Airborne Conidia Concentration Research Articles

Bremia lactucae Infection Efficiency in Lettuce is Modulated by Temperature and Leaf Wetness Duration Under Quebec Field Conditions.

More than 80% of Canadian lettuce production is located in the province of Quebec. Yet most of our knowledge on the epidemiology of lettuce downy mildew (Bremia lactucae) is derived from controlled-condition experiments or field experiments conducted in subtropical climates and, thus, cannot readily be applied to Quebec lettuce production. The influence of temperature and leaf wetness duration on the infection efficiency (IE) of B. lactucae was studied for 4 years (2003, 2004, 2012, and 2013) under field and growth-chamber conditions. IE was defined as the ratio of the number of lesions/leaf to the airborne conidia concentration (ACC). B. lactucae ACC was measured with rotating-arm samplers three times/week. In addition, 72 lettuce trap plants/sampling day were exposed to the potential airborne B. lactucae inoculum and disease intensity was assessed after 7 days of incubation in greenhouse. Under growth-chamber conditions, an ACC of 1 conidium/m3 was sufficient to cause 1 lesion/leaf, and IE ranged from 0.25 to 1.00. Under field conditions, an ACC of 10 to 14 conidia/m3 was required to cause 1 lesion/leaf, and IE ranged from 0.02 to 0.10, except in 2004, when IE ranged from 0.03 to 1.00. IE increased with increasing leaf wetness duration but decreased with increasing temperature. Also, considering an observed average temperature range from 10 to 20°C in the area of Quebec, 2 h of leaf wetness was sufficient for infection by B. lactucae. Therefore, under Quebec lettuce production conditions, a leaf wetness period of 2 h and an ACC of 10 to 14 conidia/m3 can be used as risk indicators to facilitate disease management decisions. Also, under typical Quebec weather conditions, measuring both morning and evening leaf wetness events could be used to improve the reliability of leaf wetness duration as a downy mildew risk indicator. Further research is needed to validate these risk indicators for integration into management strategies.

Relationship of Airborne Botrytis cinerea Conidium Concentration to Tomato Flower and Stem Infections: A Threshold for De-leafing Operations.

Gray mold, caused by Botrytis cinerea, is an important threat for tomato greenhouse producers. The influence of airborne conidia concentration (ACC) on both flower and stem-wound infections was studied in a greenhouse maintained at a temperature of 15, 20, or 25°C using diseased tomato leaves as the unique source of dry inoculum. Spore samplers were used to monitor ACC, and a previously developed real-time qPCR assay was used to quantify airborne B. cinerea conidia. The proportion of infected flowers remained low at ACC < 10 conidia/m3; above this concentration, flower infection increased with increasing ACC. The influence of ACC on proportion of infected flowers was well described by a sigmoid model (R2 = 0.90 to 0.92). The mean proportion of infected stem wounds over the three trials was 0.021; no infected wounds were observed at ACC < 100 conidia/m3. Based on logistic regression, the probability that a stem becomes infected increased rapidly with mean probabilities of 0.24 and 0.87 at ACCs of 315 and 3,161 conidia/m3, respectively. The results suggest that the amount of airborne B. cinerea inoculum in the greenhouse is often above the action threshold for flower infection and that monitoring airborne B. cinerea inoculum could help in timing de-leafing operations.

Testing an innovative device against airborne Aspergillus contamination.

Aspergillus fumigatus is a major airborne nosocomial pathogen that is responsible for severe mycosis in immunocompromised patients. We studied the efficacy of an innovative mobile air-treatment device in eliminating A. fumigatus from the air following experimental massive contamination in a high-security room. Viable mycological particles were isolated from sequential air samples in order to evaluate the device's effectiveness in removing the fungus. The concentration of airborne conidia was reduced by 95% in 18 min. Contamination was reduced below the detection threshold in 29 min, even when the machine was at the lowest airflow setting. In contrast, during spontaneous settling with no air treatment, conidia remained airborne for more than 1 h. This indoor air contamination model provided consistent and reproducible results. Because the air purifier proved to be effective at eliminating a major contaminant, it may prove useful in preventing air-transmitted disease agents. In an experimental space mimicking a hospital room, the AirLyse air purifier, which uses a combination of germicidal ultraviolet C irradiation and titanium photocatalysis, effectively eliminated Aspergillus conidia. Such a mobile device may be useful in routine practice for lowering microbiological air contamination in the rooms of patients at risk.

Spatial Pattern of Strawberry Powdery Mildew (Podosphaera aphanis) and Airborne Inoculum.

The relationship between strawberry powdery mildew and airborne conidium concentration (ACC) of Podosphaera aphanis was studied using data collected from 2006 to 2009 in 15 fields, and spatial pattern was described using 2 years of airborne inoculum and disease incidence data collected in fields planted with the June-bearing strawberry (Fragaria × ananassa) cultivar Jewel. Disease incidence, expressed as the proportion of diseased leaflets, and ACC were monitored in fields divided into 3 × 8 grids containing 24 100 m2 quadrats. Variance-to-mean ratio, index of dispersion, negative binomial distribution, Poisson distribution, and binomial and beta-binomial distributions were used to characterize the level of spatial heterogeneity. The relationship between percent leaf area diseased and daily ACC was linear, while the relationship between ACC and disease incidence followed an exponential growth curve. The V/M ratios were significantly greater than 1 for 100 and 96% of the sampling dates for ACC sampled at 0.35 m from the ground (ACC0.35m) and for ACC sampled at 1.0 m from the ground (ACC1.0m), respectively. For disease incidence, the index of dispersion D was significantly greater than 1 for 79% of the sampling dates. The negative binomial distribution fitted 86% of the data sets for both ACC1.0m and ACC0.35m. For disease incidence data, the beta-binomial distribution provided a good fit of 75% of the data sets. Taylor's power law indicated that, for ACC at both sampling heights, heterogeneity increased with increasing mean ACC, whereas the binary form of the power law suggested that heterogeneity was not dependent on the mean for disease incidence. When the spatial location of each sampling location was taken into account, Spatial Analysis by Distance Indices showed low aggregation indices for both ACCs and disease incidence, and weak association between ACC and disease incidence. Based on these analyses, it was found that the distribution of strawberry powdery mildew was weakly aggregated. Although a higher level of heterogeneity was observed for airborne inoculum, the heterogeneity was low with no distinct foci, suggesting that epidemics are induced by well-distributed inoculum. This low level of heterogeneity allows mean airborne inoculum concentration to be estimated using only one sampler per field with an overall accuracy of at least 0.841. The results obtained in this study could be used to develop a sampling scheme that will improve strawberry powdery mildew risk estimation.

A new risk indicator for botrytis leaf blight of onion caused by Botrytis squamosa based on infection efficiency of airborne inoculum

Botrytis leaf blight (BLB) caused by Botrytis squamosa is a major leaf disease of onion. Various forecasting systems have been developed to help growers manage the disease. To improve forecasting reliability, the influence of temperature and wetness duration on B. squamosa infection was quantified by inoculating onion leaves with a conidial suspension and incubating them under various combinations of temperature (10–30°C) and leaf wetness duration (0–84 h). Infection was measured as the number of lesions per cm2 of leaf and converted to the proportion of maximum infection (PMI). Regardless of leaf wetness duration, only a few lesions developed at 30°C and the number of lesions increased as the temperature rose from 10 to 20°C but decreased at 25°C. Between 10 and 25°C the number of lesions per cm2 of leaf area increased gradually with increasing leaf wetness duration from 12 to 72 h. Relative infection was modelled as a function of both temperature and wetness duration using a modified version of the Weibull equation, which provided a precise description of the response of B. squamosa (R2 = 0·88). To facilitate field validation, receiving operating characteristic curve analysis was performed to determine the accuracy of various sets of criteria for establishing the length of an infection event based on field weather data. The total number of leaf wetness and RH &gt;90% hours over a 72 h period was the best criterion, regardless of the wetness interruption pattern (sensitivity = 90·91, specificity = 84·62, area under the receiving operating curve = 0·878). The model describing the relationship of PMI to temperature and leaf wetness duration, and field observations on airborne conidium concentration (ACC) were used to calculate the risk of infection (RIBLB) as RIBLB = PMI × ACC. In 2009 and 2010, this risk index was compared to the observed rate of BLB progress (RateBLB+5 days) during the following 5 days. There was a linear relationship between RIBLB and RateBLB+5 days indicating that this new risk indicator was reliable for predicting the risk of BLB development. These findings will help to improve the timing of fungicide applications for BLB management.

Comparison of monitoring based indicators for initiating fungicide spray programs to control Botrytis leaf blight of onion

Botrytis leaf blight (BLB) of onion is a well-documented disease and several management tools are available. However, achieving good control with minimum use of fungicides is time consuming and requires dedication on the part of growers. In this study, three indicators for the initiation of fungicide spray programs were compared. The study was conducted in 2008, 2009 and 2010 at commercial onion farms located in the muck soil area southwest of Montreal. The indicators evaluated were: 1) first airborne Botrytis squamosa conidia detected; 2) cumulative airborne conidium concentration (ACC) of 15 conidia m −3; 3) first lesion caused by B. squamosa detected. The interval between subsequent fungicide applications was determined by taking into account sporulation potential, airborne conidium concentration and lesion density. Reliability of the indicators was evaluated on the basis of observed maximum disease level, area under the disease progress curve and rate of disease progress. Initiation of the fungicide spray program when the first airborne B. squamosa conidia were detected resulted in the lowest values for maximum disease level and area under the disease progress curve, followed by initiation when a cumulative concentration of 15 conidia m −3 was reached, and by initiation when the first lesion was detected. However, there was no significant difference in rate of disease progress, calculated using data from fields where spray programs were initiated based on the different indicators. The critical disease level for curative fungicide applications (ten lesions per leaf) was not reached when the spray program was initiated upon detection of the first conidia; was reached a few days after bulb initiation for a spray program initiated at a cumulative ACC of 15 conidia m −3; and was reached prior to bulb initiation for the spray program initiated when the first lesion was detected. These results suggest that to avoid yield reduction, BLB should be managed such that the disease does not reach the exponential phase. This was achieved by initiating the fungicide spray program based on first conidia detected.

Grape powdery mildew ( Erysiphe necator) risk assessment based on airborne conidium concentration

Atmospheric concentrations of Erysiphe necator conidia were monitored for 2 years in a vineyard planted with cultivars susceptible to powdery mildew in the vine production area of Quebec, Canada. The concentrations of airborne conidia were determined and compared using two types of samplers: a Burkard volumetric sampler and a rotating-arm sampler. The coefficients of correlation between the volumetric and rotating-arm sampler placed at 45 cm from the ground was r = 0.81 and 0.82 in 2000 and 2001, respectively. The relationship between incidence of powdery mildew on the leaves of three cultivars (Chancellor, Geisenheim, and Frontenac) and the cumulative concentration of airborne conidia (based on 3 d of sampling weekly) was then studied. This relationship was similar for the three cultivars, ( R 2 = 0.97, 0.95, 0.97, for cvs Chancellor, Geisenheim, and Frontenac respectively) and was well described using the cumulative form of the Weibull model. Based on this model, it was possible to establish the period of high risk (highest rate of increase in powdery mildew incidence) from 645 to 5614, 2437 to 2951, and 1052 to 3061 conidia m −3 of air for the cvs Chancellor, Geisenheim, and Frontenac, respectively. An action threshold, for timing interval between fungicide sprays, of 50 conidia m −3 d −1 was evaluated under field conditions with cv Chancellor and was as good as a calendar-based program with fewer fungicide sprays under unfavourable weather conditions. Monitoring airborne inoculum could be used as a component of a risk management system for grape powdery mildew to time intervals between fungicide sprays.

Fluctuations in Number of Cercospora beticola Conidia in Relationship to Environment and Disease Severity in Sugar Beet

Cercospora leaf spot, caused by Cercospora beticola, is the most damaging foliar disease of sugar beet in Minnesota (MN) and North Dakota (ND). Research was conducted to characterize the temporal progression of aerial concentration of C. beticola conidia in association with the environment and disease severity in sugar beet. In 2003 and 2004, volumetric spore traps were placed within inoculated sugar beet plots to determine daily dispersal of conidia at Breckenridge, MN, and St. Thomas, ND. Plots were rated weekly for disease severity. At both locations, conidia were first collected in early July 2003 and late June in 2004. Peaks of conidia per cubic meter of air were observed with maxima in late August 2003 and in early September 2004 at both locations. Peaks of airborne conidium concentration were significantly correlated with the average temperature of daily hours when relative humidity was greater than 87%. Weekly mean hourly conidia per cubic meter of air was significantly (P < 0.01) associated with disease severity during both years and across locations. This study showed that C. beticola conidial numbers may be used to estimate potential disease severity that, with further research, could be incorporated in a disease forecasting model to rationalize Cercospora leaf spot management.

Comparison of monitoring- and weather-based risk indicators of botrytis leaf blight of onion and determination of action thresholds

Botrytis leaf blight (Botrytis squamosa) is the key disease for scheduling fungicide sprays in many onion-growing areas. Various disease predictors have been developed to identify the best time to initiate fungicide spray programs or to time spray intervals. However, individual predictors have not been rigorously evaluated based on their predictive accuracy. Receiver operating characteristic (ROC) curve analysis was used to evaluate the reliability of the following seven predictors at various damage thresholds: (1) the number of lesions on the oldest leaves; (2) the number of lesions on the youngest leaves; (3) the airborne conidia concentration (ACC); (4) the sporulation index (SI); (5) the inoculum production index (IPI); (6) the infection probability (IP); and (7) a disease severity value (DSV). Data on disease intensity were concurrently collected in sprayed and unsprayed plots. The analysis was conducted with data from the entire season (185 samplings) and for data after the critical disease level of 1 lesion per leaf was reached (107 samplings). At damage thresholds of 1 or 5 lesions per leaf, predictors based on biological monitoring generally were more reliable at predicting disease risk than weather-based predictors. At a damage threshold of 10 lesions per leaf, most pairwise comparisons showed that there were no significant differences among the areas under the ROC curves (AUCs) for any predictors except for the AUC for ACC which was significantly higher and for and IP that were significantly higher and significantly lower, respectively, than AUC for most other predictors. The difference between the two types of predictors was smaller when only data after the first fungicide spray were analysed. Best action thresholds were 8.06 to 13.79 conidia per m3 of air, 2.73 to 3.56 lesions per oldest leaf, and 0.16 to 0.43 lesions per youngest leaf. The most reliable weather-based predictor was SI at an action threshold of 82.55 to 86.46 SI value. Best time to initiate the spray program could be predicted using monitoring-based predictors at the lowest action threshold. The interval between sprays could be best predicted using monitoring-based predictors at a higher threshold combined with SI values above 80. Key words: disease management, epidemiology, forecasting systems, risk management. La brûlure de la feuille de l’oignon (Botrytis squamosa) est une maladie clef dans la gestion des fongicides dans la plupart des régions de production. Plusieurs indicateurs de risque ont été développés afin de déterminer le meilleur moment pour débuter les applications de fongicides ou pour déterminer l’intervalle entre les pulvérisations. Toutefois, ces indicateurs n’on jamais été évalués pour leur fiabilité à estimer les risques de maladie. L’analyse ROC « receive operating characteristic curve » a été utilisée pour évaluer la fiabilité de sept indicateurs en fonction de différents seuils de dommages: (1) nombre moyen de lésions sur la feuille du bas (OLD); (2) nombre moyen de lésions sur la feuille du haut (YNG); (3) la concentration aérienne de conidies (ACC); (4) l’indice de sporulation (SI); (5) l’indice de production d’inoculum (IPI); (6) la probabilité d’infection (IP); et (7) la valeur de sévérité (DSV). Des données sur l’intensité de la maladie ont été recueillies simultanément dans des parcelles traitées et non traitées avec des fongicides. L’analyse a été faite sur les données comprenant toute la saison et seulement la période après l’atteinte du seuil critique de une lésion par feuille pour un total de 185 et 107 échantillonnages. Au seuil de dommage de 1 et 5 lésions par feuille, les indicateurs basés sur le dépistage se sont avérés plus fiables que ceux basés sur des prédictions. Par contre, au seuil de 10 lésions par feuille, il n’y a pas de différences significatives entre les aires sous la courbe ROC (AUC) pour chaque paire d’indicateurs à l’exception de ACC dont l’AUC était significativement plus élevée et de IP dont l’AUC était significativement plus faible. Peu de différence entre les AUCs ont été observées pour les données recueillies après l’atteinte du seuil de une lésion par feuille. Le meilleur seuil d’intervention a été établit à 8,06 à 13,79 conidies par m3 d’air, 2,73 à 3,56 lésions par feuille du bas et 0,16 à 0,43 lésions par feuille du haut. Parmi les autres indicateurs, l’indice de sporulation (SI) s’est avéré être le plus fiable à un seuil d’intervention de 82,55 à 86,46. Le meilleur moment pour débuter les applications de fongicides devraient être déterminées en fonction du dépistage et par la suite l’intervalle entre les pulvérisations devrait être déterminé en fonction du dépistage avec un seuil plus élevé combiné avec un indice de sporulation (SI) au seuil de 80. Mots-clés : gestion des maladies, épidémiologie, lutte raisonnée, systèmes prévisionnels, gestion des risques.

Spatiotemporal Relationships Between Disease Development and Airborne Inoculum in Unmanaged and Managed Botrytis Leaf Blight Epidemics

Comparatively little quantitative information is available on both the spatial and temporal relationships that develop between airborne inoculum and disease intensity during the course of aerially spread epidemics. Botrytis leaf blight and Botrytis squamosa airborne inoculum were analyzed over space and time during 2 years (2002 and 2004) in a nonprotected experimental field, using a 6 x 8 lattice of quadrats of 10 x 10 m each. A similar experiment was conducted in 2004 and 2006 in a commercial field managed for Botrytis leaf blight using a 5 x 5 lattice of quadrats of 25 x 25 m each. Each quadrat was monitored weekly for lesion density (LD) and aerial conidium concentration (ACC). The adjustment of the Taylor's power law showed that heterogeneity in both LD and ACC generally increased with increasing mean. Unmanaged epidemics were characterized in either year, with aggregation indices derived from SADIE (Spatial Analysis by Distance Indices). For LD, the aggregation indices suggested a random pattern of disease early in the season, followed by an aggregated pattern in the second part of the epidemic. The index of aggregation for ACC in 2002 was significantly greater than 1 at only one date, while it was significantly greater than 1 at most sampling dates in 2004. In both years and for both variables, positive trends in partial autocorrelation were observed mainly for a spatial lag of 1. In 2002, the overall pattern of partial autocorrelations over sampling dates was similar for LD and ACC with no significant partial autocorrelation during the first part of the epidemic, followed by a period with significant positive autocorrelation, and again no autocorrelation on the last three sampling dates. In 2004, there was no significant positive autocorrelation for LD at most sampling dates while for ACC, there was a fluctuation between significant and non-significant positive correlation over sampling dates. There was a significant spatial correlation between ACC at given date (t(i)) and LD 1 week later (t(i + 1)) on most sampling dates in both 2002 and 2004 for the unmanaged and managed sites. It was concluded that LD and ACC were not aggregated in the early stage of epidemics, when both disease intensity and airborne conidia concentration were low. This was supported by the analysis of LD and ACC from a commercial field, where managed levels of disease were low, and where no aggregation of both variables was detected. It was further concluded that a reliable monitoring of airborne inoculum for management of Botrytis leaf blight is achievable in managed fields using few spore samplers per field.

Effect of Microsphaeropsis ochracea on Production of Sclerotia-borne and Airborne Conidia of Botrytis squamosa

Onion leaf blight, caused by Botrytis squamosa (Walker), is a destructive disease of onion. Conidia produced on overwintered sclerotia are the main source of initial inoculum, and those produced on lesions are responsible for secondary inoculum build-up. The biological control agent Microsphaeropsis ochracea (Carisse & Bernier) was evaluated for its ability to control sclerotia-borne inoculum, to colonize onion leaves and reduce the production of conidia under field conditions. Colonisation by M. ochracea of onion leaves at different growth stages was monitored and its effect on B. squamosa sporulation on necrotic leaves was evaluated. Onion plots were treated with either Dithane® or with M. ochracea at 7–10-day intervals and according to inoculum production index (IPI). The concentration of airborne conidia and the number of lesions per leaf, on 20 plants per plot, were evaluated throughout the cropping season. The number of conidia produced per sclerotium treated with M. ochracea, was reduced by 75.5%. In the field, M. ochracea colonised only senescent or necrotic leaves and reduced the production of conidia on these leaves by an average of 82% as compared with untreated leaves. Best disease control was obtained by Dithane®, followed by M. ochracea applied at 7–10-day intervals. For the three years of the study, there were no significant differences in airborne concentrations of conidia in plots treated at 7–10-day intervals with Dithane® or M. ochracea. Fall application of M. ochracea could be used as a sanitation practice to reduce initial inoculum or as a part of an IPM program during the season.

Quantification of Airborne Inoculum as an Aid in the Management of Leaf Blight of Onion Caused by Botrytis squamosa.

Botrytis leaf blight, caused by Botrytis squamosa, is a common and frequently damaging disease of onion crops, but the severity of epidemics varies widely from year to year. The disease is initiated and spread by airborne conidia. The relationship between airborne conidium concentration (ACC) and lesion development was studied in the field. A linear relationship was found between ACC and number of lesions per leaf: ACC values of 10 to 15 and 25 to 35 conidia m-3 were associated with 1 and 2.5 lesions per leaf, respectively. In 2000 and 2001, at three sites, four different criteria were used to start a fungicide spray program and their effect on epidemic development was compared with that of a grower's conventional schedule. The criteria were: at the fourth-true-leaf growth stage; according to an inoculum production index; when the ACC reached 10 to 15 conidia m-3; and when the ACC reached 25 to 35 conidia m-3. A nonsprayed control plot was included in the trial. Fungicide programs started when the ACC reached 10 to 15 conidia m-3 were as effective as the conventional program, but used fewer sprays. A fungicide spray program based on measurements of ACC and disease severity was evaluated in 2002 and 2003 in five and three commercial onion fields, respectively. At each site, half of the field was sprayed according to the grower's schedule and, in the other half, fungicide sprays were initiated when a threshold of 10 to 15 conidia m-3 or five lesions on the lower leaf (whichever came first) was reached. Overall, the number of fungicide applications was reduced by 75 and 56% in 2002 and 2003, respectively, without causing significant yield reduction. In both years, the reduction in number of fungicide applications was due mainly to the delay in initiation of the fungicide program.

Ochratoxin A in airborne dust and fungal conidia.

Farm workers are often exposed to high concentrations of airborne organic dust and fungal conidia, especially when working with plant materials. The purpose of this investigation was to study the possibility of exposure to the mycotoxin ochratoxin A (OTA) through inhalation of organic dust and conidia. Dust and aerosol samples were collected from three local cowsheds. Aerosol samples for determination of total conidia and dust concentrations were collected by stationary sampling on polycarbonate filters. Total dust was analysed by gravimetry, and conidia were counted using scanning electron microscopy. A method was developed for extraction and determination of OTA in small samples of settled dust. OTA was extracted with a mixture of methanol, chloroform, HCl, and water, purified on immunoaffinity column, and analysed by ion-pair HPLC with fluorescence detection. Recovery of OTA from spiked dust samples (0.9-1.0 microg/kg) was 74% (quantitation limit 0.150 microg/kg). OTA was found in 6 out of 14 settled dust samples (0.2-70 microg/kg). The total concentration of airborne conidia ranged from < 1.1 x 10(4) to 3.9 x 15(5) per m3, and the airborne dust concentration ranged from 0.08 to 0.21 mg/m3. Conidia collected from cultures of Penicillium verrucosum and Aspergillus ochraceus contained 0.4-0.7 and 0.02-0.06 pg OTA per conidium, respectively. Testing of conidial extracts from these fungi in a Bacillus subtilis bioassay indicated the presence of toxic compounds in addition to OTA. The results show that airborne dust and fungal conidia can be sources of OTA. Peak exposures to airborne OTA may be significant, e.g., in agricultural environments.

Seasonal fluctuations in concentration of airborne conidia of Cercospora zebrina and incidence of Cercospora disease in subterranean clover

Seasonal fluctuations in concentration of airborne conidia of Cercospora zebrina and incidence of Cercospora disease in subterranean clover