Modelling the development of powdery mildew in cut-roses

MSc-thesis abstract (submitted 13 March 2017): Powdery mildew (Podosphaera pannosa var. rosa) is the most severe disease in a cut-rose canopy.
The development of powdery mildew can be described with the disease triangle which consists of the availability of a susceptible host, presence of a virulent pathogen and favourable environmental conditions. The pathogen life cycle phases are germination, lesion growth and sporulation. These phases are influenced by temperature and relative humidity. Till now, no model has been developed for predicting the development of powdery mildew, which is the main objective of this research project.

Experiments were conducted to quantify the effect of temperature and relative humidity on the development of powdery mildew severity on leaves of cut-rose stems. Due to a low disease pressure and the effect of leaf age on resistance against powdery mildew, no conclusions could be drawn from these experiments.

An explanatory model was developed based on data available in literature which combines the effect of temperature and relative humidity on the powdery mildew life cycle phases. The predicted optimal conditions for powdery mildew development were at a temperature of 24°C and a relative humidity of 86%. Fluctuating temperature and relative humidity conditions resulted in a lower development rate.

Differences between data from macroclimate and microclimate conditions resulted slight in a different predicted development of powdery mildew due to leaf wetness.
The developed model provided a basic framework to see the effect of constant temperature and relative humidity on the development of powdery mildew.

Keywords: Podosphaera pannosa var. rosa, cut-rose, disease development model, temperature, relative humidity, macroclimate, microclimate