Monitoring of plant and airborne inoculum of Sclerotinia sclerotiorum in spring oilseed rape using real-time PCR

Sclerotinia stem rot of spring oilseed rape (Brassica napus) is caused by Sclerotinia sclerotiorum. In Sweden, the disease leads to severe crop damage that varies from year to year. A real-time PCR assay was developed and used to determine the incidence of S. sclerotiorum DNA on petals and leaves of spring oilseed rape as well as in air samples, with the aim of finding tools to improve precision in disease risk assessment. Five field experiments were conducted from 2008 to 2010 to detect and study pathogen development. Assessments of stem rot showed significant differences between experimental sites. The real-time PCR assay proved fast and sensitive and the relationship between percentage of infected petals determined using a conventional agar test and the PCR assay was linear (R 2 > 0.76). There were significant differences in S. sclerotiorum incidence at different stages of flowering. The incidence of S. sclerotiorum DNA on the leaves varied (0-100%), with significantly higher incidence on leaves at lower levels. In one field experiment, S. sclerotiorum DNA was not detected on petals during flowering, whereas the pathogen was detected on leaves, with a corresponding stem rot incidence of 7%. The amount of S. sclerotiorum DNA in sampled air revealed that spore release did not coincide with flowering on that experimental site. Thus, using a real-time PCR assay to determine the incidence of S. sclerotiorum on oilseed rape leaves, rather than on petals, could potentially improve disease risk assessment.