Characterization of Sclerotinia sclerotiorum airborne inoculum, the widespread agent of white mould disease

Abstract

A means to rationalize the use of fungicides for crop protection and to make agriculture friendlier to environment and human health is to develop disease-risk forecasting systems based on the assessment of airborne inoculum abundance. Sclerotinia sclerotiorum, the pandemic agent of white mould disease, is disseminated via the atmosphere in the form of ascospores. These airborne spores are the primary source of inoculum initiating the majority of epidemics. However for witloof chicory (Cichorium intybus var. foliosum L.), there is no data about airborne inoculum of S. sclerotiorum, which makes it difficult to develop a forecasting model. In the present study we characterized the temporal evolution of the abundance and of the genetic characteristics of S. sclerotiorum inoculum on a witloof chicory field located in Northern France over a three-year period. To our knowledge, this study provides the first quantification of viable airborne populations of this fungus in witloof chicory crops. Moreover it provides the first genetic characterization of S. sclerotiorum airborne inoculum. The results show that viable ascospores were present through 80% of the sampling dates. A significant correlation between abundance of airborne ascospores and local relative humidity suggests a local origin of inoculum. However, the existence of a slight genetic differentiation between isolates carried by air masses coming from the West and from the North-West is compatible with the hypothesis of a distant origin of S. sclerotiorum inoculum. We discuss the additional studies that are envisioned to clarify the origin of S. sclerotiorum airborne inoculum in witloof chicory fields.

Predicting field diseases caused by Sclerotinia sclerotiorum: A review

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“…For ascospores, most studies report no association with 3‐day to 3‐week air temperature summaries (Fall, Willbur, et al, 2018; Foster et al, 2011; Qandah & del Río Mendoza, 2011) or with air temperatures from the same day to 3 days prior to ascospore counts (Bourdôt et al, 2001; Leyronas et al, 2019). However, some studies found significant correlations with air temperature.…”

“…However, some studies found significant correlations with air temperature. In witloof chicory, daily ascospore concentration was positively correlated with the minimum air temperature (ρ = 0.2) on the day of and day before ascospore release (Leyronas et al, 2019). In a non‐peer‐reviewed study, hourly and daily air temperature metrics were negatively correlated with ascospore release in dry bean (Archer & Yuan, 2004).…”

“…For ascospores, the influence of precipitation was more variable: mean quantities of precipitation at 3-to 7-day intervals were positively associated with days of peak ascospore release in canola (Qandah & del Río Mendoza, 2011), but precipitation was negatively correlated with ascospore release the following day in seed alfalfa fields (Reich et al, 2017). Other summaries of precipitation over periods of zero days to 3 weeks before spore counts found no relationship in carrot (Foster et al, 2011), seed alfalfa (authors' unpublished data) and witloof chicory (Leyronas et al, 2019).…”

“…Pearson r values ranging from 0.26 to 0.52 (Leyronas et al, 2019). In seed alfalfa, daily ascospore levels were strongly associated with changes in RH the day before ascospore collection; furthermore, high RH the day before ascospore collections was associated with lower numbers of ascospores (Reich et al, 2017).…”