Showing papers in "Plant Disease in 2019"

First Report of Tomato Brown Rugose Fruit Virus Infecting Greenhouse Tomato in the United States

Abstract: Tomato (Solanum lycopersicum L.), an important vegetable widely grown in the world, is susceptible to a number of emerging viruses. In September 2018, a severe disease outbreak, including mosaic, mottling, and plant stunting was observed on grafted tomato plants in a greenhouse (∼8 acres) in southern California, U.S.A. Initial screening of one symptomatic sample was positive for pepino mosaic virus (PepMV) and tomato mosaic virus (ToMV) using enzyme-linked immunosorbent assay (ELISA). Two types of particles, including flexuous potexvirus-like and rigid tobamovirus-like, were observed under an electron microscope. However, reverse transcription polymerase chain reaction (RT-PCR) tests using primers that are specific to ToMV and tomato mottle mosaic virus (ToMMV) (Sui et al. 2017) were negative. We suspected an involvement of tomato brown rugose fruit virus (ToBRFV) (Luria et al. 2017; Salem et al. 2016). A new set of primers specific to ToBRFV was designed and validated to have no cross-reaction to ToMV and ToMMV. Using this primer set, ToBRFV-F (5503), 5′-GAAGTCCCGATGTCTGTAAGG-3′, and ToBRFV-R (6344), 5′-GTGCCTACGGATGTGTATGA-3′, an expected PCR product (842 bp) was amplified and sequenced from this first sample but not from a healthy control. In a follow-up survey, 20 additional symptomatic samples were tested and shown to be all positive for PepMV using ELISA or qRT-PCR (Ling et al. 2007), and 14 of the same 20 samples were also positive for ToBRFV using RT-PCR. Sanger sequencing of seven amplicons generated a consensus 799-bp sequence after primer trimming, designated as isolate ToBRFV-US (accession no. MK109002). A BLASTn search of the NCBI database revealed 99.6% sequence identity to two other ToBRFV isolates from Jordan (KT383474; Salem et al. 2016) and Israel (KX619418; Luria et al. 2017). In pathogenicity assays, four tomato ‘Moneymaker’ seedlings (susceptible to tobamoviruses) were mechanically inoculated with an inoculum prepared from the first positive sample, and similar symptoms were visible. The target pathogen, ToBRFV, was confirmed on the inoculated plants using RT-PCR and amplicon sequencing. In a separate survey in October 2018, a disease outbreak was also observed on tomato in a greenhouse facility in Baja California, Mexico. An extensive crop health screen using ELISA tests to a panel of 19 common tomato viruses, as well as appropriate RT-PCR tests, were conducted on 14 collected samples. A mixed infection of multiple viruses was identified. In addition to common infection by PepMV, seven of 14 samples were also positive for tomato spotted wilt virus. Weak positives were observed for tobacco mosaic virus or ToMV when tested by ELISA. However, when using genus-specific primers for tobamoviruses (Tob-Uni1 and Tob-Uni2) (Letschert et al. 2002), amplicon sequencing revealed the presence of ToBRFV in both Californian and Mexican samples, with no mixed infection by any other tobamoviruses. Using the ToBRFV-specific primers, amplicon sequences from 14 Mexican samples resulted in a consensus sequence, designated as isolate ToBRFV-MX (accession no. MK109003). The two sequences of ToBRFV-US and ToBRFV-MX isolates were similar (99.6% sequence identity). Identification of the first ToBRFV outbreaks in the United States and Mexico, as well as its ability to break the Tm-2² resistance (Luria et al. 2017), raises a serious concern for the tomato industry. How ToBRFV was introduced to the United States and/or Mexico has not been determined. Eradication efforts are underway in both countries to contain and control these disease outbreaks.

Molecular Mapping of Stripe Rust Resistance Gene Yr81 in a Common Wheat Landrace Aus27430.

Abstract: The deployment of diverse sources of resistance in new cultivars underpins durable control of rust diseases. Aus27430 exhibited a moderate level of stripe rust resistance against Puccinia striiformis f. sp. tritici (Pst) pathotypes currently prevalent in Australia. Aus27430 was crossed with the susceptible parent Avocet S (AvS) and subsequent filial generations were raised. Monogenic segregation observed among Aus27430/AvS F3 families was confirmed through stripe rust screening of an F6 recombinant inbred line (RIL) population, and the resistance locus was temporarily named YrAW5. Selective genotyping using an Illumina iSelect 90K wheat SNP bead chip array located YrAW5 in chromosome 6A. Genetic mapping of the RIL population with linked 90K SNPs that were converted into PCR-based marker assays, as well as SSR markers previously mapped to chromosome 6A, confirmed the chromosomal assignment for YrAW5. Comparative analysis of other stripe rust resistance genes located in chromosome 6A led to the formal designation of YrAW5 as Yr81. Tests with a marker linked with Yr18 also demonstrated the presence of this gene in Aus27430. Yr18 interacted with Yr81 to produce stripe rust responses lower than those produced by RILs carrying these genes individually. Although gwm459 showed higher recombination with Yr81 compared with the other flanking marker KASP_3077, it amplified the AvS allele in 80 cultivars, whereas KASP_3077 amplified AvS allele in 67 cultivars. Both markers can be used in marker-assisted selection after confirming parental polymorphism.

Identification of a New Haplotype of 'Candidatus Liberibacter solanacearum' in Solanum tuberosum.

Abstract: In 2017, potato tubers suspected of being infected with the bacterium ‘Candidatus Liberibacter solanacearum’ were received from the Animal and Plant Health Inspection Service in the United States. ...

Ecology and Epidemiology of Diseases of Nut Crops and Olives Caused by Botryosphaeriaceae Fungi in California and Spain

Abstract: In recent decades, the cultivated area and production of nuts and olives have increased, driven by an increasing consumer interest in healthier food Diseases of almond, pistachio, olive, and walnut crops caused by species belonging to the Botryosphaeriaceae family have caused concern worldwide Although considerable progress has been made in elucidating the etiology of these diseases, scientific knowledge of other aspects of these diseases is more limited In this article, we present an overview of the most important diseases caused by Botryosphaeriaceae fungi affecting almond, pistachio, olive, and walnut crops by focusing on ecology and epidemiology, primarily in California and Spain

Ready-to-Eat Salad Crops: A Plant Pathogen's Heaven.

Abstract: The ready-to-eat salad sector, also called fresh-cut or bagged salads, is a fast-growing segment of the fresh-food industry. The dynamism and specialization of this sector, together with the lack of adequate crop rotation, the globalization of the seed market, and climate change, are the main causes of the development of many new diseases that cause severe production losses. Newly detected diseases of the most important crops grown (lettuce, wild and cultivated rocket, lamb's lettuce, chicory, endive, basil, spinach, and Swiss chard) are critically discussed. The management of these diseases represents a formidable challenge, since few fungicides are registered on these minor-use crops. An interesting feature of the ready-to-eat salad sector is that most crops are grown under protection, often in soilless systems, which provide an environment helpful to the implementation of innovative control methods. Current trends in disease management are discussed, with special focus on the most sustainable practices.

Wheat Virus Identification Within Infected Tissue Using Nanopore Sequencing Technology

Abstract: Viral diseases are a limiting factor to wheat production. Viruses are difficult to diagnose in the early stages of disease development and are often confused with nutrient deficiencies or other abiotic problems. Immunological methods are useful to identify viruses, but specific antibodies may not be available or require high virus titer for detection. In 2015 and 2017, wheat plants containing Wheat streak mosaic virus (WSMV) resistance gene, Wsm2, were found to have symptoms characteristic of WSMV. Serologically, WSMV was detected in all four samples. Additionally, High Plains wheat mosaic virus (HPWMoV) was also detected in one of the samples. Barley yellow dwarf virus (BYDV) was not detected, and a detection kit was not readily available for Triticum mosaic virus (TriMV). Initially, cDNA cloning and Sanger sequencing were used to determine the presence of WSMV; however, the process was time-consuming and expensive. Subsequently, cDNA from infected wheat tissue was sequenced with single-strand, Oxford Nanopore sequencing technology (ONT). ONT was able to confirm the presence of WSMV. Additionally, TriMV was found in all of the samples and BYDV in three of the samples. Deep coverage sequencing of full-length, single-strand WSMV revealed variation compared with the WSMV Sidney-81 reference strain and may represent new variants which overcome Wsm2. These results demonstrate that ONT can more accurately identify causal virus agents and has sufficient resolution to provide evidence of causal variants.

Pharmacological Characteristics and Control Efficacy of a Novel SDHI Fungicide Pydiflumetofen Against Sclerotinia sclerotiorum.

Abstract: Pydiflumetofen is a novel succinate dehydrogenase inhibitor fungicide. In the current research, we determined the sensitivity of 166 Sclerotinia sclerotiorum strains to pydiflumetofen using mycelial growth inhibition method. The results suggest that pydiflumetofen exhibited a strong inhibitory activity against S. sclerotiorum and the EC50 values ranged from 0.0058 to 0.0953 μg ml-1, with a mean EC50 value of 0.0250 μg ml-1. However, the baseline sensitivity was not normally distributed because of a high variation factor. After treatment with pydiflumetofen, cell membrane permeability increases, but exopolysaccharide and oxalic acid production decreases, which contributes to reduced virulence of S. sclerotiorum and leads to failure of disease infection. In addition, protective and curative activity was performed on detached oilseed rape leaves by artificial inoculation. Pydiflumetofen exhibited excellent protective and curative effects against S. sclerotiorum on oilseed rape, and the protective effect was better than the curative effect. Further, field trials were conducted to evaluate the potential of pydiflumetofen in controlling Sclerotinia stem rot (SSR) caused by S. sclerotiorum on oilseed rape. Compared with the currently used fungicides, pydiflumetofen not only exhibited excellent control efficacy against SSR, but also dramatically reduced the dosage of fungicides in the field. Thus, this study provides important references for revealing pharmacological mechanism of pydiflumetofen against S. sclerotiorum and managing SSR on oilseed rape caused by benzimidazole- and dicarboximide-resistant populations.

Colletotrichum acutatum and C. gloeosporioides Species Complexes Associated with Apple in Brazil.

Abstract: Glomerella leaf spot (GLS) is an apple disease that concerns growers due to the increases in severity over the years and the difficulties in control. Species within the Colletotrichum acutatum and C. gloeosporioides species complexes cause GLS, but the proportion of species within each complex in Brazilian apple orchards is not known. The objectives of this study were to identify isolates of Colletotrichum causing GLS on apple orchards in the main Brazilian producing regions to the species level. Two hundred and seven isolates were obtained in orchards in Sao Paulo (SP), Parana (PR), Santa Catarina (SC), and Rio Grande do Sul (RS) states. Genomic DNA was extracted, and the ITS, GAPDH, CHS-1, and TUB2 genes were amplified and sequenced. The phylogenetic trees were generated using a concatenated alignment. One hundred and fourteen isolates were identified as belonging to the C. acutatum species complex (Cac) and 93 to the C. gloeosporioides species complex (Cgc). Five phylogenetic species were identified: C. melonis (1.9%), C. nymphaeae (47.4%), C. paranaense (2.4%), C. limetticola (3.4%), and C. fructicola (44.9%). In SC, Cgc predominates, but in the states of SP, PR, and RS, Cac was predominant. This is the first report of C. limetticola from apple.

Evaluation of Selected Nonfumigant Nematicides on Increasing Inoculation Densities of Meloidogyne incognita on Cucumber.

Abstract: The southern root-knot nematode (RKN), Meloidogyne incognita, is particularly difficult to manage because of high susceptibility of all commercial cucumber (Cucumis sativus) cultivars to this nematode. Growers have conventionally relied on nematicide applications to control RKN. Two microplot experiments were conducted in which four nonfumigant nematicides, oxamyl, fluopyram, fluensulfone, and fluazaindolizine, were examined for their efficacy in reducing gall severity and postharvest soil nematode numbers in microplots inoculated with increasing inoculation densities (1,000, 5,000, 10,000, and 20,000 nematodes/microplot), and improving growth and yield of cucumber. Nematicides were applied 1 day prior to transplanting cucumber seedlings, except fluensulfone, which was applied 7 days before transplanting. At harvest, root gall indices differed significantly (P < 0.0001) among nematode inoculation densities and nematicides. All four nematicides were effective in reducing the root gall index when compared with the untreated control on a consistent basis at all M. incognita inoculation densities. At the lowest inoculation density, no significant difference in gall index or final population density was observed among nematicides; however, gall index increased with increasing nematode inoculation densities in nematicide-treated microplots. Correlations between gall index and inoculation density clearly showed that soil treatment with fluensulfone, fluazaindolizine, or fluopyram was more effective in reducing gall severity than treatment with oxamyl. Regression analysis also indicated no significant effect of nematode inoculation densities on yield of cucumber treated with these nematicides. Results of this study will provide guidance for improving nematicide efficiencies in soil with varying inoculation densities of RKN.

QTLs for Resistance to Fusarium Ear Rot in a Multiparent Advanced Generation Intercross (MAGIC) Maize Population

Abstract: Alternative approaches to linkage and association mapping using inbred panels may allow further insights into loci involved in resistance to Fusarium ear rot and lead to the discovery of suitable markers for breeding programs. Here, the suitability of a maize multiparent advanced-generation intercross population for detecting quantitative trait loci (QTLs) associated with Fusarium ear rot resistance was evaluated and found to be valuable in uncovering genomic regions containing resistance-associated loci in temperate materials. In total, 13 putative minor QTLs were located over all of the chromosomes, except chromosome 5, and frequencies of favorable alleles for resistance to Fusarium ear rot were, in general, high. These findings corroborated the quantitative characteristic of resistance to Fusarium ear rot in which many loci have small additive effects. Present and previous results indicate that crucial regions such as 210 to 220 Mb in chromosome 3 and 166 to 173 Mb in chromosome 7 (B73-RefGen-v2) contain QTLs for Fusarium ear rot resistance and fumonisin content.

Characterization and Fungicide Sensitivity of Colletotrichum spp. from Different Hosts in Shandong, China.

Abstract: Anthracnose, caused by Colletotrichum species, can severely infect the fruits and leaves of more than 30 plants and thus results in great yield and quality losses. To identify the major Colletotrichum species infecting walnut fruits, strawberry leaves, grape fruits, and tea leaves in Shandong Province, China, 101 strains were collected and isolated. The morphological characteristics of all isolates were observed, and multilocus phylogenetic analyses (ITS, GAPDH, ACT, TUB2, CAL, CHS-1, and HIS3) were conducted on the representative isolates. The strains were identified as five Colletotrichum species, namely, C. gloeosporioides sensu stricto, C. fructicola, C. camelliae, C. acutatum sensu stricto, and C. viniferum. Among them, C. viniferum was reported for the first time from walnut fruits and strawberry leaves in Shandong Province, China. Corresponding leaves or fruits were used as a model to clarify the pathogenicity of these isolates. The results showed that C. fructicola obtained from strawberry leaves was more aggressive than C. viniferum. All of the isolates obtained from various hosts were highly sensitive to pyraclostrobin, difenoconazole, fludioxonil, tebuconazole, pyrisoxazole, and tetramycin in terms of mycelial growth inhibition (EC50 values of 0.07 to 1.63 mg/liter). The fastest mycelial growth was observed in the temperature range of 25-28°C for all isolates. In addition, anthracnose symptoms occur frequently under these conditions. Overall, this study can improve the understanding of Colletotrichum species causing anthracnose in walnut fruits, strawberry leaves, grape fruits, and tea leaves and can provide a solid foundation for the effective control of this disease in different hosts.

Characterization and Pathogenicity of Botryosphaeriaceae Species Obtained from Avocado Trees with Branch Canker and Dieback and from Avocado Fruit with Stem End Rot in Chile.

Abstract: Several species of the Botryosphaeriaceae family have been associated with branch canker, dieback, and stem end rot in avocado (Persea americana Mill.). In Chile, the incidence of diseases affecting the avocado tree increased from 2011 to 2016, which coincided with a severe drought that affected avocado production. Moreover, distant countries importing avocados from Chile also reported an increase of stem end rot of ripe avocados. Therefore, the aims of this study were to identify the pathogen species associated with branch canker, dieback, and stem end rot of avocado in Chile and to study their pathogenicity. This study was conducted between 2015 and 2016 in 'Hass' avocado orchards located in the main avocado-producing regions in Chile. A diverse collection of fungal species was recovered from both necrotic woody tissue and necrotic tissue on harvested ripe fruit. On the basis of morphology and phylogenetic analyses of the internal transcribed spacer region (ITS1-5.8S-ITS2) and the translation elongation factor 1-α (TEF1-α) gene, eight species in the Botryosphaeriaceae family were identified: Diplodia mutila, D. pseudoseriata, D. seriata, Dothiorella iberica, Lasiodiplodia theobromae, Neofusicoccum australe, N. nonquaesitum, and N. parvum. For each of these species, pathogenicity studies were conducted on 1-year-old healthy Hass avocado plants. All isolates produced brown gum exudate and caused necrosis in the vascular system 3 weeks after inoculation. N. nonquaesitum, N. parvum, and D. pseudoseriata were the most virulent species. Necrotic lesions and cavities with white mycelia near the peduncle union were observed on Hass avocado fruit inoculated postharvest. L. theobromae, N. australe, and N. parvum were significantly more virulent than the other tested species in the Botryosphaeriaceae family. This study identified and characterized the pathogenicity of Botryosphaeriaceae species in Chile, which will prove useful to future research on these pathogens directed at establishing effective control strategies in avocado.

Detection and Characterization of Fusarium oxysporum f. sp. vasinfectum VCG0114 (Race 4) Isolates of Diverse Geographic Origins.

Abstract: A highly virulent cotton wilt pathogen, Fusarium oxysporum f. sp. vasinfectum VCG0114 (race 4) was found in West Texas in 2017, after being known in California since 2001. Isolates obtained from wilted plants collected in 2017 from Texas, in 2015 from China, and during 2001 to 2014 from California and isolates from historical collections including the race 4 reference isolate were characterized by soil-infestation pathogenicity assays, DNA sequence analysis, and vegetative compatibility analysis. All obtained F. oxysporum f. sp. vasinfectum isolates belonged to VCG0114. All of these isolates, except one isolate from China, caused disease in a soil-infestation assay without nematodes. Thus, they belong to the nematode-independent pathotype. Texas isolates were significantly more virulent than were isolates from China or California on Gossypium barbadense 'Pima S-7'. Four different genotypes (N, T, MT, and MiT) were identified based on the transposable element Tfo1 insertion into the PHO gene and independent MULE or MITE insertions into the Tfo1 transposon. Some significant differences in virulence were detected among the genotypes in some locations. No differences in pathogenicity were observed between the California and China collection isolates on Pima S-7, and the virulence of the major genotypes was similar on the Gossypium hirsutum cultivar 'Stoneville 474' or the Barbren 713 germplasm line. Simple polymerase chain reaction (PCR) methods were developed to specifically determine and detect the four genotypes within VCG0114. A specific PCR method to detect all VCG0114 isolates was also developed. These methods will facilitate the timely identification of infested fields and seed lots and the elucidation of evolutionary relationships among the isolates. This should help to closely monitor the movement of the pathogen and reduce dissemination of these devastating pathogens.

Evaluating Azoxystrobin Seed Coating Against Maize Late Wilt Disease Using a Sensitive qPCR-Based Method

Abstract: Harpophora maydis, a phytopathogenic fungus, causes late wilt, a severe vascular maize disease characterized by relatively rapid wilting of maize plants near fertilization The disease is currently controlled using resistant varieties Here, we evaluated seed coating efficiency with azoxystrobin against H maydis in a series of in vitro and in vivo trials A real-time polymerase chain reaction (qPCR)-based method was developed and proved to be a sensitive, accurate tool for monitoring H maydis DNA inside infected seeds, sprouts, and tissues of mature plants In the early growth stages, the chemical coating drastically reduced the pathogen DNA prevalence in host tissues and minimized the suppressing effect on the plants' biomass and development In an infested field, the qPCR assay identified the pathogen 20 days after seeding, up to a month before conventional PCR detection In the resistant fodder maize cultivar 32D99, which showed only minor disease symptoms, the seed coating blocked fungal progression and increased cob and plant weight by 39 and 60%, respectively Nevertheless, this treatment was unable to protect a sensitive maize hybrid, cultivar Prelude, at the disease wilting breakout (60 days after sowing) These results encourage further examination of azoxystrobin and other fungicides in the field using the qPCR detection method to evaluate their efficiency

Virulence of Puccinia triticina, the Wheat Leaf Rust Fungus, in the United States in 2017

Abstract: Samples of wheat leaves infected with the leaf rust fungus, Puccinia triticina, were obtained in 2017 from agricultural experiment station plots, demonstration plots, and farm fields in the Great Plains, the Ohio Valley, the southeastern states, California, and Washington in order to determine the prevalent virulence phenotypes present in the United States. A total of 65 virulence phenotypes were identified among the 469 single uredinial isolates that were tested on 20 near-isogenic lines of Thatcher wheat that differ for leaf rust resistance genes. Virulence phenotypes MBTNB at 11.3% of the overall population, and MCTNB at 7.0%, were the first and third most common phenotypes. Both phenotypes were found mostly in the southeastern states and Ohio Valley region. Phenotype TFTSB at 10.9% was the second most common phenotype and was found mostly in southern Texas. Virulence to leaf rust resistance gene Lr39, which is present in hard red winter wheat cultivars, was highest in the Great Plains region. Virulence to Lr11 and Lr18, which are present in soft red winter wheat cultivars, was highest in the southeastern states and Ohio Valley region. Virulence to Lr21, which is present in hard red spring wheat cultivars, was highest in the northern Great Plains region. The predominate P. triticina phenotypes from the soft red winter wheat regions of the southeastern states and Ohio Valley area differed from those in the hard red winter and hard red spring wheat areas of the Great Plains region. Collections from Washington had unique virulence phenotypes that had not been previously detected.

Loop-Mediated Isothermal Amplification for the Diagnostic Detection of Meloidogyne chitwoodi and M. fallax.

Abstract: Meloidogyne chitwoodi is a root-knot nematode that parasitizes a broad range of plants. In the Pacific Northwest (PNW) of the United States, M. chitwoodi is a major potato pest. The nematodes infec...

Characterization of Ti Ringspot-Associated Virus, a Novel Emaravirus Associated with an Emerging Ringspot Disease of Cordyline fruticosa.

Abstract: Ti ringspot is an emerging foliar disease of the ti plant (Cordyline fruticosa) in Hawaii that is quickly spreading throughout the islands. Symptoms include small chlorotic ringspots on leaves that...

Spinach Downy Mildew: Advances in Our Understanding of the Disease Cycle and Prospects for Disease Management.

Abstract: Downy mildew on spinach is caused by Peronospora effusa, an oomycete pathogen that poses a challenge to spinach production worldwide, especially in organic production. Following infection, P. effus...

First report of cotton leafroll dwarf virus associated with cotton blue disease symptoms in Alabama

Abstract: During 2017, virus-like symptoms were observed in cotton fields in six counties in coastal Alabama infested by the whitefly Bemisia tabaci (Genn.) and cotton aphid Aphis gossypii (Glover). Symptoms consisted of foliar distortion, curling, rolling, bluish-green discoloration, vein-clearing, shortened internodes that resulted in dwarfing of plants, and reduced boll set. Disease incidence ranged from 3 to 30% per field, resulting in approximately 560 kg/ha loss, and affecting 25% of the 50,585-ha cotton crop in Alabama, with a loss of approximately 50,000 bales valued at $19 million dollars. Leaves were collected from symptomatic cotton plants, Gossypium hirsutum L., in Barbour County, AL, during late August and mid-September 2017. Total DNA and RNA were purified and subjected to DNA and RNAseq Illumina Hi-Seq 2500 (paired-end reads, 150-bp) shotgun sequencing. The de novo-assembled DNA contigs yielded no match to viral sequences available in the GenBank database. However, one RNA de novo assembled contig (103 reads) of 1,143 bp (accession no. MK015625), located at nucleotide coordinates 3,830 to 4,972, was annotated as Cotton leafroll dwarf virus (CLRDV) “atypical isolate” from Argentina (KF359947), based on 97.1% similarity and 98% coverage (BLASTn, GenBank). The CLRDV belongs to the genus Polerovirus (family Luteoviridae), which are aphid-transmitted, positive-sense single-stranded RNA viruses with a genome size of approximately 5.8 kb (Distefano et al. 2010). To verify the Illumina results, the polymerase chain reaction (PCR) primer pair AL674F-5′-CCGTAGCGGTCATCGTCTTT/AL1407R-5′-TAACCCTGACACAGTGGGGA was designed based on a region conserved in all six available CLRDV genome sequences (GenBank accession nos. GU167940, HQ827780, KF359946, KF359947, KF906260, and KF906261) and was used in parallel with the primers CLRDV3675F and Pol3982R (Sharman et al. 2015) for reverse transcription PCR amplification of total RNA purified from symptomatic cotton plants (n = 3). An amplicon of the expected size was obtained using the primers, AL674F/1407R and CLRDV3675F/Pol3982R, at 733 bp (accession no. MH883237) and 310 bp (accession no. MH883236) in size, respectively. Bidirectional Sanger sequencing confirmed CLRDV presence, based on the BLASTn score of 94 and 99% similarity for the AL674F/1407R and CLRDV3675F/Pol3982R amplicon sequence, having a closest match to CLRDV from South America (GU167940 and KF906261, respectively). Pairwise distance analysis (Muhire et al. 2014) of the CLRDV fragments from Alabama cotton plants with the analogous genome fragment from the six available CLRDV isolates from South America indicated 91.5 to 99.3% nt sequence identity, suggesting the U.S. isolate is closely related, but not identical, to previously reported CLRDV isolates. The CLRDV, endemic to Africa and/or Asia-Pacific (Ray et al. 2016), was introduced into Brazil and Argentina during 2005 and 2010, respectively (Correa et al. 2005; Distefano et al. 2010). Although A. gossypii is the only known CLRDV vector, and it was prevalent in symptomatic cotton fields during 2017, aphid transmissibility of the Alabama isolate has not been verified. Whether the CLRDV becomes established and causes economic losses to the U.S. cotton crop remains to be determined. To our knowledge, this is the first detection of the exotic CLRDV in U.S. cotton plantings.

Comprehensive Disease Survey of U.S. Sunflower: Disease Trends, Research Priorities and Unanticipated Impacts.

Abstract: Between 2002 and 2015, a comprehensive survey of sunflower fields across seven Midwestern U.S. states was conducted 12 times and continues to be conducted every other year. The surveyors collected data on yield, agronomic management factors, disease, insect, weed, and bird damage. All surveyors were volunteers and came from universities (extension and research staff), USDA-ARS, and seed and chemical companies. In the 12 years the survey was conducted, data from 2,267 fields were collected. The results are presented annually at the National Sunflower Association Research Forum and are used to set sunflower research priorities. While 10 diseases are surveyed annually, we focus this article on the importance, findings, implications, and impacts of the five most important: downy mildew, Phomopsis stem canker, rust, Rhizopus head rot, and Sclerotinia head rot. This survey is unique among field crops in both scope and scale, and this manuscript discusses salient and clandestine benefits of intense and long-term disease surveys.

Integrated Effects of Genetic Resistance and Prothioconazole + Tebuconazole Application Timing on Fusarium Head Blight in Wheat.

Abstract: Integrated Fusarium head blight (FHB) management programs consisting of different combinations of cultivar resistance class and an application of the fungicide prothioconazole + tebuconazole at or after 50% early anthesis were evaluated for efficacy against FHB incidence (INC; percentage of diseased spikes), index (IND; percentage of diseased spikelets per spike), Fusarium damaged kernel (FDK), deoxynivalenol (DON) toxin contamination, grain yield, and test weight (TW) in inoculated field trials conducted in 11 U.S. states in 2014 and 2015. Mean log response ratios and corresponding percent control values for INC, IND, FDK, and DON, and mean differences in yield and TW relative to a nontreated, inoculated susceptible check (S_CK), were estimated through network meta-analyses as measures of efficacy. Results from the analyses were then used to estimate the economic benefit of each management program for a range of grain prices and fungicide applications costs. Management programs consisting of a moderately resistant (MR) cultivar treated with the fungicide were the most efficacious, reducing INC by 60 to 69%, IND by 71 to 76%, FDK by 66 to 72%, and DON by 60 to 64% relative to S_CK, compared with 56 to 62% for INC, 68 to 72% for IND, 66 to 68% for FDK, and 58 to 61% for DON for programs with a moderately susceptible (MS) cultivar. The least efficacious programs were those with a fungicide application to a susceptible (S) cultivar, with less than a 45% reduction of INC, IND, FDK, or DON. All programs were more efficacious under conditions favorable for FHB compared with less favorable conditions, with applications made at 50% early anthesis being of comparable efficacy to those made 2 to 7 days later. Programs with an MS cultivar resulted in the highest mean yield increases relative to S_CK (541 to 753 kg/ha), followed by programs with an S cultivar (386 to 498 kg/ha) and programs with an MR cultivar (250 to 337 kg/ha). Integrated management programs with an MS or MR cultivar treated with the fungicide at or after 50% early anthesis were the most likely to result in a 50 or 75% control of IND, FDK, or DON in a future trial. At a fixed fungicide application cost, these programs were $4 to $319/MT more economically beneficial than corresponding fungicide-only programs, depending on the cultivar and grain price. These findings demonstrate the benefits of combining genetic resistance with a prothioconazole + tebuconazole treatment to manage FHB, even if that treatment is applied a few days after 50% early anthesis.

Occurrence and Molecular Variability of Kiwifruit Viruses in Actinidia deliciosa 'Xuxiang' in the Shaanxi Province of China.

Abstract: Kiwifruit (Actinidia spp.) is an economically substantial fruit crop with China the main producer. China is the primary source of wild kiwifruit and the largest producer of kiwifruit in terms of both production and planting area, and Shaanxi province is the largest kiwifruit producer in China. Previous studies reported presence of kiwifruit viruses in Actinidia chinensis. In this study, six viruses were identified in kiwifruit 'Xuxiang' (A. deliciosa) in Shaanxi, China. The incidence, distribution, and genetic diversity of these viruses were studied. The results showed that Actinidia virus A (AcVA), Actinidia virus B (AcVB), Actinidia chlorotic ringspot-associated virus (AcCRaV), cucumber mosaic virus (CMV), apple stem grooving virus (ASGV), and potato virus X (PVX) were the main viruses infecting Xuxiang kiwifruit in Shaanxi, China. Incidence of the various viruses with both single and multiple infection varied with different kiwifruit-growing counties. For single virus infection, the highest and the lowest numbers of samples infected were about 22 for AcCRaV and 0 for AcVB in Meixian out of 170 samples, 12 for AcVA and 0 for CMV in Zhouzhi out of 120 samples, 10 for AcVA and 0 for AcVB, AcCRaV, ASGV, PVX, and CMV in Yangling out of 70 samples, and 8 for AcCRaV and CMV and 0 for AcVA, AcVB, ASGV, and PVX in Hanzhong out of 80 samples, respectively. Samples which were multiply infected with two or more viruses were also detected. Analysis of the phylogenetic tree of these viruses showed some genetic variability in the AcVA, AcVB, and AcCRaV isolates of Shaanxi kiwifruit. There was no obvious molecular variation in the coat protein genes of ASGV, CMV, and PVX virus isolates from Shaanxi kiwifruit. The present study is the first large-scale survey of kiwifruit viruses in Shaanxi, China. To our knowledge, this is the first report of PVX infecting kiwifruit and the first report of molecular variability of AcVA, AcVB, and AcCRaV. These results provide important data for studying the genetic evolution of AcVA, AcVB, AcCRaV, ASGV, CMV, and PVX.

Phytophthora Species Are Common on Nursery Stock Grown for Restoration and Revegetation Purposes in California.

Abstract: Phytophthora tentaculata was detected for the first time in North America in 2012 in a nursery on sticky monkeyflower plant (Diplacus aurantiacus) and again in 2014 on outplanted native plants At that time, this species was listed as a federally actionable and reportable pathogen by the USDA As a result of these detections, California native plant nurseries were surveyed to determine the prevalence of Phytophthora species on native plant nursery stock A total of 402 samples were collected from 26 different native plant nurseries in California between 2014 and 2016 Sampling focused on plants with symptoms of root and crown rot Symptomatic tissue was collected and tested by immunoassay, culture, and molecular techniques (PCR) Identifications were made using sequences from the internal transcribed spacer (ITS) rDNA region, a portion of the trnM-trnP-trnM, or the atp9-nad9 mitochondrial regions Phytophthora was confirmed from 149 of the 402 samples (37%), and from plants in 22 different host families P tentaculata was the most frequently detected species in our survey, followed by P cactorum and members of the P cryptogea complex Other species include P cambivora, P cinnamomi, P citricola, P hedraiandra, P megasperma, P multivora, P nicotianae, P niederhauserii, P parvispora, P pini, P plurivora, and P riparia A few Phytophthora sequences generated from mitochondrial regions could not be assigned to a species Although this survey was limited to a relatively small number of California native plant nurseries, Phytophthora species were detected from three quarters of them (77%) In addition to sticky monkeyflower, P tentaculata was detected from seven other hosts, expanding the number of associated hosts During this survey, P parvispora was detected for the first time in North America from symptomatic crowns and roots of the nonnative Mexican orange blossom (Choisya ternata) Pathogenicity of P parvispora and P nicotianae was confirmed on this host These findings document the widespread occurrence of Phytophthora spp in native plant nurseries and highlight the potential risks associated with outplanting infested nursery-grown stock into residential gardens and wildlands

QTL Mapping of Adult-Plant Resistance to Leaf and Stripe Rust in Wheat Cross SW 8588/Thatcher using the Wheat 55K SNP Array.

Abstract: Wheat leaf rust (caused by Puccinia triticina) and stripe rust (caused by Puccinia striiformis f. sp. tritici) cause large production losses in many regions of the world. The objective of this study was to identify quantitative trait loci (QTL) for resistance to leaf rust and stripe rust in a recombinant inbred line population derived from a cross between wheat cultivars SW 8588 and Thatcher. The population and parents were genotyped with the Wheat 55K SNP Array and SSR markers and phenotyped for leaf rust severity at Zhoukou in Henan Province and Baoding in Hebei Province. Stripe rust responses were also evaluated at Chengdu in Sichuan Province, and at Baoding. Seven and six QTL were detected for resistance to leaf rust and stripe rust, respectively. Four QTL on chromosomes 1BL, 2AS, 5AL, and 7BL conferred resistance to both rusts. The QTL on 1BL and 2AS were identified as Lr46/Yr29 and Lr37/Yr17, respectively. QLr.hebau-2DS from Thatcher, identified as Lr22b that was previously thought to be ineffective in China, contributed a large effect for leaf rust resistance. QLr.hebau-5AL/QYr.hebau-5AL, QLr.hebau-3BL, QLr.hebau-6DS, QYr.hebau-4BS, and QYr.hebau-6DS are likely to be new QTL, but require further validation. Kompetitive allele-specific PCR (KASP) markers for QLr.hebau-2DS and QLr.hebau-5AL/QYr.hebau-5AL were successfully developed and validated in a diverse wheat panel from Sichuan Province, indicating their usefulness under different genetic backgrounds. These QTL and their closely linked SNP and SSR markers will be useful for fine mapping, candidate gene discovery, and marker-assisted selection in breeding for durable resistance to both leaf and stripe rusts.

Comparative Evaluation of LAMP, qPCR, Conventional PCR, and ELISA to Detect Ralstonia solanacearum in Kenyan Potato Fields.

Abstract: Bacterial wilt caused by Ralstonia solanacearum is considered among the most damaging diseases of potato in Sub-Saharan Africa and the most significant biotic constraint of potato production alongside late blight. Unlike late blight, which can be managed by chemical means, R. solanacearum can only be managed through cultural methods and clean seed. Laboratory testing to certify seed before planting is required to confirm the absence of the pathogen in Kenya. A loop-mediated isothermal amplification (LAMP) assay was developed using the UDP-(3-O-acyl)-N-acetylglucosamine deacetylase gene (IpxC) to screen seed potato for R. solanacearum strains. The assay was assessed using DNA extracted from R. solanacearum and other soil and potato pathogens to demonstrate specificity and sensitivity. The LAMP assay was validated using field samples from different potato growing regions of Kenya collected over two growing seasons and compared with established nucleic acid and protein-based assays. The IpxC LAMP assay was found to be specific and sensitive to R. solanacearum, detecting as low as 2.5 pg/µl of R. solanacearum DNA. Of the 47 potentially infected field samples collected, both IpxC LAMP and quantitative polymerase chain reaction (PCR) detected R. solanacearum DNA in 90% of the samples, followed by conventional PCR (86%) and ELISA (75%). This IpxC LAMP assay is a promising diagnostic tool to rapidly screen for R. solanacearum in seed potato with high sensitivity in Kenya. Copyright © 2019 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .

Polyphenols from Rheum Roots Inhibit Growth of Fungal and Oomycete Phytopathogens and Induce Plant Disease Resistance

Abstract: A growing world population requires an increase in the quality and quantity of food production. However, field losses due to biotic stresses are currently estimated to be between 10 and 20% worldwide. The risk of resistance and strict pesticide legislation necessitate innovative agronomical practices to adequately protect crops in the future, such as the identification of new substances with novel modes of action. In the present study, liquid chromatography mass spectrometry was used to characterize Rheum rhabarbarum root extracts that were primarily composed of the stilbenes rhaponticin, desoxyrhaponticin, and resveratrol. Minor components were the flavonoids catechin, epicatechin gallate, and procyanidin B1. Specific polyphenolic mixtures inhibited mycelial growth of several phytopathogenic fungi and oomycetes. Foliar spray applications with fractions containing stilbenes and flavonoids inhibited spore germination of powdery mildew in Hordeum vulgare with indications of synergistic interactions. Formulated extracts led to a significant reduction in the incidence of brown rust in Triticum aestivum under field conditions. Arabidopsis thaliana mutant and quantitative reverse-transcription polymerase chain reaction studies suggested that the stilbenes induce salicylic acid-mediated resistance. Thus, the identified substances of Rheum roots represent an excellent source of antifungal agents that can be used in horticulture and agriculture.