A new method for quantifying Puccinia triticina airborne inoculum in wheat fields

Wheat leaf rust caused by Puccinia triticina is regularly observed in Belgian fields. This fungus produces wind-dispersed urediospores that infect host plants many kilometers from their source plant, but the origin of the primary inoculum and the factors involved in disease occurrence and severity are not well known. In order to better understand the epidemiology of the disease, the dispersion of P. triticina airborne inoculum and the relationship with disease incidence in fields were studied. A network of Burkard 7-day spore-recording traps was set up in wheat fields in the Walloon region, in Belgium, to monitor the airborne inoculum. Total DNA from each fragment of spore trap tape, corresponding to 1 day of sampling, was extracted and the quantity of P. triticina on the tape fragments was assessed using a specific real-time polymerase chain reaction (PCR) assay. The disease incidence was recorded weekly in experimental fields from March 2012 onwards. Initial analysis revealed that the real-time PCR assay was highly specific and repeatable. The detection threshold was determined at 7 spores per m3. This method (spore traps coupled with real-time PCR) allows airborne inoculum to be detected before the appearance of symptoms in fields and could be used in a model for predicting wheat leaf rust epidemics.