Factors Affecting Airborne Concentrations of Podosphaera xanthii Conidia and Severity of Gerbera Powdery Mildew

To determine the factors affecting airborne conidial concentrations of Podosphaera xanthii Braun and Shishkoff and powdery mildew severity in greenhouse-grown potted gerbera (Gerbera jamesonii H. Bolus), airborne concentrations of conidia were monitored in a glass and polyethylene greenhouse. Temperature, relative humidity, and leaf wetness were recorded onsite, and the percentage of foliage with visible disease was assessed weekly at the glasshouse and every 2 weeks at the polyethylene greenhouse (1 to 10 visual rating scale). Peak airborne conidial concentrations occurred at 0800/1600 and 0900/1400 HR at the glasshouse and polyethylene greenhouses, respectively. Few conidia were sampled between 2200 and 0500 HR at either greenhouse. Worker activity was associated with conidial release in the glasshouse, but not in the larger polyethylene greenhouse, and worker activity may have influenced the daily periodicity of conidial concentrations. Airborne conidial concentrations were not related to environmental conditions in the same hour as conidial detection. An increase in disease severity was positively related to relative humidity and negatively related to leaf wetness at both greenhouses; in addition, temperature was negatively related to an increase in disease severity in the glasshouse. In light of the results of this study, frequent scouting and fungicide applications for powdery mildew are advised. Wide plant spacing and adequate ventilation are also recommended to reduce relative humidity in the microclimate. Michigan ranks fifth in potted gerbera daisy (Gerbera jamesonii H. Bolus) production after Texas, California, Illinois, and Florida and also produces a limited number of gerbera daisies for cut flowers. Potted gerbera production represents a $730,000 industry to Michigan growers (USDA, 2010). Powdery mildew is the most common foliar disease of gerbera daisies, although they are also susceptible to other foliar diseases including Botrytis blight (Kerssies, 1993). Powdery mildew on gerbera may be caused by Golovinomyces (syn. Erysiphe) cichoracearum (DC.) V.P. Heluta (Troisi et al., 2010) or Podosphaera xanthii Braun and Shishkoff (syn. P. fusca) (Chen et al., 2007; Kloos et al., 2004). Both of these fungi form a network of hyphae over the plant surface, and entire leaves may be covered with white, talcum-like colonies; lower leaves may drop and stems and flowers may display pathogen signs during severe infections (Chase et al., 1995) (Fig. 1). Because the signs of powdery mildew are conspicuous and unsightly, infected plants may become unmarketable in a short period of time (Daughtrey et al., 1995). In a greenhouse environment, conidia are responsible for epidemic initiation and secondary spread occurs through air currents and water splash (Daughtrey et al., 1995). It was hypothesized that P. xanthii releases conidia to the air after an increase or decrease in relative humidity or exposure to infrared radiation (Jarvis et al., 2002), but direct studies have not been completed to confirm or reject this hypothesis. Studies with other powdery mildew pathogens have associated airborne conidial concentrations with changes in relative humidity in the field (Adams et al., 1986; Jarvis et al., 2002) and in a greenhouse environment (Byrne et al., 2000). A daily fluctuation in airborne conidial concentrations has been observed for many powdery mildew pathogens including Oidium sp. in greenhouses (Byrne et al., 2000), Podosphaera leucotricha in an apple orchard (Sutton and Jones, 1979), and Podosphaera aphanis in strawberry fields (Blanco et al., 2004) with many conidia being released midday and few at night. Although the relationships between environmental conditions and conidial release have been reported for several powdery mildew pathosystems (Byrne et al., 2000; Childs, 1940; Sutton and Jones, 1979), these relationships have not been established for the P. xanthii–gerbera pathosystem. Some commercial gerbera cultivars are tolerant to powdery mildew, but the most popular commercial gerbera cultivars are susceptible (Hausbeck et al., 2003; Sconyers and Hausbeck, 2005). Powdery mildew of gerbera daisies in the greenhouse is currently managed using frequent fungicide applications. Growers begin fungicide applications before the appearance of disease symptoms on gerbera and reapply fungicides frequently. Systemic fungicides such as azoxystrobin and myclobutanil provide better control than contact fungicides (Hausbeck et al., 2003, 2006). Although frequent scouting of the gerbera crop for early signs of powdery mildew may be useful to time fungicide applications, scouting is time-consuming and difficult, especially for large-scale growers. Powdery mildew diseases on flowering potted plants are thought to develop best at greater than 95% relative humidity and at an optimum temperature of 20 C in a glasshouse environment (Daughtrey et al., 1995). Better understanding the effects of various environmental factors on spore release and disease development could provide a framework for an integrated pest management approach that may reduce the number of fungicide sprays needed to produce a healthy crop. A reduction in fungicide applications or better timing of fungicide applications are desirable because fungicides can be costly and pose a potential human health and environmental risk. The objective of this study was to determine the influence of environmental conditions (temperature, relative humidity, leaf wetness, and worker activity) on airborne P. xanthii conidial concentrations and severity of powdery mildew in greenhouse-grown potted gerbera. Materials and Methods Plant material. Seven-week-old ‘Festival Dark Eye Golden Yellow’ gerbera plugs highly susceptible to powdery mildew (Sconyers and Hausbeck, 2005) were obtained from a commercial supplier and transplanted into 15-cm diameter plastic pots containing commercial soilless potting mix composed of 40% perlite and 60% sphagnum peatmoss (Baccto Professional Planting Mix; Michigan Peat Company, Houston, TX). Plants (375 at the glasshouse, 209 at the polyethylene greenhouse) were placed with pots touching each other on three adjacent benches in each of two research greenhouses at Michigan State University (MSU). One greenhouse was located on the main campus of MSU at the Plant Science Greenhouses and was glass (9.0 · 8.5 m). The other greenhouse was located at the Horticultural Farm in Holt, MI, and was covered with polyethylene (6.5 · 36.7 m). A 14-h artificial photoperiod was used to promote plant growth and flowering (Ball, 1985). Flowers were cut periodically to maintain plant vigor. Plants were hand-watered using Received for publication 27 Dec. 2011. Accepted for publication 15 June 2012. This study was supported by funding from the American Floral Endowment and the Floriculture and Nursery Research Initiative of the Agricultural Research Service under Agreement #58-1907-0-096. We thank S. Linderman, B. Harlan, M. Ziolkowski, A. Worth, R. Merrill, J. Witer, A. Selden, K. Bigoness, and K. Smith for their technical assistance with this project. Visiting Research Associate. Former Visiting Research Associate. Professor and Extension Specialist. To whom reprint requests should be addressed; e-mail [email protected]. 1068 HORTSCIENCE VOL. 47(8) AUGUST 2012 a hose and water breaker as needed; care was taken to avoid wetting the foliage. Plants were fertilized during watering with 200 ppm of Peter’s 20N–20P–20K general purpose fertilizer (Scotts-Sierra Horticultural Products Co., Marysville, OH) at 2to 3-d intervals. Substrate pH was maintained between 5.8 and 6.2 and electrical conductivity between 1.2 and 1.5 mmhos/cm by treating water as necessary with phosphoric acid. Temperatures in both greenhouses were set at 20 to 22 C and venting occurred when temperature exceeded 22 C. To manage root rot at the polyethylene greenhouse, drenches of etridiazole/thiophanate-methyl 0.8 g product/liter (Banrot 40WP; ScottsSierra Crop Protection Co.) and mefenoxam 0.2 mL product/L (Subdue MAXX 21.3EC; Syngenta Crop Protection, Inc., Greensboro, NC) were applied in alternation every 6 weeks (three treatments). Drenches were not applied at the glasshouse, because all of the leaves displayed P. xanthii signs before 6 weeks following exposure to inoculum. Monitoring airborne conidia. Concentrations of airborne conidia were monitored in each greenhouse using a 7-d volumetric spore sampler (Burkard Mfg. Co. Ltd., Rickmansworth, Hertfordshire, U.K.) from 2 Mar. to 12 Aug. 2004 in the glasshouse and from 13 July to 8 Nov. 2004 in the polyethylene greenhouse. The spore sampler was placed in the center of the center greenhouse bench with the orifice 0.5 m above the bench surface and operated at a flow rate of 10 L min. Conidia were impacted onto Melanex tape (Burkard Scientific, U.K.), which had been coated with a polyvinyl alcohol (Airvol; Burkard Scientific) and phenol mixture (35 g polyvinyl alcohol, 25 mL glycerol, 50 mL distilled water, 2 g phenol), dried overnight, and subsequently coated with an adhesive mixture of petroleum jelly and paraffin (9:1, wt/wt) dissolved in sufficient toluene ( 3 mL per 50 g of mixture) to result in a viscous consistency. Tapes were removed weekly, cut into 48-mm lengths, scored at hourly intervals, lightly stained with aniline blue in lactic acid (28 mg aniline blue, 20 mL distilled water, 10 mg glycerol, and 10 mL 85% lactic acid, diluted with 5 drops to 25 mL of distilled water), and mounted on glass slides beneath 22 · 50-mm coverslips. Each hourly interval on each slide was scanned vertically at 100· magnification. P. xanthii conidia were identified based on morphological characteristics including the presence of inclusion bodies (Braun et al., 2002) and enumerated. Conidial counts were converted to conidia per cubic meter of air sampled per hour. Collection of environmental and disease data. Temperature ( C), relative humidity (%), and leaf wetness (0 to 15 scale) were recorded in each greenhouse every 15 min by a WatchDog 450 data logger (Spectrum Technologies Inc., Plainfield, IL) and an external leaf wetness sensor set at a 45 angle facing north placed within the plant canopy. Hourly averages were calculated for temperature and relative humidity. Before entering greenhouses, personnel documented date, time of day, and activity performed (applying pesticides, watering, pruning, assessing plant status). Plants were exposed to inoculum on 27 Feb. 2004 (glasshouse) and 12 July 2004 (polyethylene greenhouse) by placing one severely infected gerbera plant ( 100% of foliage showed pathogen signs) with actively sporulating P. xanthii colonies in the center of each greenhouse bench adjacent to healthy gerberas. Inoculum-bearing plants remained on the greenhouse bench for the duration of the experiment. Powdery mildew disease severity was assessed every week in the glasshouse (six ratings, 375 plants) and every 2 weeks (seven ratings, 209 plants) in the polyethylene greenhouse based on a visual estimation and rated on a scale of 1 to 10, where 1 = no disease, 2 = trace to 10%, 3 = 11% to 20%, 4 = 21% to 30%, 5 = 31% to 40%, 6 = 41% to 50%, 7 = 51% to 60%, 8 = 61% to 70%, 9 = 71% to 80%, and 10 = 81% to 100% of foliage infected. Ratings were conducted more frequently in the glasshouse because disease progressed more quickly in this greenhouse than in the polyethylene greenhouse. Statistical analysis. Statistical analyses were performed using the SAS statistical package Version 9.1 (SAS Institute, Inc., Cary, NC). The GAM procedure of SAS used b-spline and local regression methods to model smoothed seasonal trends for daily conidial concentrations and daily temporal trends for non-zero hourly conidial concentrations in each greenhouse. The smoothing parameter was chosen to minimize the generalized crossvalidation criterion. To determine if conidial release was more likely during periods with worker activity, the FREQ procedure was used to conduct a one-sided Fisher’s exact test to determine if the probability of conidial release and detection was greater in the activity group vs. the time periods without worker activity, and an odds ratio was used to estimate relative risk of conidial dispersal during periods of worker activity. Hourly conidial concentrations and corresponding environmental conditions were temporally autocorrelated time series (7). Hence, time series analyses were used to study changes in these variables through time and uncover patterns and relationships in the data. Using the ARIMA procedure, regression analyses were completed, regressing conidial concentrations on covariates (environmental conditions) assuming an autoregressive (AR) moving average (MA) error structure (6, 42). The AR portion of the error model accounts for the dependency of the current value of airborne conidial concentrations on past values, and the persistence of error terms beyond one observation for conidial concentrations is the MA portion of the error model (Box and Jenkins, 1976). The mean was subtracted from each time series before analysis and first-order differencing was completed for all of the variables (including covariates) to induce stationarity [ARIMA (p,1,q)]. Differencing removes trends from the time series by taking the difference of the series from one period to another (I = the number of lags) (8). Stationarity was evaluated using plots of the expected values of the series and its autocorrelation function to ensure temporal trends were removed. Lags, time periods before the current time period, of up to 5 h were permitted for each environmental factor to allow for the influence of environment on pathogen sporulation in the hours before conidial dispersal. Series of environmental variables were prewhitened using an autoregressive moving average (ARMA) filter to achieve a white noise (random) residual series before crosscorrelation (Box and Jenkins, 1976). The goodness of fit of models was measured using the Akaike information criterion (Akaike, 1974). Variables with P # 0.05 were considered statistically significant. The GAM procedure was used to develop models relating the increase in disease severity Fig. 1. Colonies of powdery mildew (Podosphaera xanthii) on (A) leaves of gerbera and (B) a gerbera flower. (C) Chains of Podosphaera