern of Cercospora zeae-maydis was analogous to C. beticola. In contrast, the genome of more distantly related Mycosphaerella graminicola did not contain MAT fragments. Although fragments occurred in syntenic regions of the C. beticola and C. zeae-maydis genomes, each MAT fragment was more closely related to the intact MAT gene of the same species. Taken together, these data suggest MAT genes fragmented after divergence of M. graminicola from the remaining taxa, and concerted evolution functioned to homogenize MAT fragments and MAT genes in each species.
An epidemic of aphid-transmitted potato virus Y (PVY) in seed potato grown in the Red River Valley of Minnesota and North Dakota was the reason we began a study to determine which aphid species were ...found in traps placed near potato fields. Knowledge of which aphid species are present and which of these are known vectors of PVY is needed to develop management strategies that minimize virus spread. The spread of PVY to healthy indicator plants and captures of elate aphids in tile traps were monitored weekly for 3 yr consecutively (1992-1994) throughout the Red River Valley. Thirty-four aphid species were identified from green and yellow tile traps in 1992, 25 in 1993, and 26 in 1994. Yellow tile traps caught significantly more aphids overall than green tile traps and were significantly preferred by Aphis helianthi Monell, Capitophorus elaeagni (del Guercio), and Rhopalosiphum maidis (Fitch). Intervals of greatest aphid capture in green traps were generally between mid-July and mid-August at all sites in all 3 yr. Aphid captures at all sites were 3-25 times greater in 1992 and 1994 than in 1993. PVY infection of indicator plants exposed at the trapping sites also was greater in 1992 (25 plants) and 1994 (18 plants) than in 1993 (2 plants). Eighty-nine percent of PVY spread to indicator plants occurred between 8 July and 19 August. Eight species comprised 89.9% of the aphids collected in green traps during intervals of PVY transmission to indicator plants: Acyrthosiphon pisum (Harris), pea aphid; A. helianthi; C. elaeagni; Lipaphis erysimi (Kaltenbach), turnip aphid; R. maidis, corn leaf aphid; R. padi (L.), bird cherry-oat aphid; Schizaphis graminum (Rondani), greenbug; and Sitobion avenae (F.), English grain aphid. Seven of these species were previously known PVY vectors. We found that A. helianthi transmitted PVY with low efficiency under greenhouse conditions. The 8 most common aphid species were associated with crops and weeds common in the Red River Valley
Cercospora leaf spot caused by Cercospora beticola is one of the most damaging foliar diseases of sugar beet. The sterol demethylation inhibitor (DMI) fungicide tetraconazole is widely-used to manage ...Cercospora leaf spot. However, there has been an increase in prevalence of tetraconzole-resistant isolates in recent years. Knowledge about the stability of tetraconazole resistance in tetraconzole-resistant isolates after exposure to cold temperatures in the absence of the selection pressure imposed by tetraconazole application would be important information for fungicide resistance management. To explore this, we inoculated sugar beet plants with two known tetraconazolesensitive and two known tetraconazole-resistant isolates of C. beticola. Four weeks after inoculation, symptomatic leaves were harvested and subsequently exposed to six different temperature/time treatments: -20°C (4 weeks), 4°C (4 weeks), 20°C (4 weeks), -20°C (2 weeks)/4°C (2 weeks), -20°C (1 week)/4°C (1 week)/-20°C (1 week)/4°C (1 week), and -20°C (1 week)/20°C (1 week)/-20°C (1 week)/20°C (1 week). Subsequently, spore production, spore germination, radial mycelial growth, sensitivity to tetraconazole, and disease severity were evaluated for each isolate and compared to the control (the same fungal isolates used in the inoculation series and maintained on CV8 media). After exposure to all temperature/time treatments, all tested isolates were found sTablefor the parameters evaluated. However, the tetraconazole-resistant isolate 09-347 after exposure to -20°C and -20°C/4°C/-20°C/4°C treatments became 38.6 and 32.8 times more sensitive to tetraconazole, respectively. Taken together, cold temperatures do not appear to impart a significant fitness penalty in C. beticola.
Cercospora leaf spot (CLS) of sugar beet is caused by the fungus Cercospora beticola. CLS management practices include the application of the sterol demethylation inhibitor (DMI) fungicides ...tetraconazole, difenoconazole, and prothioconazole. Evaluating resistance to DMIs is a major focus for CLS fungicide resistance management. Isolates were collected in 1997 and 1998 (baseline sensitivity to tetraconazole, prothioconazole, or difenoconazole) and 2007 through 2010 from the major sugar-beet-growing regions of Minnesota and North Dakota and assessed for in vitro sensitivity to two or three DMI fungicides. Most (47%) isolates collected in 1997-98 exhibited 50% effective concentration (EC
) values for tetraconazole of <0.01 μg ml
, whereas no isolates could be found in this EC
range in 2010. Since 2007, annual median and mean tetraconazole EC
values have generally been increasing, and the frequency of isolates with EC
values >0.11 μg ml
increased from 2008 to 2010. In contrast, the frequency of isolates with EC
values for prothioconazole of >1.0 μg ml
has been decreasing since 2007. Annual median difenoconazole EC
values appears to be stable, although annual mean EC
values generally have been increasing for this fungicide. Although EC
values are important for gauging fungicide sensitivity trends, a rigorous comparison of the relationship between in vitro EC
values and loss of fungicide efficacy in planta has not been conducted for C. beticola. To explore this, 12 isolates exhibiting a wide range of tetraconazole EC
values were inoculated to sugar beet but no tetraconazole was applied. No relationship was found between isolate EC
value and disease severity. To assess whether EC
values are related to fungicide efficacy in planta, sugar beet plants were sprayed with various dilutions of Eminent, the commercial formulation of tetraconazole, and subsequently inoculated with isolates that exhibited very low, medium, or high tetraconazole EC
values. The high EC
isolate caused significantly more disease than isolates with medium or very low EC
values at the field application rate and most reduced rates. Because in vitro sensitivity testing is typically carried out with the active ingredient of the commercial fungicide, we investigated whether loss of disease control was the same for tetraconazole as for the commercial product Eminent. The high EC
isolate caused more disease on plants treated with tetraconazole than Eminent but disease severity was not different between plants inoculated with the very low EC
isolate.
Fusarium graminearum, a known producer of trichothecene mycotoxins in cereal hosts, has been recently documented as a cause of dry rot of potato tubers in the United States. Due to the uncertainty of ...trichothecene production in these tubers, a study was conducted to determine the accumulation and diffusion of trichothecenes in potato tubers affected with dry rot caused by F. graminearum. Potato tubers of cv. Russet Burbank were inoculated with 14 F. graminearum isolates from potato, sugar beet, and wheat and incubated at 10 to 12 degrees C for 5 weeks to determine accumulation of trichothecenes in potato tubers during storage. Twelve of the isolates were classified as deoxynivalenol (DON) genotype and two isolates were as nivalenol (NIV) genotype. Trichothecenes were detected only in rotted tissue. DON was detected in all F. graminearum DON genotype isolates up to 39.68 microg/ml in rotted potato tissue. Similarly, both NIV genotype isolates accumulated NIV in rotted potato tissue up to 18.28 microg/ml. Interestingly, isolates classified as genotype DON accumulated both DON and NIV in the dry rot lesion. Potato tubers were then inoculated with two isolates of F. graminearum chemotype DON and incubated up to 7 weeks at 10 to 12 degrees C and assayed for DON diffusion. F. graminearum was recovered from >53% of the isolations from inoculated tubers at 3 cm distal to the rotted tissue after 7 weeks of incubation but DON was not detected in the surrounding tissue. Based in this data, the accumulation of trichothecenes in the asymptomatic tissue surrounding dry rot lesions caused by F. graminearum is minimal in cv. Russet Burbank potato tubers stored for 7 weeks at customary processing storage temperatures.
Silver scurf has become a major reason for rejection of fresh and processing potatoes in recent years. Control of the disease by chemical or cultural practices or resistant cultivars has been ...difficult. Observations have shown spread and increase of disease of potatoes in storage, but this has not been extensively studied. The objective of this study was to document Helminthosporium solani conidia production, dispersal, and tuber infection in potato storages. Spore samplers placed in seed, processing, and table stock storages collected conidia ranging from 0 to 12,000 conidia per day in seed and table stock storages (4 degree C), and from 0 to 24,000 conidia per day in processing storages (10 degree C). Conidia were detected soon after tubers entered storage and increased progressively during the storage period, with the maximum conidia numbers found during the time of tuber handling. Greenhouse-produced minitubers placed in storages for 1, 2, 3, or 4 weeks were infected by H. solani spores. Infection was significantly higher in those exposed for 4 weeks than in those exposed for 1 week. Results document the buildup of H. solani spores throughout the storage period, and that this inoculum is important in disease epidemiology. Control of this inoculum could lead to disease reduction.
An epidemic of purple top disease of potato (Solanum tuberosum) occurred in the Columbia Basin Region of Washington and Oregon in 2002 and 2003, causing great economic loss in the potato industry ...(1). Symptoms of potato purple top (PPT) were characterized by upright terminal shoots, upward leaf rolling, chlorosis, red or purplish discoloration of new leaves, proliferation of axillary shoots with basal swelling, and the formation of aerial tubers. Preliminary studies on PPT disease suggested phytoplasma as a possible cause (1). In this study, 78 potato samples (including five asymptomatic) were collected from five fields throughout the region. A nested polymerase chain reaction (PCR) with primer pair P1/P7 in the first amplification followed with primer pair R16F2n/R16R2 was performed to detect the presence of phytoplasmas in infected plants (2). Restriction fragment length polymorphism (RFLP) and phylogenetic analyses of amplified 16S rDNA sequences were used for phytoplasma identification. Eighty-four percent (63% in the first amplification) of the symptomatic samples and 60% (0% in the first amplification) of the asymptomatic samples tested positive. Low phytoplasma titers and the presence of PCR inhibitors accounts for the low detection rate in the first PCR amplifications. RFLP analyses of 16S rDNA with enzymes MseI, AluI, HhaI, RsaI, and HpaII indicated that the phytoplasma associated with PPT belonged to the clover proliferation (CP) group (16SrVI) subgroup A (16SrVI-A) (2). 16SrVI-A currently consists of three members, CP (GenBank Accession No. AY500130), potato witches'-broom (GenBank Accession No. AY500818), and vinca virescence (VR) (GenBank Accession No. AY500817), a strain of beet leafhopper-transmitted virescence agent (BLTVA) phytoplasma (2). The taxonomic affiliation of PPT phytoplasma was confirmed by phylogenetic analysis of cloned 16S rDNA (GenBank Accession Nos. PPT4, AY496004; PPT8, AY496005). The 16S rDNA sequences of the PPT strains were closely related to VR with 99.7% sequence homology compared with 99.2% with CP. A high correlation between the symptoms and the presence of 16SrVI-A phytoplasmas in the potato plants suggests that these phytoplasmas play an etiological role in PPT disease. To gain further evidence, a modified test of Koch's postulates was conducted. Infected tissues from four phytoplasma-positive potato samples (including PPT4 and PPT8) were grafted onto healthy potato seedlings. Within 60 days after grafting, the potato seedlings developed symptoms similar to those in the original diseased samples. The newly infected plants were maintained through cuttings. RFLP analysis of 16S rDNA indicated that the phytoplasmas detected in each of the seedlings and cuttings were identical to those in the scions. These results confirmed the probable etiological role of CP group, subgroup 16SrVI-A phytoplasma strains in PPT disease in Washington and Oregon. There are two other confirmed cases of phytoplasmas (BLTVA and aster yellows phytoplasma) associated with PPT disease in Utah (4) and Mexico (3). References: (1) P. B. Hamm et al. Potato Prog. Vol. 3, No. 1, 2003. (2) I.-M. Lee et al. Int. J. Syst. Bacteriol. 48:1153, 1998. (3) N. E. Leyva-Lopez et al. Can. J. Microbiol. 48:1062, 2002. (4) C. D. Smart et al. Phytopathology 83:1399, 1993.
Forty-two strains of Gibberella pulicaris (anamorph: Fusarium sambucinum, synonym F. sulphureum) were obtained from dry-rotted potato tubers collected in North America between 1963 and 1991. ...Twenty-four of 25 strains collected in 1990 and 1991 were resistant to the fungicide thiabendazole (TBZ), which is widely used to control potato dry rot. The 17 strains collected between 1963 and 1986 were all very sensitive to TBZ. In laboratory tests, most TBZ-resistant and TBZ-sensitive strains were virulent on potato tubers and produced trichothecene mycotoxins in liquid culture and in potato tubers. All 42 strains were characterized for sexual compatibility by crosses with tester strains and for vegetative compatibility by complementation of nitrate-nonutilizing mutants. Twenty-one (50%) of the strains belonged to one widespread vegetative compatibility group (VCG 01). Twelve strains (26%) belonged to two additional overlapping groups (VCG 03 and 04). Forty strains were mating type 1. Two strains were mating type 2 and belonged to a unique group (VCG 02). All TBZ-resistant strains were vegetatively compatible with TBZ-sensitive strains collected in previous years. Genetic analysis indicated that TBZ resistance was stable and inherited as a single gene or as closely linked genes, and that resistance mutations of independently isolated field strains were allelic. These results suggest that TBZ-resistant strains are competitive and have the potential to spread and persist in the G. pulicaris population that causes potato tuber dry rot in North America
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