William W. Turechek

A Two-Phase Resistance Response of Venturia inaequalis Populations to the QoI Fungicides Kresoxim-Methyl and Trifloxystrobin

The class of fungicides acting as respiration inhibitors by binding to the Qo center of cyto-chrome b (QoIs) are in wide use for the management of apple scab caused by Venturia inaequalis. In order to assess responses of V. inaequalis populations to treatments with QoIs, sensitivities of isolates were determined for germinating conidia or for mycelial colonies developing from germinating conidia. Under both test conditions, inhibitory potencies of kresoxim-methyl and trifloxystrobin were largely equivalent. V. inaequalis populations treated with QoIs in a commercial and an experimental orchard both responded with significant shifts toward declining QoI sensitivities. However, the population responses were quantitative in nature, and highly resistant isolates indicative of a cytochrome b target site mutation were not detected. V. inaequalis populations from both orchards investigated also were fully resistant to sterol de-methylation-inhibiting fungicides (DMIs) such as fenarimol and myclobutanil, but isolate sensitivities to QoIs and DMIs were largely unrelated. Performance tests with kresoxim-methyl and trifloxystrobin at the experimental orchard diagnosed as DMI-resistant revealed that the quantitative shift toward declining QoI sensitivities did not constitute the status of practical QoI resistance. In contrast to these quantitative responses, emergence of qualitative QoI resistance was documented for V. inaequalis in an orchard in North Germany, which had been treated intensively with a total of 25 QoI applications over four consecutive seasons. Isolates retrieved from the orchard were highly resistant to both kresoxim-methyl and trifloxystrobin and were characterized as G143A cytochrome b mutants. The results indicated that the paths of QoI resistance can be both quantitative and qualitative in nature. A similar phenomenon has not been described before. Circumstantial evidence suggests that the quantitative phase of V. inaequalis population responses to QoIs might be succeeded by a quantitative selection of highly resistant G143A target-site mutants.

Evaluating Predictors of Apple Scab with Receiver Operating Characteristic Curve Analysis

ABSTRACT Apple scab (Venturia inaequalis) is a perennial threat to apple production in temperate climates throughout the world. In the eastern United States, apple scab is managed almost exclusively through the regular application of fungicides. Management of the primary phase of disease is focused on preventing infection by ascospores. Management of secondary cycles of infection is largely dependent on how well primary infections were controlled. In this study, we used receiver operating characteristic curve analysis to evaluate how well mid-season assessments of the incidence of apple scab on cluster leaves, clusters (i.e., the whorl of cluster leaves), or immature fruit can serve as predictors of apple scab on harvested fruit (harvest scab) and whether these mid-season assessments of scab could be used reliably to manage scab under various damage thresholds. Results showed that assessment of scab on immature fruit was superior at predicting harvest scab than were assessments made on clusters or cluster leaves at all damage thresholds evaluated. A management action threshold of 7% scab incidence on immature fruit was identified by Youdens index as the optimal action threshold to prevent harvest scab incidence from exceeding 5%. Action thresholds could be higher or lower than 7% when economic assumptions were factored in to the decision process. The utility of such a predictor is discussed.

Emergence of Groundnut ringspot virus and Tomato chlorotic spot virus in Vegetables in Florida and the Southeastern United States

Groundnut ringspot virus (GRSV) and Tomato chlorotic spot virus (TCSV) are two emerging tospoviruses in Florida. In a survey of the southeastern United States, GRSV and TCSV were frequently detected in solanaceous crops and weeds with tospovirus-like symptoms in south Florida, and occurred sympatrically with Tomato spotted wilt virus (TSWV) in tomato and pepper in south Florida. TSWV was the only tospovirus detected in other survey locations, with the exceptions of GRSV from tomato (Solanum lycopersicum) in South Carolina and New York, both of which are first reports. Impatiens (Impatiens walleriana) and lettuce (Lactuca sativa) were the only non-solanaceous GRSV and/or TCSV hosts identified in experimental host range studies. Little genetic diversity was observed in GRSV and TCSV sequences, likely due to the recent introductions of both viruses. All GRSV isolates characterized were reassortants with the TCSV M RNA. In laboratory transmission studies, Frankliniella schultzei was a more efficient vector of GRSV than F. occidentalis. TCSV was acquired more efficiently than GRSV by F. occidentalis but upon acquisition, transmission frequencies were similar. Further spread of GRSV and TCSV in the United States is possible and detection of mixed infections highlights the opportunity for additional reassortment of tospovirus genomic RNAs.

Pre- and Post-Infection Activity of Pyraclostrobin for Control of Anthracnose Fruit Rot of Strawberry Caused by Colletotrichum acutatum

The effect of pre- and post-infection-period applications of pyraclostrobin (Cabrio EG) on the development of anthracnose fruit rot was characterized in a controlled-climate study and validated in field studies in New York and Florida. Plants of the day-neutral cv. Tristar were inoculated with C. acutatum and placed into mist chambers at 14, 22, or 30°C. The plants were removed from the chambers after 3, 6, 12, or 24 h of misting and placed on greenhouse benches to allow disease development. The fungicide pyraclostrobin was applied to the berries at a concentration equivalent to 168 g a.i./ha at 3, 8, 24, and 48 h prior to inoculation and exposure to their wetting period, or 3, 8, 24, and 48 h following inoculation and exposure to their wetting period. All pyraclostrobin treatments suppressed disease compared with the corresponding untreated control treatments. The highest incidence of disease occurred on plants exposed to the longest wetness durations (12 and 24 h) or highest temperature treatments (22 and 30°C). Post-infection applications of pyraclostrobin provided significant control when applications were made within 3 and often up to 8 h after wetting, but generally were less effective than protective sprays. We further tested the ability of pyraclostrobin to control anthracnose when applied as a protectant or as an after-infection application in inoculated field plots exposed to a short (8 h) or long (24 h) wetting period in Florida and in New York. In three of the four experimental plots, disease control equivalent to or better than the protective spray was achieved when pyraclostrobin was applied up to 24 h after infection for long and short wetting periods. In the remaining plot, conditions for disease development were exceptionally favorable. The protective treatment provided approximately 75% control, whereas the best post-infection treatment provided only 50% control. Our study indicates that for short wetting events, such as those associated with seasonal thunderstorms, growers can wait until after such an infection event before applying pyraclos-trobin and achieve control equivalent to a protective application.

Spatial pattern analysis of hop powdery mildew in the pacific northwest: Implications for sampling

ABSTRACT The spatial pattern of hop powdery mildew was characterized using 3 years of disease incidence data collected in commercial hop yards in the Pacific Northwest. Yards were selected randomly from yards with a history of powdery mildew, and two to five rows were selected for sampling within each yard. The proportion of symptomatic leaves out of 10 was determined from each of N sampling units in a row. The binomial and the beta-binomial frequency distributions were fit to the N sampling units observed in each row and to SigmaN sampling units observed in each yard. Distributional analyses indicated that disease incidence was better characterized by the beta-binomial than the binomial distribution in 25 and 47% of the data sets at the row and yard scales, respectively, according to a log-likelihood ratio test. Median values of the beta-binomial parameter theta, a measure of small-scale aggregation, were near 0 at both sampling scales, indicating that disease incidence was close to being randomly distributed. The variability in disease incidence among rows sampled in the same yard generally increased with mean incidence at the yard scale. Spatial autocorrelation analysis, used to measure large-scale patterns of aggregation, indicated that disease incidence was not correlated between sampling units over several lag distances. Results of a covariance analysis showed that heterogeneity of disease incidence was not dependent upon cultivar, region, or time of year when sampling was conducted. A hierarchical analysis showed that disease incidence at the sampling unit scale (proportion of sampling units with one or more diseased leaves) increased as a saturation-type curve with respect to incidence at the leaf level and could be described by a binomial function modified to account for the effects of heterogeneity through an effective sample size. Use of these models permits sampling at the sampling unit scale while allowing inferences to be made at the leaf scale. Taken together, hop powdery mildew was nearly randomly distributed with no discernable foci, suggesting epidemics are initiated from a well-distributed or readily dispersible overwintering population. Implications for sampling are discussed.

Effect of scale on plant disease incidence and heterogeneity in a spatial hierarchy

Abstract The effect of patch scale on disease incidence and heterogeneity was assessed in a three-scaled spatial hierarchy using data collected from strawberry leaf blight epidemics and with Monte-Carlo simulation studies. Cluster sampling was used to collect disease incidence data from several strawberry fields at two commercial farms in Ohio over 2 years. Sampling units consisted of five leaves of three leaflets each. Leaflets represented the lowest scale of the hierarchy, leaves represented the intermediate scale, and the sampling units represented the upper scale. Results from data randomizations indicated that diseased leaflets were nearly maximally aggregated among leaves within sampling units, resulting in close to the minimum possible incidence and heterogeneity of disease among sampling units at the leaf scale. Based on Monte-Carlo simulations, incidence and heterogeneity of disease at the leaflet (i.e. lowest) scale had the largest effect on incidence and heterogeneity of disease at the leaf scale, respectively. However, analysis of variance of simulation results indicated that arrangement of diseased leaflets among leaves within sampling units, sampling unit size (=number of leaves or leaflets per sampling unit), leaf complexity (leaflets per leaf), and all interactions affected disease incidence and heterogeneity at the leaf scale. For a given level of intensity of leaflet disease incidence and heterogeneity, incidence and heterogeneity of disease at the leaf scale were most affected by sampling unit size when leaflets were arranged in an aggregated fashion among leaves; increasing the number of leaves per sampling unit was associated with decreasing disease incidence and increasing heterogeneity at the leaf scale. However, leaf complexity had the greater effect on leaf disease incidence and heterogeneity when diseased leaflets were arranged randomly or uniformly among leaves within sampling units; greater leaf complexity was associated with greater incidence and heterogeneity. Combined results indicated that incidence and heterogeneity of incidence of Phomopsis leaf blight varied between scales in a patch hierarchy. This variability was shown to be a function of lower-scale disease incidence and heterogeneity, sampling unit size, and leaf complexity.

Distribution of four viruses in single and mixed infections within infected watermelon plants in Florida.

Whitefly-transmitted Squash vein yellowing virus (SqVYV) and Cucurbit leaf crumple virus (CuLCrV) and aphid-transmitted Papaya ringspot virus type W (PRSV-W) have had serious impact on watermelon production in southwest and west-central Florida in the past 5 years. Tissue-blot nucleic acid hybridization assays were developed for simple, high-throughput detection of these three viruses as well as Cucurbit yellow stunting disorder virus (CYSDV), which was first reported in Florida in 2008. To determine virus distribution within plants, we collected 80 entire plants just before or during the harvest period in a systematic sample, 20 each on 11 April, 18 April, 26 April, and 3 May 2007, from a fruiting commercial watermelon field near Immokalee, FL showing symptoms of infection by SqVYV, CuLCrV, and PRSV-W and, possibly, CYSDV. This was followed by a sampling of five plants collected at harvest showing symptoms of virus infection on 11 October 2007 in a different commercial planting located in Duette, FL. Tissue prints were made from cross sections of watermelon plants from the crowns through the tips at 0.6-m intervals on nylon membranes and nucleic acid hybridization assays were used for virus detection. Results from testing crown tissue showed that SqVYV, CuLCrV, and PRSV-W were present in ≈37, 44, and 54%, respectively, of the 80 plants collected over the four sampling dates from the first field. For individual vines diagnosed with SqVYV, the distribution of SqVYV in vine tissue decreased proportionately with distance from the crown. The probability of detecting SqVYV was 70% at the base of the vine compared with 23% at the tip of the vine. In contrast, CuLCrV tended to be more evenly distributed throughout the plant, with ≈10% higher probability of detection at the growing tip relative to the crown of the plant. The distribution of PRSV-W resembled that of SqVYV but with ≈20% higher probability of detection at the tip of the vine. Similar trends were detected in the smaller sampling; however, CYSDV was also detected in three of the plants. Overall, the results indicated that SqVYV and PRSV-W were distributed differently than CuLCrV in watermelon plants, and this difference has implications on how samples should be collected and may affect vector acquisition and transmission of these viruses.

Incidence, Distribution, and Association of Spongospora subterranea and Potato mop-top virus in Costa Rica

A survey was conducted in 39 potato (Solanum tuberosum) fields in Costa Rica to determine incidence and association of Spongospora subterranea f. sp. subterranea and Potato mop-top pomovirus (PMTV). The fields were located in Costa Ricas two major potato-production regions and were further characterized by their altitude. In all, 633 paired samples of leaf tissue and corresponding tubers were collected, assessed visually for disease, and subsequently assayed by enzyme-linked immunosorbent assay (ELISA). S. subterranea presence in tuber tissue was tested by double-antibody sandwich (DAS)-ELISA and PMTV presence in leaf and tuber tissues was tested by triple-antibody sandwich (TAS)-ELISA. Moreover, soil samples were collected from 10 fields surveyed and were evaluated for both pathogens via ELISA and bioassay. The incidence of both diseases ranged from 0 to 100% within individual fields, with incidences lower than 40% occurring in more than 70% of the fields. Higher incidences were found in fields located at higher altitudes. Of the 633 paired samples, 179 and 146 were positive for PMTV and S. subterranea, respectively, according to ELISA in either the foliage or tubers. A low correlation was found for PMTV visual symptoms and ELISA test results. Only 14 of the 81 foliar samples testing positive for PMTV had visual symptoms; the remaining 67 samples were asymptomatic. Conversely, comparison of visual evaluation with detection of S. subterranea by ELISA on tubers showed that 70% of the results were coincident. S. subterranea was detected in 4 of 10 soil samples tested by ELISA. Soilborne PMTV was detected by ELISA in roots of bait plants sown in these soil samples. Co-occurrence of both pathogens was detected in 64 samples. A significant but low degree of association for vector and virus was determined, and data suggests that S. subterranea is participating in the transmission of PMTV in Costa Rica in low frequency.

Site-Specific Risk Factors for Ray Blight in Tasmanian Pyrethrum Fields

Ray blight of pyrethrum (Tanacetum cinerariifolium), caused by Phoma ligulicola var. inoxydablis, can cause defoliation and reductions of crop growth and pyrethrin yield. Logistic regression was used to model relationships among edaphic factors and interpolated weather variables associated with severe disease outbreaks (i.e., defoliation severity ≥40%). A model for September defoliation severity included a variable for the product of number of days with rain of at least 0.1 mm and a moving average of maximum temperatures in the last 14 days, which correctly classified (accuracy) the disease severity class for 64.8% of data sets. The percentage of data sets where disease severity was correctly classified as at least 40% defoliation severity (sensitivity) or below 40% defoliation severity (specificity) were 55.8 and 71%, respectively. A model for October defoliation severity included the number of days with at least 1 mm of rain in the past 14 days, stem height in September, and the product of the number of days with at least 10 mm of rain in the last 30 days and September defoliation severity. Accuracy, sensitivity, and specificity were 72.6, 73.6, and 71.4%, respectively. Youdens index identified predictive thresholds of 0.25 and 0.57 for the September and October models, respectively. When economic considerations of the costs of false positive and false negative decisions and disease prevalence were integrated into receiver operating characteristic (ROC) curves for the October model, the optimal predictive threshold to minimize average management costs was 0 for values of disease prevalence greater than 0.2 due to the high cost of false negative predictions. ROC curve analysis indicated that management of the disease should be routine when disease prevalence is greater than 0.2. The models developed in this research are the first steps toward identifying and weighting site and weather disease risk variables to develop a decision-support aid for the management of ray blight of pyrethrum.

Spatial Heterogeneity of the Incidence of Powdery Mildew on Hop Cones

The spatial heterogeneity of the incidence of hop cones with powdery mildew (Podosphaera macularis) was characterized from transect surveys of 41 commercial hop yards in Oregon and Washington from 2000 to 2005. The proportion of sampled cones with powdery mildew ( p) was recorded for each of 221 transects, where N = 60 sampling units of n = 25 cones assessed in each transect according to a cluster sampling strategy. Disease incidence ranged from 0 to 0.92 among all yards and dates. The binomial and beta-binomial frequency distributions were fit to the N sampling units in a transect using maximum likelihood. The estimation procedure converged for 74% of the data sets where p > 0, and a loglikelihood ratio test indicated that the beta-binomial distribution provided a better fit to the data than the binomial distribution for 46% of the data sets, indicating an aggregated pattern of disease. Similarly, the C(α) test indicated that 54% could be described by the beta-binomial distribution. The heterogeneity parameter of the beta-binomial distribution, θ, a measure of variation among sampling units, ranged from 0.01 to 0.20, with a mean of 0.037 and a median of 0.015. Estimates of the index of dispersion ranged from 0.79 to 7.78, with a mean of 1.81 and a median of 1.37, and were significantly greater than 1 for 54% of the data sets. The binary power law provided an excellent fit to the data, with slope and intercept parameters significantly greater than 1, which indicated that heterogeneity varied systematically with the incidence of infected cones. A covariance analysis indicated that the geographic location (region) of the yards and the type of hop cultivar had little effect on heterogeneity; however, the year of sampling significantly influenced the intercept and slope parameters of the binary power law. Significant spatial autocorrelation was detected in only 11% of the data sets, with estimates of first-order autocorrelation, r1, ranging from -0.30 to 0.70, with a mean of 0.06 and a median of 0.04; however, correlation was detected in only 20 and 16% of the data sets by median and ordinary runs analysis, respectively. Together, these analyses suggest that the incidence of powdery mildew on cones was slightly aggregated among plants, but patterns of aggregation larger than the sampling unit were rare (20% or less of data sets). Knowledge of the heterogeneity of diseased cones was used to construct fixed sampling curves to precisely estimate the incidence of powdery mildew on cones at varying disease intensities. Use of the sampling curves developed in this research should help to improve sampling methods for disease assessment and management decisions.