Botany
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Item Taxonomic complexity of powdery mildew pathogens found on lentil and pea in the U.S. Pacific Northwest(Phytopathology, 2008) Attanayake, R.N.; Glawe, D.A.; McPhee, K.E.; Dugan, F.M.; Chen, W.and field production conditions in the U.S. Pacific Northwest was investigated using morphological and molecular characters. Isolates collected from lentil plants grown in the greenhouse or field displayed morphologies in substantial agreement with descriptions of Erysiphe trifolii, but sometimes with more extensively branched chasmothecial appendages resembling those of E. diffusa. ITS sequences of the lentil fungi were identical to each other, and more similar to GenBank accessions of E. trifolii (99.5%) than of E. diffusa (97%). The data suggest there may be more than one Erysiphe species causing lentil powdery mildew. The fungus on field-grown pea plants was determined to be E. pisi. However, powdery mildew samples obtained from greenhouse pea plants were either E. pisi or E. trofolii depending on the time of sampling and greenhouse location. Therefore, the powdery mildews infecting lentil and pea are more diverse than previously assumed. Powdery mildews from black medic (Medicago sp.) and sweet clover (Melilotus sp.) found near the greenhouses exhibited ITS sequences with 99.9 to 100% similarity to isolates from lentil and pea in the greenhouses, and to isolates from lentil from the field. These weedy legumes could be inoculum sources for powdery mildew of lentil and pea in the greenhouses, and serve as alternative hosts for cultivated legumes.Item Potential alternative hosts for a powdery mildew on pea(Phytopathology, 2009) Attanayake, R.N.; Glawe, D.A.; Dugan, F.M.; Chen, W.Powdery mildew of pea (Pisum sativum) is an important disease in the field and in the greenhouse. The most widely documented powdery mildew on pea is Erysiphe pisi, but E. trifolii and E. baeumleri have also been reported. From greenhouse-grown peas, we obtained powdery mildew samples with rDNA ITS sequences nearly identical to previously deposited sequences of E. trifolii. Because detailed studies on host range of this pea powdery mildew in the US Pacific Northwest were lacking, we tested common legume plants from the region as potential alternative hosts. Eleven species were used in greenhouse cross inoculation studies: Lens culinaris, Glycine max, Melilotus albus, M. officinalis, Medicago polymorpha, M. lupulina, M. scutellata, Lathyrus latifolius, Trifolium pratense, Vicia cracca, and V. faba. Except for Glycine max, all the plant species tested developed powdery mildew lesions in 10?14 days after inoculation. Susceptibilities of two of these species (L. culinaris and M. albus) were also confirmed with detached leaf assays. Results showed that all the above legumes (except soybean) are potential alternative hosts for the E. trifolii found on pea. Powdery mildews found on wild legumes (Meliotus albus and Medicago lupulina) were also confirmed to be E. trifolii, suggesting that the wild legumes could be inoculum sources of powdery mildew on greenhouse pea plants during winter months. These findings have implications in managing powdery mildew of pea.Item Characterization of Ascochyta rabiei for population structure, mating type and pathogenic variability from Pakistan and United States(III international Ascochyta workshop, Cordoba, Spain, 2012) Hinaali, A.S.S.; Attanayake, R.N.; Rahman, M.; Chen, W.Chickpea production is greatly hampered by blight causing fungal pathogen Ascochyta rabiei (AR) in chickpea growing regions of the world. Genetic variability and mating type frequency of thirty two AR isolates from six geographical regions of Pakistan were compared with a US-AR population. Pakistani AR (PAR) population had an apparent skewed (3 Mat1-2: 1 Mat1-1) distribution, although Chi-square tests showed non significant deviation from equal distribution due to small sample sizes and the US-population showed a 1:1 distribution. The results showed that sexual reproduction is rare in PAR due to either unavailability of both mating types or lack of conductive environment but statistical analysis showed panmixia which may be due to past recombinational events. Genetic variation at six microsatellite loci was assessed and each isolate was assigned to a microsatellite haplotype. Population structure using Bayesian analyses differentiated isolates into three distinct clusters, two clusters of PAR and one of the US isolates. However, few isolates from US shared same genetic background with one cluster of the PAR isolates, providing a link of inter-continental migration of the pathogen due to import of seeds. Additionally, the two clusters of Pak-isolates are not strictly linked to the geographic locations in Pakistan, suggesting frequent gene flow of AR among different locations. Pathogenic variability of nineteen PAR collected from two different provinces was assessed. The results based on the reaction of isolates with differential lines showed that aggressive and highly aggressive pathotypes II and III respectively are prevalent in Pakistan as compared to least aggressive pathotype I. It is interesting to note that highly aggressive pathotypes III and IV have only beenreported from Syria and Pakistan where we assume less frequency of sexual reproduction due to predominance of one mating type, in contrast to other countries where both mating types are present in equal ratio hence, this issue needs further investigations.Item Pair-wise linkage disequilibrium decay among linked loci suggests meiotic recombination in natural populations of Sclerotinia sclerotiorum(Asilomar Conference Grounds, 2013) Attanayake, R.N.; Chen, W.Both clonal and recombining population structures have been reported in Sclerotinia sclerotiorum populations around the world. Association of independent and putatively unlinked markers indicates clonal population structure, whereas random association of the markers suggests recombination and outcrossing. However, high mutation rates of markers used for inference, like certain microsatellite markers, could interfere and compromise the inferences of recombination and outcrossing. To test if the recombination is due to outcrossing or mutation, we used 12 microsatellite loci distributed over four chromosomes to genotype 230 isolates sampled from seven populations in the USA and China from a variety of crops. All the isolates produced single alleles for each of the loci tested, indicating all the isolates were homokaryotic for the microsatellite loci in consideration. Pair-wise linkage disequilibrium (LD) tests (Hedrick?s D, Fishers exact test and IA) between physically linked loci showed relationships ranging from linked to random association with increasing distance between loci on three chromosomes. For the three loci on chromosome four, LD decay with increasing physical distance between loci was found in six of the seven populations.Item Genetic diversity and population differentiation of Sclerotinia sclerotiorum collected from canola in China and in U.S.A.(Phytopathology, 2011) Attanayake, R.N.; del Río-Mendoza, L.; Chen, W.; Jiang, D.Sclerotinia sclerotiorum is an important pathogen of canola and many other crops worldwide. Genetic diversity and population differentiation of S. sclerotiorum collected from canola fields in Anhui Province, China (30 isolates) and in North Dakota, U.S.A. (29 isolates) were investigated in terms of genetic variation in 8 simple sequence repeat (SSR) marker loci, mycelial compatibility groups (MCGs) and three phenotypic traits: sensitivity to fungicides benomyl, iprodione and fluzinam, oxalic acid production, and pathogenicity. Significant genetic differences were observed; there were no shared SSR haplotypes and no shared MCGs between the two populations. Population differentiation was significant (p = 0.000) indicating lack of gene flow between the two populations. There were also significant differences between the two populations in oxalic acid production and in fungicide sensitivity. The Chinese population displayed high levels of insensitivity (faster growth rate) to benomyl and fluzinam and higher levels of oxalic acid production per unit dry weight of mycelium than did the U.S. population. However, there was no significant difference in pathogenicity between the two populations as measured by colonization of detached canola leaves. Data Vol. 101, No. 6 (Supplement), 2011 S11 suggest that despite geographic and genetic isolation the two populations of S. sclerotiorum were equally adapted to colonizing canola plants, and pathogenicity is under different selection pressure than the other genetic and phenotypic traits.Item Population structure and mating type distribution of the chickpea blight pathogen Ascochytarabiei form Pakistan and the United States(Journal of plant pathology, 2012) Ali, H.; Alam, S.S.; Attanayake, R.N.; Rahman, M.; Chen, W.Ascochyta blight caused by the fungus Ascochyta rabiei (AR) depresses chickpea production in Pakistan and worldwide. Thirty two AR isolates representing six geographical regions of Pakistan were compared with a US-AR population for mating type frequency and genetic variation. Mating type results showed that the Pakistani AR (PAR) population had an apparent skewed (3 Mat1-2: 1 Mat1-1) distribution, although Chi-square tests showed non-significant deviation from equal distribution due to small sample sizes. The US population showed a 1:1 distribution of the two mating types. The uneven distribution of mating types indicates that sexual reproduction among the PAR is rare due to either unavailability of both mating types or lack of conducive environment, but statistical analysis showed that panmixia is there reflecting past recombinational events. Genetic variation at six microsatellite loci was assessed and each isolate was assigned to a microsatellite haplotype. Population structure of the isolates was inferred using Bayesian analyses implemented in the structure software which differentiated isolates into three distinct clusters, two clusters of PAR and one of the US isolates. However, few isolates from the US shared the same genetic background with one cluster of the PAR isolates, providing a link of inter-continental migration of the pathogen. Additionally, the two clusters of PAR-isolates are not strictly associated with geographic locations in Pakistan, suggesting frequent gene flow of AR among different locations. Future studies should extend the sampling of representative populations to overcome the limitations of the small sample size for more accurate assessment of population structure.Item Detection of intrachromosomal recombination in Sclerotinia sclerotiorum populations(Phytopathology, 2012) Attanayake, R.N.; Chen, W.Genetic structure and reproductive mode of the homothallic fungal pathogen Sclerotinia sclerotiorum have been widely studied using linkage disequilibrium (LD) tests with putatively unlinked molecular markers. We previously observed random association between linked loci in S. sclerotiorum populations suggesting intrachromosomal recombination or high mutation rates at these loci. This study was aimed at testing intrachromosomal recombination using 12 microsatellite loci distributed over four chromosomes. Two hundred thirty isolates sampled from seven populations in the USA and China from a variety of crops were genotyped. Each isolate carried a single allele for each of the 12 loci suggesting the isolates were haploid and homokaryotic. Pairwise LD tests of all the intrachromosomal loci showed relationship ranged from linked to random association, and in many cases LD declined with increasing physical distance between loci. Thus the random associations of alleles cannot be simply attributed to random mutation. Majority of the isolates were mycelially incompatible, likely minimizing the possibility of heterokaryon formation and mitotic recombination. Thus the observed high intrachromosomal recombination is most likely due to meiotic recombination following outcross in these populations.Item The Importance of Reporting New Host-Fungus Records for Ornamental and Regional Crops(Peer-Reviewed by Plant Health Progress, 2009) Dugan, F.M.; Glawe, D.A.; Attanayake, R.N.; Chen, W.Accurate and timely reports of new host-fungus records are essential for diagnostics and identification, management, and prevention of plant diseases. Important also are venues to publish these reports in a timely manner and the ability to rapidly search for the information contained in these reports. Presented herein are examples of first reports of fungal pathogens on regional crops, including ornamentals and turf grasses, which illustrate how first reports contribute to preparedness, accurate diagnostics, and knowledge of biogeography and host range. We provide a guide to sources of host-fungus records, discuss venues for publishing new records, and review the information important in a new record, including deposition of voucher specimens. We appeal to plant health professionals to increase their efforts of discovering, documenting, and reporting new records.Item First Report of Powdery Mildew of Chickpea (Cicer arietinum) Caused by Leveillula taurica in Washington State(Plant Health Progress, 2008) Chen, W.; Attanayake, R.N.; Glawe, D.A.; McPhee, K.E.; Dugan, F.M.In Oct. 2007, powdery mildew was found in chickpea fields in an experimental farm near Pullman, Whitman County, Washington. Although disease signs were observed on all chickpea cultivars in the fields, high incidence was seen only on cvs. Dwelley and Spanish White. To our knowledge this is the first record of powdery mildew caused by Leveillula taurica on chickpea in WA. The pathogen has also been reported from chickpea in California and elsewhere, e.g., Ethiopia, India, Iran, Morocco, Pakistan, Sudan, Turkey, and the former USSR.Item Erysiphe trifolii Causing Powdery Mildew of Lentil (Lens culinaris)(Plant Disease, 2009) Attanayake, R.N.; Glawe, D.A.; Dugan, F.M.; Chen, W.The taxonomy of the powdery mildew fungus infecting lentil in the Pacific Northwest (PNW) of the United States was investigated on the basis of morphology and rDNA internal transcribed spacer (ITS) sequences. Anamorphic characters were in close agreement with descriptions of Erysiphe trifolii. However, teleomorphs formed chasmothecial appendages with highly branched apices, whereas E. trifolii has been described as producing flexuous or sometimes loosely branched appendages. Branched appendages have been described in Erysiphe diffusa, a fungus reported from species of Lens, Glycine, and Sophora, raising the possibility that the PNW fungus could be E. diffusa. Examination of morphological characters of an authentic specimen of E. trifolii from Austria determined that it included chasmothecial appendages resembling those seen in PNW specimens. Furthermore, ITS sequences from five powdery mildew samples collected from lentils in PNW greenhouses and fields from 2006 to 2008 were identical to one another, and exhibited higher similarity to sequences of E. trifolii (99%) than to those of any other Erysiphe spp. available in GenBank. Parsimony analysis grouped the lentil powdery mildew into a clade with Erysiphe baeumleri, E. trifolii, and E. trifolii?like Oidium sp., but indicated a more distant relationship to E. diffusa. In greenhouse inoculation studies, the lentil powdery mildew fungus did not infect soybean genotypes known to be susceptible to E. diffusa. The pathogenicity of E. trifolii on lentil was confirmed using modified Koch's postulates. This is the first report of E. trifolii infecting lentil. E. diffusa and E. trifolii have different host ranges, so the discovery of E. trifolii on lentil has implications both for determining species of powdery mildews on cool-season grain legumes, and in disease management.