Ecology of Powdery Mildews – Influence of Abiotic Factors on their Development and Epidemiology

Abstract Powdery mildews are some of the most common and dangerous biotrophic plant pathogens. They attack more than 10, 000 plant species, and can be found mainly in temperate and sub-tropical zones. This review evaluates the effects of most important abiotic conditions on powdery mildew namely temperature, humidity, light quality, air composition (mainly CO2 and ozone concentration) and movement. With the most intensively studied factors, temperature and humidity, powdery mildew species vary in their requirements, this variation occurring in different phases of their life cycle. Generally, temperatures between 13 and 30 °C were optimal for their development, with conidial germination being the least and sporulation the most affected part of the life cycle and lower marginal temperatures only prolonging the latent period. The role of moisture in their development is more elusive; free moisture inhibits dispersal and germination of conidia and extension of hyphae of most powdery mildews. However, for further development high relative humidity is preferred and free water is required for release and dispersal of ascospores. Light most affects the pathogen indirectly through its effect on the host. Although germination and appressorial maturation is possible under low illumination and darkness, light is needed for completion of the disease cycle. A suitable photoperiod (alternating day and night) favors optimal development, e.g., continuous light reduces infection. The effect of CO2 concentration is complex; sometimes an increased concentration of CO2 causes more intensive disease, sometimes less or no effect at all. Most environmental factors also affect the host thus affecting the pathogen indirectly; other factors (e.g. UV or CO2) mainly directly affect the pathogen. Hypotheses on the possible effect of predicted climate change on pathosystems are discussed.