Development of molecular detection methods for early warning of the hazelnut powdery mildew pathogen Erysiphe corylacearum

Official URL: https://dx.doi.org/10.17660/ActaHortic.2023.1379.50

Powdery mildews are an agronomically important group of fungal plant pathogens, which cause serious diseases on numerous crop species. European hazel (Corylus avellana L.) has long been known to be colonized by the mildew fungus Phyllactinia guttata, producing relatively mild symptoms late in the growing season. However, a new and more destructive powdery mildew disease was first observed in orchards in eastern Turkey in 2013, and was subsequently found to be caused by Erysiphe corylacearum, a fungus previously observed only on wild relatives of C. avellana. Mildew disease caused by E. corylacearum develops early in the growing season, reducing yields and quality by causing premature leaf and nut fall. Since its first appearance the disease has become endemic throughout Turkey and other hazelnut growing areas around the Black Sea, and also spread to most of Europe. Protection of affected orchards requires regular fungicide treatment, ideally starting before the first symptoms appear. Therefore, we aimed to develop rapid molecular methods for the early detection of this pathogen to facilitate efficient treatment. The internal transcribed spacer (ITS) regions of the rRNA are commonly used for discrimination between fungal species by the ‘DNA barcoding’ method. E. corylacearum fungi were collected from several heavily mildewed orchards and their ITS regions sequenced. These were then compared with ITS regions from other Erysiphe species, and the most diverse segments used to design species-specific quantitative PCR assays. Using this approach, the presence of the pathogen could be specifically detected from ~50 genome copies, approximately 100 times fewer than when symptoms become visible to the naked eye. This technique was validated by testing samples collected in multiple orchards early in the growing season, before and during the development of disease symptoms.