Detecting airborne inoculum to forecast oilseed rape diseases

Abstract: Many fungal diseases of oilseed rape are caused by air-dispersed spores. The timing of spore release changes each season due to the weather because the maturation and release of spores is usually affected by moisture (rain, dew and relative humidity) and temperature. Most airborne spores are dispersed only short distances from a source but many others still travel long distances, although spore concentrations and viability in air reduces over time. Spore deposition results in individual disease foci when occurrence of viable inoculum and infection conditions are rare; as a gradient in a crop when a large number of spores are produced from a nearby source; or as a uniform infection when there is a large but distant source or multiple local sources of inoculum. Epidemics of common monocyclic diseases of widespread crops, such as Phoma stem canker and Sclerotinia stem rot of oilseed rape are usually initiated by airborne spores produced either a long distance from the crop or from multiple sites throughout a region. Therefore it should be possible to predict such epidemics regionally using suitably located spore samplers in order to enhance integrated disease control methods. Appropriate DNA-based diagnostic methods can be integrated with many different types of air samplers and are now providing new information about species that previously could not be identified accurately by visual microscopy methods. For example, a new diagnostic for Sclerotinia sclerotiorum (Rogers et al., 2008) has shown potential for warning of the presence of airborne inoculum. Furthermore, where reliable climate-based disease forecasts have been developed (e.g for Leptosphaeria maculans), air sampling integrated with DNA-based diagnostics can also provide useful information at the sub-species level, to monitor populations for traits such as the development of fungicide resistance (in a similar way to that found with strobilurin resistance in Mycosphaerella graminicola (Fraaije et al., 2005) or changes to the pathogen race-structure in response to deployment of resistant cultivars. The optimal location of air samplers depends on how widespread the host crop is and how common the pathogen is, since air samples, particularly at ground level are heavily weighted in favour of spore types produced nearby. Further work is required to investigate the spatial variability in spore numbers in air at different sites; how changes in numbers of airborne spores at particular heights above or distances away from crops are related to subsequent regional disease severity; and to develop methods to analyse samples and disseminate results rapidly.