Tritici Populations Research Articles

Virulence and Diversity of Blumeria graminis f. sp. tritici Populations in China

Wheat powdery mildew, caused by Blumeria graminis f. sp. tritici, is an important disease in China. To characterize the virulence and diversity of the pathogen, 1082 isolates were obtained from 8 major wheat-growing regions during the spring growing season in 2011. The virulence test was performed by inoculation on detached leaves of 22 differential lines with known Pm genes. Frequencies of virulence on these genotypes ranged from 0 to 97.4%. None of the 1082 isolates was compatible to Pm21 and less than 20.0% were virulent to the genotype carrying Pm13. In contrast, the virulence frequencies of each population was more than 50.0% to differentials carrying Pm1a, Pm3b, Pm3c, Pm3f, Pm5a, Pm6 and Pm8. In total, 1028 pathotypes were detected, of which 984 were unique. Phenotypic diversity indices revealed a high level of diversity within populations. Genetic distance between different populations correlated significantly with geographical distance (Rb=0.494, P≤0.001). In addition, isolates from Xinjiang appear to form a separate group. Significant positive or negative associations between alleles at pairs of virulence loci were detected in 57 allele pairs to Pm genes. Virulence and diversity of the 8 populations suggested that varieties with effective resistance gene combinations should be developed at a regional level.

Characterization of Ustilago segetum tritici causing loose smut of wheat in northwestern India

Loose smut of wheat, caused by Ustilago segetum tritici, occurs in India wherever the crop is grown. Chemical seed treatments used by seed producers and farmers result in a low incidence of the disease. However, the deployment of loose smut resistant cultivars could lead to reduced use of seed treatment chemicals in the wheat crop. Knowledge of the virulence of natural populations of U. segetum tritici is necessary to identify effective host genes for resistance. Since the late 1980s, there have been sporadic reports on the virulence characteristics of the U. segetum tritici population in India, but the current status is unknown. Thirty-five isolates of U. segetum tritici were collected from cultivars grown at various locations in five states of northwestern India from 2008–2011. The collected isolates were characterized for virulence on the standard Canadian differential set developed by Nielsen. The isolates represented 6 races, representing 43% of the 35 isolates, with race T34 being the most common. Two new races, T59 and T60, were identified. Two individual isolates collected from Triticum aestivum representing 2 different races, T1 and T57, originated from geographically diverse regions. Race T1 is from Mansa (210 m above mean sea level) and T57 from Palampur (1220 m above mean sea level). The isolates corresponding to the other races were not restricted to a particular area or wheat type.

Development of a rapid multiplex SSR genotyping method to study populations of the fungal plant pathogen Zymoseptoria tritici.

BackgroundZymoseptoria tritici is a hemibiotrophic ascomycete fungus causing leaf blotch of wheat that often decreases yield severely. Populations of the fungus are known to be highly diverse and poorly differentiated from each other. However, a genotyping tool is needed to address further questions in large collections of isolates, regarding regional population structure, adaptation to anthropogenic selective pressures, and dynamics of the recently discovered accessory chromosomes. This procedure is limited by costly and time-consuming simplex PCR genotyping. Recent development of genomic approaches and of larger sets of SSRs enabled the optimization of microsatellite multiplexing.FindingsWe report here a reliable protocol to amplify 24 SSRs organized in three multiplex panels, and covering all Z. tritici chromosomes. We also propose an automatic allele assignment procedure, which allows scoring alleles in a repeatable manner across studies and laboratories. All together, these tools enabled us to characterize local and worldwide populations and to calculate diversity indexes consistent with results reported in the literature.ConclusionThis easy-to-use, accurate, repeatable, economical, and faster technical strategy can provide useful genetic information for evolutionary inferences concerning Z. tritici populations. Moreover, it will facilitate the comparison of studies from different scientific groups.

First Report of Resistance to QoI Fungicides in North American Populations of Zymoseptoria tritici, Causal Agent of Septoria Tritici Blotch of Wheat.

The G143A mutation in cytb (cytochrome b gene) is associated with high levels of resistance to quinone outside inhibitor (QoI or strobilurin) fungicides that disrupt electron transport during cellular respiration (1). The G143A mutation in Zymoseptoria tritici (synonyms: Mycosphaerella graminicola and Septoria tritici), the causal agent of septoria tritici blotch of wheat (Triticum aestivum), was first reported in Europe in 2001 (1). Although Z. tritici has a global distribution (3), G143A mutants of Z. tritici have not been reported outside of Europe. We used PCR-RFLP (4) to estimate the frequencies of G143A mutants in Z. tritici populations at two locations in the Willamette Valley of western Oregon: the Hyslop Crop Science Field Research Laboratory (Hyslop Farm, HF), Benton County (44°37'52.85″ N, 123°11'55.19″ W) and research plots planted in a commercial wheat field in Washington County (45°33'58.53″ N, 123°00'11.78″ W) (North Valley Farm, NVF). Isolates originated from flag leaf collections from two cultivars ('Bobtail' and 'Tubbs 06') made in April and June of 2012 from plants in a replicated fungicide-treatment experiment, with isolates collected from both sprayed and unsprayed plots. Sixteen of the 169 isolates (9.5%) from HF possessed the G143A mutation (7 of 132 isolates from plots not receiving a QoI fungicide and 9 of 37 isolates collected from plots receiving two applications of the QoI azoxystrobin). One hundred forty six of the 175 isolates (83.4%) from NVF were G143A mutants (101 of 129 isolates from plots receiving no QoI fungicide and 45 of 46 isolates from plots receiving two applications of azoxystrobin). Results of phenotypic assays of a subset of 10 isolates from each location (5 mutants, 5 wild types from each location; 20 isolates altogether) supported a high level of resistance to azoxystrobin only in the G143A mutants. All 10 G143A mutants developed colonies after 8 days of growth on YMA plates amended with SHAM (2) and 1 ppm or 10 ppm azoxystrobin, with nine and eight G143A mutant isolates developing colonies on plates amended with 1 ppm and 10 ppm azoxystrobin, respectively. None of the wild-type isolates developed colonies on plates amended with SHAM and 1 ppm azoxystrobin, nor on plates amended with SHAM and 10 ppm azoxystrobin. All 20 isolates developed colonies on YMA plates lacking azoxystrobin, and treatments produced identical results across three replicates. These results are consistent with findings of higher levels of azoxystrobin resistance in G143A mutants compared to wild types in European populations (1). Isolates from HF and NVF differ in their previous exposure to QoI fungicides. The majority of the wheat area at HF is planted to breeding plots that are not sprayed with fungicide. Plots at NVF were planted in a commercial wheat field in a county where most wheat fields were treated with two to three applications of strobilurins each year over the past 4 years. Future monitoring for G143A mutants of Z. tritici throughout its range in North America will be necessary to assess whether strobilurin resistance will spread via wind-dispersal of ascospores or emerge de novo in treated fields. In Europe, stobilurins were first applied to wheat in 1996. G143A mutants of Z. tritici emerged de novo several times (4) and were widespread by 2007.

Identification of Eighteen Berberis Species as Alternate Hosts of Puccinia striiformis f. sp. tritici and Virulence Variation in the Pathogen Isolates from Natural Infection of Barberry Plants in China

ABSTRACT The wheat stripe rust pathogen (Puccinia striiformis f. sp. tritici) population in China has been reported to be a distinct genetic group with higher diversity than those in many other countries. Genetic recombination in the P. striiformis f. sp. tritici population has been identified with molecular markers but whether sexual reproduction occurs in China is unknown. In this study, we surveyed barberry plants for infection by rust fungi in the stripe rust "hotspot" regions in Gansu, Sichuan, and Shaanxi provinces; collected barberry plants and inoculated plants of 20 Berberis spp. with germinated teliospores under controlled greenhouse conditions for susceptibility to P. striiformis f. sp. tritici; and tested P. striiformis f. sp. tritici isolates obtained from aecia on naturally infected barberry plants on the wheat genotypes used to differentiate Chinese P. striiformis f. sp. tritici races to determine virulence variations. Different Berberis spp. were widely distributed and most surveyed plants had pycnia and aecia of rust fungi throughout the surveyed regions. In total, 28 Berberis spp. were identified during our study. From 20 Berberis spp. tested with teliospores of P. striiformis f. sp. tritici from wheat plants, 18 species were susceptible under greenhouse conditions. Among 3,703 aecia sampled from barberry plants of three species (Berberis shensiana, B. brachypoda, and B. soulieana) under natural infections in Gansu and Shaanxi provinces, four produced P. striiformis f. sp. tritici uredinia on susceptible wheat 'Mingxian 169'. Sequence of the internal transcribed spacer (ITS) regions of the four isolates from barberry shared 99% identity with the P. striiformis f. sp. tritici sequences in the National Center for Biotechnology Information database. The four isolates had virulence patterns different from all previously reported races collected from wheat plants. Furthermore, 82 single-uredinium isolates obtained from the four barberry isolates had high virulence diversity rates of 9.0 to 28.1%, indicating that the diverse isolates were produced through sexual reproduction on barberry plants under natural conditions. In addition to P. striiformis f. sp. tritici, sequence analysis of polymerase chain reaction products of the ITS regions and inoculation tests on wheat identified P. graminis (the stem rust pathogen). Our results indicated that P. striiformis f. sp. tritici can infect some Berberis spp. under natural conditions, and the sexual cycle of the fungus may contribute to the diversity of P. striiformis f. sp. tritici in China.

Virulence Characterization of International Collections of the Wheat Stripe Rust Pathogen, Puccinia striiformis f. sp. tritici.

Wheat stripe rust (yellow rust [Yr]), caused by Puccinia striiformis f. sp. tritici, is an economically important disease of wheat worldwide. Virulence information on P. striiformis f. sp. tritici populations is important to implement effective disease control with resistant cultivars. In total, 235 P. striiformis f. sp. tritici isolates from Algeria, Australia, Canada, Chile, China, Hungary, Kenya, Nepal, Pakistan, Russia, Spain, Turkey, and Uzbekistan were tested on 20 single Yr-gene lines and the 20 wheat genotypes that are used to differentiate P. striiformis f. sp. tritici races in the United States. The 235 isolates were identified as 129 virulence patterns on the single-gene lines and 169 virulence patterns on the U.S. differentials. Virulences to YrA, Yr2, Yr6, Yr7, Yr8, Yr9, Yr17, Yr25, YrUkn, Yr28, Yr31, YrExp2, Lemhi (Yr21), Paha (YrPa1, YrPa2, YrPa3), Druchamp (Yr3a, YrD, YrDru), Produra (YrPr1, YrPr2), Stephens (Yr3a, YrS, YrSte), Lee (Yr7, Yr22, Yr23), Fielder (Yr6, Yr20), Tyee (YrTye), Tres (YrTr1, YrTr2), Express (YrExp1, YrExp2), Clement (Yr9, YrCle), and Compair (Yr8, Yr19) were detected in all countries. At least 80% of the isolates were virulent on YrA, Yr2, Yr6, Yr7, Yr8, Yr17, YrUkn, Yr31, YrExp2, Yr21, Stephens (Yr3a, YrS, YrSte), Lee (Yr7, Yr22, Yr23), and Fielder (Yr6, Yr20). Virulences to Yr1, Yr9, Yr25, Yr27, Yr28, Heines VII (Yr2, YrHVII), Paha (YrPa1, YrPa2, YrPa3), Druchamp (Yr3a, YrD, YrDru), Produra (YrPr1, YrPr2), Yamhill (Yr2, Yr4a, YrYam), Tyee (YrTye), Tres (YrTr1, YrTr2), Hyak (Yr17, YrTye), Express (YrExp1, YrExp2), Clement (Yr9, YrCle), and Compair (Yr8, Yr19) were moderately frequent (>20 to <80%). Virulence to Yr10, Yr24, Yr32, YrSP, and Moro (Yr10, YrMor) was low (≤20%). Virulence to Moro was absent in Algeria, Australia, Canada, Kenya, Russia, Spain, Turkey, and China, but 5% of the Chinese isolates were virulent to Yr10. None of the isolates from Algeria, Canada, China, Kenya, Russia, and Spain was virulent to Yr24; none of the isolates from Algeria, Australia, Canada, Nepal, Russia, and Spain was virulent to Yr32; none of the isolates from Australia, Canada, Chile, Hungary, Kenya, Kenya, Nepal, Pakistan, Russia, and Spain was virulent to YrSP; and none of the isolates from any country was virulent to Yr5 and Yr15. Although the frequencies of virulence factors were different, most of the P. striiformis f. sp. tritici isolates from these countries shared common virulence factors. The virulences and their frequencies and distributions should be useful in breeding stripe-rust-resistant wheat cultivars and understanding the pathogen migration and evolution.

Comparative virulence phenotypes and molecular genotypes of Puccinia striiformis f. sp. tritici, the wheat stripe rust pathogen in China and the United States

Stripe rust (yellow rust) of wheat, caused by Puccinia striiformis f. sp. tritici, is one of the most important diseases in both China and the United States. The Chinese and US populations of the stripe rust fungus were compared for their virulence phenotypes on wheat cultivars used to differentiate races of the pathogen in China and the US and molecular genotypes using simple sequence repeat (SSR) markers. From 86 Chinese isolates, 54 races were identified based on reactions on the 17 Chinese differentials and 52 races were identified based on the 20 US differentials. The selected 51 US isolates, representing 50 races based on the US differentials, were identified as 41 races using the Chinese differentials. A total of 132 virulence phenotypes were identified from the 137 isolates based on reactions on both Chinese and US differentials. None of the isolates from the two countries had identical virulence phenotypes on both sets of differentials. From the 137 isolates, SSR markers identified 102 genotypes, of which 71 from China and 31 from the US. The virulence data clustered the 137 isolates into 20 virulence groups (VGs) and the marker data clustered the isolates into seven molecular groups (MGs). Virulence and SSR data had a low (r = 0.34), but significant (P = 0.01) correlation. Principal component analyses using either the virulence data or the SSR data separated the isolates into three groups: group a consisting of only Chinese isolates, group b consisting of both Chinese and US isolates and group c consisting of mostly US isolates. A neighbour-joining tree generated using the molecular data suggested that the P. striiformis f. sp. tritici populations of China and the US in general evolved independently.

Virulence, Frequency, and Distribution of Races of Puccinia striiformis f. sp. tritici and P. striiformis f. sp. hordei Identified in the United States in 2008 and 2009.

Puccinia striiformis f. sp. tritici and P. striiformis f. sp. hordei, the causal agents of stripe rust on wheat and barley, respectively, can change rapidly in virulence, and such changes may overcome resistance in cultivars and result in severe epidemics. To monitor virulence changes in the pathogen populations, isolates obtained from stripe rust samples collected by the authors and collaborators from 17 U.S. states in 2008 and 13 states in 2009 were tested on 20 wheat and 12 barley differential lines to identify races of P. striiformis f. tritici and P. striiformis f. sp. hordei, respectively. In 2008, 33 P. striiformis f. tritici (PST) races were detected, including a new race, PST-138, which was similar to previously identified PST-127 (virulent on wheat differentials 1, 2, 3, 4, 6, 8, 9, 10, 11, 12, 13, 15, 16, 17, 18, 19, and 20) but not virulent on differential 8. The five most frequent races were PST-114 (virulent on differentials 1, 3, 4, 8, 9, 10, 11, 12, 14, 16, 17, 18, 19, and 20), PST-100 (virulent on differentials 1, 3, 8, 9, 10, 11, 12, 16, 17, 18, 19, and 20), PST-116 (similar to PST-114 plus virulent on differential 5), PST-101 (similar to PST-100 plus virulent on differential 2), and PST-98 (similar to PST-100 but not virulent on differential 9). In 2009, 26 P. striiformis f. tritici races were identified, including two new races, PST-139 and PST-140. PST-139 was similar to PST-127 but not virulent on differentials 16 and 20. PST-140 was similar to PST-114 but not virulent on differential 9. The five most frequent races were PST-139 (19%), PST-140 (14%), PST-114 (11%), PST-116 (10%), and PST-127 (9%). However, the most widely distributed races were PST-98 (in 10 of the 14 states) and PST-102 (in 7 of the 14 states). Differential genotype AvSYr5NIL (Yr5) was the only one among the 20 differentials that remained resistant to all of the identified races. Virulence diversity of the P. striiformis f. tritici populations was higher west of the Rocky Mountains. For barley stripe rust, P. striiformis f. sp. hordei (PSH)-33 (virulent on barley differentials 1 and 7) was the most common (46%) of the 11 races detected in 2008, including a new race, PSH-82 (virulent only on barley differentials 1 and 11). In 2009, six previously identified races were detected, of which five (PSH-16, PSH-38, PSH-46, PSH-54, and PSH-71) were detected in Washington and two (PSH-54 and PSH-70) in Oregon. The information on P. striiformis f. sp. tritici and P. striiformis f. sp. hordei races should be useful in selecting genes for developing cultivars with effective stripe rust resistance.

Life Table Parameters of the Predatory Mite, Phytoseiulus persimilis Athias-Hcnriot on Four Tetranychid Prey Species (Phytoseiidae-Tetranychidac)

Developmental duration, predation rate, reproduction and life table parameters of Phytoseiulus persimilis Athias-Hcmiot on the four tetranychid mites: Tetranychus urticae Koch., Oligonychus sayedi Zaher, Gomaa & EI-Enany, Eotelranychus zacheri Zaher, Gomaa & EI-Enany, and Petrobia tritici Kandeel, EI-Naggar & Mohamed. were studied at 25 ± 2°C and 70 ± 5% R.H. P. persimilis was able to feed and complete its development on the four tested preys. The predator exhibit a particularly high capacity for increase when fed on T. urticae and O. sayedi, feeding on P. tritici, enhanced poor predator performance as significant longer life cycle and decrease in adult female longevity and no eggs laying occurred. The period of adult female longevity increased to 24.1 days when fed on T urticae comparing to 23.4 & 14.2 days when fed on O. sayedi and E. zacheri, respectively. The result showed that low capacity of predator population growth suggests poor ability of P. persimilis to suppress E. zacheri and P. tritici populations on commercial crops. The daily multiplication rate (R0/T) and the intrinsic rate increase (rm) revealed that highest rate of P. persimilis fed on T. urticae which averaged 2.51 and 0.12, respectively.

Population densities of wheat thrips, Haplothrips tritici Kurdjumov (Thysanoptera: Phlaeothripidae), on different wheat and barley cultivars in the province of Kahramanmara, Turkey

Population densities of Haplothrips tritici (H. tritici) Kurdjumov (Thysanoptera: Phlaeothripidae) were studied in KahramanmaraAŸ, Turkey. In 2002, bread wheat cultivars, Bocro 4, Pehlivan and YureAŸir 89, durum wheat cultivars, Balcali 85, Ceylan 95 and Harran 95 and barley cultivars, Esterel and Pacific, were used. In 2003, Sham IV and Firat 93 were used instead of Pehlivan and Ceylan 95. Hege was also added to the barley cultivars. H. tritici (mostly second-stage larvae) population densities on Bocro 4, Pehlivan and YureAŸir 89 on the 6th of June in the first year were 54.8, 59.5 and 33.0/ear, respectively. Populations on durum wheat reached their highest value twice (during the egg and second larvae stages) in the first year. The first one was on the 10th May, when H. tritici densities in Balcali 85, Ceylan 95 and Harran 95, were 42.1, 52.0 and 40.5/ear, respectively, and the second one was on the 6th of June, when they were 47.5, 60.9 and 58.9/ear, respectively. In the second year, H. tritici on Bocro 4 reached their highest level (26.9/ear) on 17th May and H. tritici populations on Sham IV and YureAŸir 89 on 29th May were also at their highest level with 48.4 and 20.1/ear, respectively. Population densities on durum wheat cultivars were not as high as in the first year and changed between 0.2 and 9.7/ear. H. tritici on barley also stayed at low levels for the first (0.1 to 3.2/ear) and second (0.1 to 1.2/ear) years. Key words: Thysanoptera, Haplothrips tritici, population density, bread wheat, durum wheat, barley, cultivars.

Genetic Characterization of Puccinia graminis f.sp. tritici Populations from Ethiopia by SSRs

AbstractThe genetic structure of Puccinia graminis f.sp. tritici isolates sampled in three different regions of Ethiopia in 2006 and 2007 was investigated using 20 SSRs assuming sexual as well as asexual reproduction. Assays showed a high level of genetic diversity within P. graminis f.sp. tritici populations in Ethiopia in both modes of reproductions. Tests for population subdivision revealed the absence of genetic differentiation among the populations based on geographical separation as reflected by a low coefficient of genetic differentiation (≤0.046) and a single cluster consisting of all isolates. Gene flow among populations was estimated to be high (10 per generation). This study shows that the pathogen population of Ethiopia is characterized by a high genetic diversity within each population and absence of segregation among populations. Information obtained from this study may serve as a basis to develop better strategies for deployment of resistance genes, e.g. using marker‐assisted combination of resistance alleles to achieve better control of wheat stem rust in Ethiopia.

First Detection of Virulence in Puccinia striiformis f. sp. tritici in China to Resistance Genes Yr24 (=Yr26) Present in Wheat Cultivar Chuanmai 42.

Stripe rust disease of wheat caused by Puccinia striiformis f. sp. tritici was observed on previously resistant bread wheat (Triticum aestivum L.) cv. Chuanmai 42 during the 2008-2009 crop season in Pi County, Sichuan Province, China. More than 10 single pustules were isolated from the diseased leaf samples collected in the field and inoculated on 7-day-old susceptible wheat seedlings cv. Mingxian 169. After 18 to 24 h of incubation at 100% relative humidity in darkness, the plants were moved into the greenhouse, maintained at 15 to 18°C, and supplemented with 10,000 lx of fluorescent light for 10 h per day. The second leaves were clipped when chlorotic spots appeared on leaves (~7 days postinoculation), and plants were covered with glass cylinders to prevent cross contamination. Urediniospores of each isolate were collected 16 days after inoculation and temporarily kept in a dryer at low temperature (3 to 4°C). The virulence spectra of the isolates were tested on Chinese differentials and wheat lines with known Yr genes at the seedling stage (1). A new Yr24 (=Yr26) virulent pathotype, different from currently known pathotypes in China, was identified. To our knowledge, this is the first detection of Yr24 virulence in Puccinia striiformis f. sp. tritici populations on Chuanmai 42. In addition, the new pathotype was also virulent to Lantian 17, Guinong 22 (Chinese differential), and 92R137 derived wheat lines Nannong 04Y10 and Nannong 05Y628, known to carry Yr24 (2,3). The avirulence/virulence formula of the new pathotype is Yr1, 3, 4, H46, 5, 6, 15, 17, 18, 32, Sp, Sd/Yr2, 8, 9, 10, 12, 24 (=26), 31, and Su. Wheat cultivars and breeding materials, previously protected by Yr24 gene, are now vulnerable to stripe rust epidemics in the region. Pure isolates of the new pathotype (Accession No. 09-6-16-3) are stored in the Chinese Academy of Agricultural Sciences (CAAS; Beijing) stripe rust collection.

Development of Race-Specific SCAR Markers for Detection of Chinese Races CYR32 and CYR33 of Puccinia striiformis f. sp. tritici.

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici, is a devastating disease in China. Races CYR32 and CYR33 have been predominant in the recent P. striiformis f. sp. tritici population. To develop molecular markers for these races, initially 86 isolates, most of which were collected in 2007 throughout China, were tested on the set of wheat genotypes for differentiating Chinese P. striiformis f. sp. tritici races, and their genomic DNA were amplified with 94 random amplified polymorphic DNA (RAPD) primers. Twelve isolates were identified as CYR33, 14 as CYR32, and 60 as 13 other races. A 320-bp band was identified to be associated with CYR32 with primer S1271 (5'-CTTCTCGGTC-3'), and a 550-bp band was identified to be specific to CYR33 with primer S1304 (5'-AGGAGCGACA-3'). The two bands were cloned and sequenced. Based on the sequences, sequence characterized amplified region (SCAR) markers CYR32sp1/sp2 and CYR33sp1/sp2 were developed to differentiate CYR32 and CYR33, respectively, from other races. The SCAR markers were validated with DNA samples from wheat leaves inoculated with selected isolates from the 86 isolates and urediniospore DNA samples from an additional 63 isolates collected from 2006 to 2009. The detection of CYR32 and CYR33 with the SCAR markers was completely consistent with the results of the race identification with the set of differential wheat genotypes. Thus, the markers are highly reliable for identification of the two races.

Population Genetic Analysis of an Eastern U.S. Wheat Powdery Mildew Population Reveals Geographic Subdivision and Recent Common Ancestry with U.K. and Israeli Populations

The structure of the U.S. wheat powdery mildew population (Blumeria graminis f. sp. tritici) has not been previously investigated, and the global evolutionary history of B. graminis f. sp. tritici is largely unknown. After gathering 141 single-ascosporic B. graminis f. sp. tritici isolates from 10 eastern U.S. locations, 34 isolates from the United Kingdom, and 28 isolates from Israel, we analyzed pathogen population structure using presumptively neutral markers. DNA was extracted from conidia, primers for 12 "housekeeping" genes were designed, and amplicons were examined for polymorphism. Four genes were found to contain a total of 12 single-nucleotide polymorphisms in the U.S. population and were also analyzed in the U.K. and Israeli populations. In total, 25 haplotypes were inferred from the four concatenated genes, with 2 haplotypes comprising over 70% of the U.S. population. Using Hudson's tests and analysis of molecular variance, we found the wheat mildew isolates subdivided into four groups corresponding to distinct regions: the mid-Atlantic United States, the southern United States, the United Kingdom, and Israel. Genotypic diversity was greatest in samples from the United Kingdom, Israel, Virginia, and Kingston, NC. Using rarefaction, a procedure that compensates for differing sample sizes when estimating population richness and diversity, we found that cooler locations with greater conduciveness to regular powdery mildew epidemics had the greatest haplotype richness. Our results suggest that the eastern U.S. B. graminis f. sp. tritici population is young, descended recently from Old World populations with isolation and genetic drift, and is currently subdivided into northern and southern subpopulations.

Virulence analysis of Puccinia graminis f.sp. tritici populations in Ethiopia with special consideration of Ug99

Wheat stem rust samples were collected in 2006 and 2007 in the Arsi, Bale, Shewa and northwest regions of Ethiopia to determine virulence diversity and race distribution in Puccinia graminis f.sp. tritici populations. Stem rust incidence was high in Arsi, Bale and east Shewa. In northwest Ethiopia, and north and west Shewa, stem rust was prevalent at low levels. A total of 152 isolates was analysed and 22 races were identified. Races TTKSR (Ug99), TTHSR and RRTTR were predominant, with frequencies of 26·6, 17·7 and 11·1%, respectively. These races were also detected in all regions. The highly virulent race designated Ug99 was present throughout the country and dominated in all regions except northwest Ethiopia. A variant of Ug99 virulent against the stem rust resistance gene Sr24 was not detected in this study. Four stem rust resistance genes (Sr13, Sr30, Sr36 and SrTmp) were found to confer resistance to most of the races prevalent in Ethiopia. With the exception of Sr30, which is not effective against Ug99, these genes could be used in breeding for resistance to stem rust in Ethiopia.

Linear relationship between Gaeumannomyces graminis var. tritici (Ggt) genotypic frequencies and disease severity on wheat roots in the field

In order to investigate potential links existing between Gaeumannomyces graminis var. tritici (Ggt) population structure and disease development during polyetic take-all epidemics in sequences of Ggt host cereals, seven epidemics in fields with different cropping histories were monitored during the seasons 2001/2002 (two fields), 2002/2003 (two fields) and 2003/2004 (three fields). Take-all incidence and severity were measured at stem elongation and Ggt populations were characterized. The 73 isolates collected in the two fields in 2001/2002 were distributed into two multilocus genotypes, G1 and G2 according to amplified fragment length polymorphism analysis. A monolocus molecular marker amplified by F-12 random amplification polymorphism DNA primer sizing between 1.9 and 2.0 kb that gave strictly the same distinction between the two multilocus genotypes was further applied to measure G1/G2 frequencies among Ggt populations in all fields (266 isolates). The ratios of G1 to G2 differed between fields with different cropping histories. A linear relationship between G2 frequency among Ggt populations and disease severity at stem elongation was measured during the three cropping seasons. When take-all decline was observed, G2 frequencies were low in first wheat crops, highest in short-term sequences and intermediate in longer sequences of consecutive crops of Ggt host cereals. This pattern could be the result of population selection by environmental conditions, in particular by microbial antagonism during the parasitic phase of the fungus. In order to better understand take-all epidemic dynamics, the distinction between these two genotypes could be a basis to develop models that link approaches of quantitative epidemiology and advances in population genetics of Ggt.

Changes in population structure of the soilborne fungus Gaeumannomyces graminis var. tritici during continuous wheat cropping.

A method was developed to assess the genetic structure of Gaeumannomyces graminis var. tritici (Ggt) populations and test the hypothesis of an association between disease level in the field with changes in pathogen populations. A long-term wheat monoculture experiment, established since 1994, generated different take-all epidemics with varying the number of wheat crop successions in the 1999-2000 cropping season. Genetic polymorphism in Ggt populations was investigated over natural, local epidemics. Four populations of 30 isolates were isolated from necrotic wheat roots in a first, third, fourth, and sixth wheat crop in the same year. Each Ggt isolate was characterized with RAPD (Random Amplification Polymorphism DNA) markers and AFLP (Amplified Fragment Length Polymorphism) fingerprinting. Seventeen multilocus genotypes based on the combination of RAPD and AFLP markers were identified among all these populations. The 120 isolates were divided into two main groups, G1 and G2, according to bootstrap values higher than 86%, except for an unique isolate from the third wheat crop. Within each group, populations ranged between 93 and 100% similarity. Both groups included isolates collected from the first, third, fourth or sixth wheat crop. However, G1 group profiles dominated amongst isolates sampled in the first and the sixth wheat crops, whereas G2 group profiles largely dominated amongst isolates collected from the third and fourth wheat crops. Aggressiveness of group G2 (38%) was significantly greater than that of G1 (29.5%). These results suggest that changes in Ggt population structure occur during continuous wheat cropping. The distinction of two Ggt groups provides a simple basis for further spatio-temporal analysis of Ggt population during polyetic take-all decline.

The recent history of Puccinia striiformis f.sp. tritici in Denmark as revealed by disease incidence and AFLP markers

Disease observations and amplified fragment length polymorphism (AFLP) markers were used to study recent developments in the Puccinia striiformis f.sp. tritici population in Denmark. The fungus appeared spontaneously at 10 locations in Denmark in 1997 after it was not observed under natural conditions in 1996. The pattern of disease development and prevailing winds suggested that the fungus reappeared by airborne spores from the south or west. In 1998, disease incidence was more evenly distributed throughout the country. Forty‐eight single lesion isolates were collected from most crops where the disease was observed in these years; all except one from 1997 belonged to two pathotypes that were not previously detected in the country, and both possessed the newly discovered Yr17 virulence. The isolates were characterized with AFLP markers together with 28 isolates representing eight of 13 pathotypes observed prior to 1996. Initial screening of 240 PstI/MseI AFLP primer combinations on four isolates showed that a primer combination, on average, revealed 0·4 polymorphisms between any isolate pair. A selection of 21 primer combinations resulted in 28 AFLP markers, which revealed 16 AFLP phenotypes among all 76 isolates. The two Yr17‐virulent pathotypes consisted of three AFLP phenotypes, which were observed in both 1997 and 1998; the two most frequent AFLP phenotypes occurred at most sampling locations and often within the same crop. AFLP diversity was larger among samples collected prior to 1996, and also in this period most AFLP phenotypes were observed at different sampling locations. These results are consistent with the features of an entirely asexually reproducing pathogen dispersed by aerial spores across large areas.

Assessment of partial resistance to powdery mildew in Chinese wheat varieties

AbstractField trials in two cropping seasons and two locations in central China were conducted on 60 Chinese autumn‐sown wheat varieties to assess their partial resistance to powdery mildew. Mean levels of disease severity ranged from close to 0 to more than 90%. The method of inoculation and the location in which trials were conducted affected the relative performance of the varieties, but these effects were much smaller than the main effect of variety. The area under the disease progress curve was highly correlated with final disease severity, but both were poorly correlated with apparent infection rate. Disease severity was regressed against frequencies of virulence in the Blumeria graminis (syn. Erysiphe graminis) f sp. tritici populations in the trial plots. A vertical distance (D) from the mean mildew severity to the fitted line was calculated for each variety and was used to quantify partial resistance. Five of the 60 varieties, ‘Hx8541’, ‘E28547’, ‘Chuan1066’, ‘Zhe88pin6’ and ‘Lin5064’, consistently expressed relatively low levels of disease despite high frequencies of virulence in the pathogen and had consistently high D‐values. They may therefore have good levels of partial resistance.

Invasion thresholds for fungicide resistance: deterministic and stochastic analyses

Fungicide resistance is an important practical problem, but one that is poorly understood at the population level. Here we introduce a simple nonlinear model for fungicide resistance in botanical epidemics which includes the dynamics of the chemical control agent and the host population, while also allowing for demographic stochasticity in the host-parasite dynamics. This provides a mathematical framework for analysing the risk of fungicide resistance developing by including the parameters for the amount applied, longevity and application frequency of the fungicide. The model demonstrates the existence of thresholds for the invasion of the resistant strain in the parasite population which depend on two quantities: the relative fitness of the resistant strain and the effectiveness of control. This threshold marks a change from definite elimination of the resistant strain below the threshold to a finite probability of invasion which increases above the threshold. The fungicide decay rate, the amount of fungicide applied and the period between applications affect the effectiveness of control and, consequently, they influence whether or not resistance develops and the time taken to achieve a critical frequency of resistance. All three parameters are amenable to control by the grower or by coordinating the activity of a population of growers. Providing crude estimates of the effectiveness of control and relative fitness are available, the results can be used to predict the consequences of changing these parameters for the risk of invasion and the proportion of sites at which this might be expected to occur. Although motivated for fungicide resistance, the model has broader application to herbicide, antibiotic and antiviral resistance. The modelling approach and results are discussed in the context of resistance to chemical control in general.