Genome Sequencing and Transcriptome Analysis of the Hop Downy Mildew Pathogen Pseudoperonospora humuli Reveal Species-Specific Genes for Molecular Detection.

Pseudoperonospora humuli is an obligate oomycete pathogen of hop (Humulus lupulus) that causes downy mildew, an important disease in most production regions in the Northern Hemisphere. The pathogen can cause a systemic infection in hop, overwinter in the root system, and infect propagation material. Substantial yield loss may occur owing to P. humuli infection of strobiles (seed cones), shoots, and cone-bearing branches. Fungicide application and cultural practices are the primary methods to manage hop downy mildew. However, effective, sustainable, and cost-effective management of downy mildew can be improved by developing early detection systems to inform on disease risk and timely fungicide application. However, no species-specific diagnostic assays or genomic resources are available for P. humuli. The genome of the P. humuli OR502AA isolate was partially sequenced using Illumina technology and assembled with ABySS. The assembly had a minimum scaffold length of 500 bp and an N50 (median scaffold length of the assembled genome) of 19.2 kbp. A total number of 18,656 genes were identified using MAKER standard gene predictions. Additionally, transcriptome assemblies were generated using RNA-seq and Trinity for seven additional P. humuli isolates. Bioinformatics analyses of next generation sequencing reads of P. humuli and P. cubensis (a closely related sister species) identified 242 candidate species-specific P. humuli genes that could be used as diagnostic molecular markers. These candidate genes were validated using polymerase chain reaction against a diverse collection of isolates from P. humuli, P. cubensis, and other oomycetes. Overall, four diagnostic markers were found to be uniquely present in P. humuli. These candidate markers identified through comparative genomics can be used for pathogen diagnostics in propagation material, such as rhizomes and vegetative cuttings, or adapted for biosurveillance of airborne sporangia, an important source of inoculum in hop downy mildew epidemics.

Powdery Mildew Outbreaks caused by Podosphaera macularis on Hop Cultivars Possessing the Resistance Gene R6 in the Pacific Northwestern United States.

Resistant cultivars of hop (Humulus lupulus) have been grown, with the aim of helping to manage powdery mildew in the Pacific Northwest since the first report of the disease in the field in 1997 (4). A major objective of many breeding programs is development of resistance to powdery mildew, and this has generally been achieved by single resistance genes (qualitative resistance). One such gene, R6 (3), has been utilized extensively in new cultivars and has prevented epidemics of the disease in those cultivars across the Pacific Northwestern United States for approximately 15 years. In 2011, a grower in Washington State reported outbreaks of powdery mildew on cv. Apollo, which is thought to possess powdery mildew resistance derived from R6. Fungicides and cultural control measures were applied, and the grower reported no substantial crop damage from the disease. During the winter of 2012, the same grower planted rhizomes of cv. Apollo in a greenhouse in the Yakima Valley of Washington State and later found the plants to be affected by powdery mildew. Affected leaves from plants of cvs. Apollo, Newport, and Nugget (all reported [3] or assumed to possess R6 based on pedigree) grown in the same greenhouse were later provided to the authors. Conidia obtained from each affected plants were transferred to plants of the highly susceptible cv. Symphony, which is not known to contain any resistance genes. After 10 to 14 days of incubation, resultant conidia from each cultivar above (total of three isolates) were transferred to greenhouse grown plants of cvs. Nugget and Symphony and incubated at 18°C. Within 7 days, all three isolates produced powdery mildew colonies characteristic of P. macularis (2) on both cultivars. Cleistothecia did not develop in any colonies. In addition, Nugget and Symphony plants were inoculated with a field population of P. macularis originating from cultivars lacking R6 in Oregon. These inoculations on Nugget did not develop powdery mildew whereas Symphony plants did. Non-inoculated controls remained free of powdery mildew. Results were identical in two additional experiments. The sequence of the mating type idiomorph, MAT1-1, was obtained to confirm identity of the pathogen as P. macularis as described previously (1). The sequences were identical among the three isolates obtained from the greenhouse in Washington and isolates of P. macularis obtained previously from Oregon and Washington. MAT1-2 idiomorph was not detected in the isolates collected. While R6-virulent strains have been detected previously in race characterization experiments, these strains have not caused widespread epidemics of powdery mildew. The increasing prevalence of virulent strains of P. macularis and outbreaks of powdery mildew on formerly resistant cultivars necessitates changes in breeding strategies and disease management efforts to minimize damage resulting from the disease. The distribution of virulent strains of the pathogen and susceptibility of formerly resistance cultivars to powdery mildew are currently under investigation. References: (1) B. Asalfet et al. Phytopathology 103:717, 2013. (2) R. Bélanger et al. The Powdery Mildews: a Comprehensive Treatise. APS Press, St. Paul, MN, 2002. (3) P. Darby. Brew Hist. 121:94, 2005. (4) C. Ocamb et al. Plant Dis. 83:1072, 1999.

Suppression of hop looper (Lepidoptera: Noctuidae) by the fungicide pyraclostrobin.

The hop looper, Hypena humuli Harris, is a reemergent pest of hop that often requires treatment to mitigate crop damage. In 4 yr of field trials, plots treated with fungicides were observed to sustain less hop looper defoliation compared with nontreated plots. Further investigation revealed that abundance of hop looper and associated defoliation were reduced when the fungicide pyraclostrobin was applied in late July to early August. Two other fungicides possessing active ingredients in the same chemical family (quinone outside inhibitor) did not reduce abundance of hop looper or its defoliation. Pyraclostrobin is efficacious against powdery mildew diseases, and the application timing evaluated in these studies corresponds with a period of juvenile susceptibility of hop cones to the disease. Use of fungicides containing pyraclostrobin at this time may have the ancillary benefit of reducing hop looper damage, potentially obviating the need for broad-spectrum insecticides later in the season. Follow-up studies are warranted to determine whether pyraclostrobin may inhibit other lepidopteran species.

Development of biological control of Tetranychus urticae (Acari: Tetranychidae) and Phorodon humuli (Hemiptera: Aphididae) in Oregon hop yards.

The temporal development of biological control of arthropod pests in perennial cropping systems is largely unreported. In this study, the development of biological control of twospotted spider mite, Tetranychus urticae Koch, and hop aphid, Phorodon humuli (Schrank), in a new planting of hop in Oregon is described over a period of 9 yr (2005-2013). Both the abundance and diversity of natural enemies increased over time. Known predators of hop aphid (Coccinellidae and Anthocoridae) were present in all years; however, stable biological control of hop aphid was not achieved in most years and aphicides were required to suppress populations at commercially acceptable levels in 5 of 9 yr. Populations of aphidophagous coccinellids developed synchronously with hop aphid populations, and temporal correlations indicated these are the primary predatory insect associated with hop aphid regulation. However, sampling methods did not assess levels of aphid parasitoids and hyperparasitoids and their contribution to biological control was unquantified. Spider mite biological control was associated primarily with predatory mites (Phytoseiidae) and Stethorus spp. (Coccinellidae). The magnitude of temporal correlations of abundance of these predators with spider mites was found to be greatest on the same sampling dates and at lags of 7-14 d. Stable biological control of spider mites occurred after four field seasons, suppressing spider mites to levels similar to those commonly achieved with chemical control. A survey of 11 commercial hop yards in Oregon documented pest and natural enemy densities under commercial management practices over a period of 4 yr (2008-2011). Natural enemy abundance in commercial hop yards was similar to that of a 2- to 3-yr-old hop yard with limited disturbance. Whereas total reliance on biological control for hop aphid is unlikely to be successful, there appears to be unrealized potential for biological control of spider mites in commercial production. Dynamic action thresholds that consider the value of natural enemies are needed for both pests.

Sensitivity of Disease Management Decision Aids to Temperature Input Errors Associated with Sampling Interval and Out-of-Canopy Sensor Placement.

Many plant disease epidemic models, and the disease management decision aids developed from them, are created based on temperature or other weather conditions measured in or above the crop canopy at intervals of 15 or 30 min. Disease management decision aids, however, commonly are implemented based on hourly weather measurements made from sensors sited at a standard placement of 1.5 m above the ground or are estimated from off-site weather measurements. We investigated temperature measurement errors introduced when sampling interval was increased from 15 to 60 min, and when actual in-canopy conditions were represented by temperature measurements collected by standard-placement sensors (1.5 m above the ground, outside the canopy) in each of three crops (grass seed, grape, and hops) and assessed the impact of these errors on outcomes of decision aids for grass stem rust as well as grape and hops powdery mildews. Decreasing time resolution from 15 to 60 min resulted in statistically significant underestimates of daily maximum temperatures and overestimates of daily minimum temperatures that averaged 0.2 to 0.4°C. Sensor location (in-canopy versus standard-placement) also had a statistically significant effect on measured temperature, and this effect was significantly less in grape or hops than in the grass seed crop. Effects of these temperature errors on performance of disease management decision aids were affected by magnitude of the errors as well as the type of decision aid. The grape and hops powdery mildew decision aids used rule-based indices, and the relatively small (±0.8°C) differences in temperature observed between in-canopy and standard placement sensors in these crops resulted in differences in rule outcomes when actual in-canopy temperatures were near a threshold for declaring that a rule had been met. However, there were only minor differences in the management decision (i.e., fungicide application interval). The decision aid for grass stem rust was a simulation model, for which temperature recording errors associated with location of the weather station resulted in incremental (not threshold) effects on the model of pathogen growth and plant infection probability. Simple algorithms were devised to correct the recorded temperatures or the computed infection probability to produce outcomes similar to those resulting from in-canopy temperature measurements. This study illustrates an example of evaluating (and, if necessary, correcting) temperature measurement errors from weather station sensors not located within the crop canopy, and provides an estimate of uncertainty in temperature measurements associated with location and sampling interval of weather station sensors.

Sclerotinia Wilt of Hop (Humulus lupulus) Caused by Sclerotinia sclerotiorum in the Pacific Northwest United States.

In June 2009, wilted hop bines were observed in a yard in Marion County, OR. The wilt was associated with a stem rot that occurred ~1 m from the ground near the point where bines are tied together for horticultural purposes. Samples of affected stems were submitted to the Oregon State University Plant Clinic. White hyphae and large, black sclerotia were present on the stems, with a clear delineation between healthy and diseased tissue. The pathogen was identified as Sclerotinia sclerotiorum based on morphological characters. In June 2011, bine wilting was observed on the same farm but in a different hop yard (cv. Nugget) ~10 km from the 2009 occurrence. Affected plants had upward curled leaves with necrotic margins or wilted bines that were severed at the soil line. Wilted bines tended to have smaller diameters than bines with foliar symptoms only. Of 100 plants examined, 75% displayed some foliar symptoms and 66% had at least one bine that was wilted. Yield loss was estimated at 10 to 20% due to bine wilting before cone development. Unlike the 2009 occurrence, wilted bines did not display aerial signs of S. sclerotiorum. Rather, water-soaked lesions covered in white, cottony mycelium were apparent on affected stems 2.5 to 5 cm below the soil surface, some bearing large, irregularly shaped sclerotia. Isolations made onto potato dextrose agar yielded isolates with rapid growth rates and morphological characters consistent with S. sclerotiorum (1). DNA was extracted (2) and pathogen identity was confirmed by PCR amplification and sequencing of the internal transcribed spacer regions from isolates SS001 and SS002 as described before (4). The amplicons were sequenced bidirectionally and consensus sequences were 100% similar to S. sclerotiorum (GenBank No. AAGT01000678.1). Two nucleotide polymorphisms were present that differentiated the sequences from those of 12 S. trifoliorum accessions in GenBank that could be aligned (2). Greenhouse assays utilizing a toothpick inoculation procedure (3) were conducted to fulfill Koch's postulates. Stems of five 4-week-old hop plants of cv. Agate were pierced with a toothpick colonized with S. sclerotiorum. Five control plants were similarly inoculated with toothpicks without the fungus. Inoculated plants developed symptoms similar to those observed in the field within 11 days; four of five plants inoculated with isolate SS001 and two of five plants inoculated with isolate SS002 completely wilted. S. sclerotiorum was reisolated from all inoculated plants but not the control plants. To our knowledge, this is the first report of Sclerotinia wilt on hop in Oregon or the Pacific Northwest (1), where nearly all commercial hop production occurs in the United States. The disease appears to be localized to a limited number of yards, although given the widespread distribution and host range of S. sclerotiorum, it is plausible that the disease may occur in other yards. Recurrent outbreaks and spread of the disease among yards on the affected farm suggests that Sclerotinia wilt has the potential to become a perennial problem on hop and efforts to limit the introduction of S. sclerotiorum into other yards are warranted. References: (1) D. H. Gent. Page 32 in: Compendium of Hop Diseases and Pests. The American Phytopathological Society, St. Paul, MN, 2009. (2) E. N. Njambere et al. Plant Dis. 92:917, 2008. (3) M. L. Putnam. Plant Pathol. 53:252, 2004. (4) T. J. White et al. PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, 1990.

Decision aids for multiple-decision disease management as affected by weather input errors.

Many disease management decision support systems (DSSs) rely, exclusively or in part, on weather inputs to calculate an indicator for disease hazard. Error in the weather inputs, typically due to forecasting, interpolation, or estimation from off-site sources, may affect model calculations and management decision recommendations. The extent to which errors in weather inputs affect the quality of the final management outcome depends on a number of aspects of the disease management context, including whether management consists of a single dichotomous decision, or of a multi-decision process extending over the cropping season(s). Decision aids for multi-decision disease management typically are based on simple or complex algorithms of weather data which may be accumulated over several days or weeks. It is difficult to quantify accuracy of multi-decision DSSs due to temporally overlapping disease events, existence of more than one solution to optimizing the outcome, opportunities to take later recourse to modify earlier decisions, and the ongoing, complex decision process in which the DSS is only one component. One approach to assessing importance of weather input errors is to conduct an error analysis in which the DSS outcome from high-quality weather data is compared with that from weather data with various levels of bias and/or variance from the original data. We illustrate this analytical approach for two types of DSS, an infection risk index for hop powdery mildew and a simulation model for grass stem rust. Further exploration of analysis methods is needed to address problems associated with assessing uncertainty in multi-decision DSSs.

Comments regarding the binary power law for heterogeneity of disease incidence.

The binary power law (BPL) has been successfully used to characterize heterogeneity (overdispersion or small-scale aggregation) of disease incidence for many plant pathosystems. With the BPL, the log of the observed variance is a linear function of the log of the theoretical variance for a binomial distribution over the range of incidence values, and the estimated scale (?) and slope (b) parameters provide information on the characteristics of aggregation. When b = 1, the interpretation is that the degree of aggregation remains constant over the range of incidence values observed; otherwise, aggregation is variable. In two articles published in this journal in 2009, Gosme and Lucas used their stochastic simulation model, Cascade, to show a multiphasic (split-line) relationship of the variances, with straight-line (linear) relationships on a log-log scale within each phase. In particular, they showed a strong break point in the lines at very low incidence, with b considerably >1 in the first line segment (corresponding to a range of incidence values usually not observed in the field), and b being ?1 in the next segment (corresponding to the range of incidence values usually observed). We evaluated their findings by utilizing a general spatially explicit stochastic simulator developed by Xu and Ridout in 1998, with a wide range of median dispersal distances for the contact distribution and number of plants in the sampling units (quadrats), and through an assessment of published BPL results. The simulation results showed that the split-line phenomenon can occur, with a break point at incidence values of ?0.01; however, the split is most obvious for short median dispersal distances and large quadrat sizes. However, values of b in the second phase were almost always >1, and only approached 1 with extremely short median dispersal distances and small quadrat sizes. An appraisal of published results showed no evidence of multiple phases (although the minimum incidence may generally be too high to observe the break), and estimates of b were almost always >1. Thus, it appears that the results from the Cascade simulation model represent a special epidemiological case, corresponding primarily to a roughly nearest-neighbor population-dynamic process. Implications of a multiphasic BPL property may be important and are discussed.

Evaluation of airborne methyl salicylate for improved conservation biological control of two-spotted spider mite and hop aphid in Oregon hop yards.

The use of synthetic herbivore-induced plant volatiles (HIPV) to attract natural enemies has received interest as a tool to enhance conservation biological control (CBC). Methyl salicylate (MeSA) is a HIPV that is attractive to several key predators of two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), and hop aphid, Phorodon humuli (Schrank) (Homoptera: Aphididae). A 2-year study was conducted to evaluate the recommended commercial use of MeSA in hop yards in Oregon. Slow-release MeSA dispensers were stapled to supporting poles in 0.5 ha plots and these plots were compared to a paired non-treated plot on each of three farms in 2008 and 2009. Across both years, there was a trend for reduced (range 40-91%) mean seasonal numbers of T. urticae in five of the six MeSA-baited plots. Stethorus spp., key spider mite predators, tended to be more numerous in MeSA-baited plots compared to control plots on a given farm. Mean seasonal densities of hop aphid and other natural enemies (e.g., Orius spp. and Anystis spp.) were similar between MeSA-treated and control plots. Variability among farms in suppression of two-spotted spider mites and attraction of Stethorus spp. suggests that the use of MeSA to enhance CBC of spider mites in commercial hop yards may be influenced by site-specific factors related to the agroecology of individual farms or seasonal effects that require further investigation. The current study also suggests that CBC of hop aphid with MeSA in this environment may be unsatisfactory.

Effects of powdery mildew fungicide programs on twospotted spider mite (Acari: Tetranychidae), hop aphid (Hemiptera: Aphididae), and their natural enemies in hop yards.

Twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), and hop aphid, Phorodon humuli (Schrank) (Hemiptera: Aphididae), are the most important arthropod pests of hop (Humulus lupulus L.) in the Northern Hemisphere. A potential barrier for greater adoption of conservation biological control strategies for spider mites and hop aphid is the extensive use of fungicides for management of hop powdery mildew, Podosphaera macularis (Wallr.:Fr.) U. Braun & S. Takamatsu. Field studies conducted in experimental plots in Oregon and Washington in 2005 and 2006 quantified the effects of powdery mildew fungicide programs (i.e., sulfur, paraffinic oil, and synthetic fungicides) on arthropod pests and natural enemies on hop. Fungicide treatment significantly affected spider mite populations in all four studies. Multiple applications of sulfur fungicides applied before burr development resulted in 1.4-3.3-fold greater spider mite populations during summer. Near the cessation of the sulfur applications, or after a lag of 20-30 d, spider mite populations increased significantly faster on sulfur treated plants compared with water-treated plants in three of four experiments. The effect of paraffinic oil on spider mites was varied, leading to exacerbation of spider mites in Oregon and Washington in 2005, suppression of mites in Oregon in 2006, and no significant effect compared with water in Washington in 2006. Significant relative treatment effects for cone damage due to spider mite feeding were detected in Oregon in 2005 in plots treated with sulfur and paraffinic oil compared with water and synthetic fungicides. Mean populations of hop aphids were similar among treatments in Oregon, although sulfur treatment suppressed hop aphid populations in Washington in 2005 and 2006. Populations of individual predacious insect species and cumulative abundance of macropredators were not consistently suppressed or stimulated by treatments in all trials. However, predatory mite abundance in Washington was affected by fungicide treatments, with plots treated with sulfur consistently having 10-fold fewer phytoseiids per leaf compared with the other treatments. Based on the results of these studies, powdery mildew fungicide programs that minimize or eliminate applications of sulfur and paraffinic oil would tend to conserve predatory mites and minimize the severity of spider mite outbreaks. However, mechanisms other than direct or indirect toxicity to phytoseiid mites likely are associated with exacerbation of spider mite outbreaks on hop.

Stability and Resiliency of Biological Control of the Twospotted Spider Mite (Acari: Tetranychidae) in Hop.

The twospotted spider mite (Tetranychus urticae Koch) is a common pest in agricultural and ornamental crops. This pest can be controlled by resident predatory arthropods in certain situations. This research quantified the stability and resiliency of established conservation biological control of the twospotted spider mite in hop over a 5-yr period associated with nitrogen fertilization rate and use of a broad-spectrum insecticide. Biological control generally was stable and resilient over a sixfold range of nitrogen fertilization rates, and in only 1 of 5 yr did elevated nitrogen rates significantly affect populations of spider mites. In contrast, one application of the insecticide bifenthrin was associated with disruption of biological control and a severe outbreak of spider mites. The complex of natural enemies suppressed the outbreak during the same year in which bifenthrin was applied, but only after populations of spider mites exceeded levels associated with economic damage. However, in the following year the system returned to an equilibrium state where spider mites were suppressed below economically damaging levels. Therefore, conservation biological control in hop appears stable and robust to factors such as nitrogen fertilization that increase reproductive rates of spider mites but may be sensitive to factors such as nonselective insecticides that are lethal to natural enemies. Conservation biological control can be considered resilient to a single use of a nonselective insecticide in the year following the application, but not within the year of application.

Spatial and temporal stability of the estimated parameters of the binary power law.

The incidence of hop powdery mildew on leaves, caused by Podosphaera macularis, collected from 1,606 transects in 77 commercial hop yards in Oregon and Washington over 9 years was used to assess variability in heterogeneity of disease and the estimated binary power law parameters. Spatial analyses of data sets were conducted at the level of individual rows (row level) and multiple rows within a yard (yard level). The binary power law provided a good fit to all data sets, with R(2) values ranging from 0.933 to 0.993. At the row level, the intercept parameter ln(A(x)) was >0 for 8 years, but was not significantly greater than 0 in 2006. The parameter b was greater than 1 for all row-level data sets collected from 1999 to 2005, but was <1 in 2006 and not significantly different from 1 in 2007. Covariance analysis indicated the factor 'region' affected ln(A(x)) in 3 years, and b in 2 years. 'Cultivar' had an effect on ln(A(x)) in 3 years and b in year. At the yard level, ln(A(x)) was greater than 0 for 6 years, but in 2006 and 2007, ln(A(x)) was not significantly different from 0. The slope parameter b was greater than 1 in 6 years, but was not significantly different from 1 in 2006 and 2007. Differences in b among years were large enough to have practical implications for sample sizes and precision of fixed and sequential sampling. Although the binary power law parameter tended to be relatively stable, variability of the estimated parameters may have practical consequences for sampling precision and costs.

Influence of Nitrogen Fertilizer Rate on Hop Looper.

Hop looper, Hypena humuli Harris, can cause substantial defoliation and crop damage by feeding on hop leaves and cones. A 4-yr field study conducted in western Oregon evaluated the abundance of hop looper larvae and associated defoliation of leaves on plants fertilized with nitrogen rates ranging from 44.8 to 269 kg/ha. There was annual variation in abundance of hop looper and defoliation, with a tendency for increasing nitrogen rate to increase both abundance of hop looper and defoliation. A mixed model analysis with data combined from 2014 to 2017 found that abundance of hop looper was linearly related to nitrogen fertilizer rate, with a 2.5 increase in hop looper-days per kilogram of nitrogen fertilizer applied. Similarly, based on data from 2015 to 2017, defoliation associated with hop looper increased 0.031 percent with each kilogram of nitrogen fertilizer applied. Therefore, avoiding unduly high rates of nitrogen fertilizer may reduce the abundance and defoliation caused by hop looper. Further studies are needed to understand the mechanisms associated with nitrogen stimulation of hop looper.

First Report of Iris yellow spot virus on Onion and Leek in Western Oregon.

Onion (Allium cepa) and leek (Allium porrum) are grown on approximately 600 ha in western Oregon annually for bulb and seed production. During July and August of 2006, surveys of onion bulb crops and onion and leek seed crops in western Oregon found plants with symptoms of elongated to diamond-shaped, straw-colored lesions characteristic of those caused by Iris yellow spot virus (IYSV) (1-4). Symptomatic plants were collected from fields of an onion bulb crop, an onion seed crop, and two leek seed crops located in Marion County. The onion bulb crop had been planted in the spring of 2006, and the onion and leek seed crops had been planted in the fall of 2005, all direct seeded. Cultivar names were not provided for proprietary purposes. Symptomatic plants in the onion bulb crop and leek seed crop generally were found near the borders of the field. Disease incidence was less than 5% and yield losses in these crops appeared to be negligible. In the onion seed crop, symptomatic plants were found throughout the field and disease incidence was approximately 20%. Approximately 1% of the onion plants in this field had large necrotic lesions that caused the seed stalks (scapes) to lodge. The presence of IYSV was confirmed from symptomatic leaves and scapes by ELISA (Agdia Inc., Elkhart, IN) using antiserum specific to IYSV. RNA was extracted from symptomatic areas of onion leaves and scapes, and a portion of the nucleocapsid gene was amplified by reverse transcription-PCR. The amplicons were sequenced and found to share more than 99% nucleotide and amino acid sequence identity with an onion isolate of IYSV from the Imperial Valley of California (GenBank Accession No. DQ233475). In the Pacific Northwest region of the United States, IYSV has been confirmed in the semi-arid regions of central Oregon (1), central Washington (2), and the Treasure Valley of eastern Oregon and southwest Idaho (3). To our knowledge, this is the first report of the disease on a host crop in the mild, maritime region west of the Cascade Mountain Range and the first report of IYSV on leek seed crops in the United States, which complements a simultaneous report of IYSV on commercial leek in Colorado. The presence of IYSV may have implications for the iris and other ornamental bulb industries in western Oregon and western Washington. This report underscores the need for further research to determine the impact of the disease on allium crops and other hosts and the development of effective management programs for IYSV and the vector, Thrips tabaci. References: (1) F. J. Crowe and H. R. Pappu. Plant Dis. 89:105, 2005. (2) L. J. du Toit et al. Plant Dis. 88:222, 2004. (3) J. M. Hall et al. Plant Dis. 77:952, 1993. (4) H. F. Schwartz et al. Plant Dis. 91:113, 2007.

Molecular genetic testing for familial hypercholesterolemia in the Netherlands: a stepwise screening strategy enhances the mutation detection rate.

Familial hypercholesterolemia (FH) has been identified as a major risk factor for coronary vascular disease and is associated with mutations in the low-density liporotein receptor (LDLR) and apolipoprotein B (APOB) gene. The molecular basis of FH in the Dutch population is well understood. Approximately 160 different LDLR and APOB gene defects have been identified with a panel of 9 LDLR gene and 1 APOB gene frequently occurring mutations accounting for approximately 30% of all clinically diagnosed FH cases. As molecular diagnosis of FH is becoming increasingly widely applied, a variety of mutation detection rates is reported, ranging from as low as 30% and up to 80%. This variability appears to depend on the clinical criteria applied to identify patients with FH and on the strategies and methodologies used for mutation screening. In this study we describe the application of a stepwise screening approach, combining different methodologies, to detect mutations of the LDLR gene and APOB gene in 1465 patients with FH. A mutation was found in approximately 44% of the patients, which demonstrates that this is an effective strategy for the molecular diagnosis of FH.

Iris yellow spot virus on Onion in Colorado.

A new disease was found in September 2001 on greenhouse-produced onion transplants of cv. Colorado 6 grown in a field in Larimer County in northern Colorado. Symptoms included straw-colored, dry, tan, spindle- or diamond-shaped lesions on the leaves and scapes of onion plants. Infected plants were scattered (less than 5% incidence) throughout the outer perimeter of the sprinkler-irrigated field. Iris yellow spot virus (IYSV) in two collections each with 4 to 6 symptomatic onion plants was confirmed with western blot assays by James Moyer of North Carolina State University. Western blot showed a faint band from protein extracts of infected Nicotiana benethimiana. Western blot assay is the most definitive method of identification. IYSV can be mechanically transmitted to N. benethimiana, but it cannot be recovered and transmitted back to onion, and it is difficult to detect in infected onion plants. IYSV is a tospovirus that is transmitted by various species of thrips, including onion thrips and western flower thrips (1). The host range for this disease includes onion, leek, and iris. IYSV has been reported previously on onion in Israel, Brazil, and Idaho (2). There are no reports that this disease affects bulb quality or marketability; however, heavy losses of onion bulb production are reported (1). University and industry personnel in other onion-growing areas of the country are encouraged to monitor onion and other host fields for evidence and distribution of IYSV. References: (1) A. Kritzman et al. Plant Dis. 85:838, 2001. (2) L. Pozzer et al. Plant Dis. 83:345, 1999.

Population density and phenology of Tetranychus urticae (Acari: Tetranychidae) in hop is linked to the timing of sulfur applications.

The twospotted spider mite, Tetranychus urticae Koch, is a worldwide pest of numerous agronomic and horticultural plants. Sulfur fungicides are known to induce outbreaks of this pest on several crops, although mechanisms associated with sulfur-induced mite outbreaks are largely unknown. Studies were conducted during 2007-2009 in Oregon and Washington hop yards to evaluate the effect of timing of sulfur applications on T. urticae and key predators. In both regions, applications of sulfur made relatively late in the growing season (mid-June to mid-July) were associated with the greatest exacerbation of spider mite outbreaks, particularly in the upper canopy of the crop. The severity of mite outbreaks was closely associated with sulfur applications made during a relatively narrow time period coincident with the early exponential phase of spider mite increase and rapid host growth. A nonlinear model relating mean cumulative mite days during the time of sulfur sprays to the percent increase in total cumulative mite days (standardized to a nontreated plot) explained 58% of the variability observed in increased spider mite severity related to sulfur spray timing. Spatial patterns of spider mites in the Oregon plots indicated similar dispersal of motile stages of spider mites among leaves treated with sulfur versus nontreated leaves; however, in two of three years, eggs were less aggregated on leaves of sulfur-treated plants, pointing to enhanced dispersal. Apart from one experiment in Washington, relatively few predatory mites were observed during the course of these studies, and sulfur-induced mite outbreaks generally occurred irrespective of predatory mite abundance. Collectively, these studies indicate sulfur induces mite outbreaks through direct or indirect effects on T. urticae, mostly independent of predatory mite abundance or toxicity to these predators. Avoidance of exacerbation of spider mite outbreaks by sulfur sprays was achieved by carefully timing applications to periods of low spider mite abundance and slower host development, which is generally early to mid-spring for hop.