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.
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