Effects of elevated temperature and fluorine pollution on relations between the pedunculate oak (Quercus robur) and oak powdery mildew (Microsphaera alphitoides)

Effects of elevated temperature and soil pollution with fluorine on host-pathogen relations were studied in seedlings of the pedunculate oak (Quercus robur L.) inoculated with oak powdery mildew (Microsphaera alphitoides Griff. et Maubl.) and control seedlings. The plants were grown for 1month in elevated temperature (on average by 1.6°C) and soil pollution with sodium fluoride (330 ppm F). The above factors did not have any significant effect on nitrogen content of leaves or on concentrations of metabolites favourable to growth and development of the fungal pathogen (total non-structural carbohydrates, including soluble carbohydrates and starch) and those unfavourable to fungi (soluble phenols, condensed tannins and lignins). The elevated temperature and fluorine pollution did not affect the leaf infection rate. However, a significant temperature × pollution interaction was observed in inoculated seedlings. At the elevated temperature, fluorine caused a less severe infection by powdery mildew. This could be due to a direct toxic effect of fluorine on the pathogen or by an indirect influence, resulting from changes in levels of other metabolites, which were not analysed in this study. The inoculation of oak seedlings with powdery mildew caused a decline in the carbohydrate content of leaves but did not have any significant effect on levels of other analysed metabolites. However, it significantly affected the distribution of phenols and lignins in oak leaves. Those compounds accumulated within necrotic lesions and in adjacent cells. Our results do not enable drawing definite conclusions on effects of a slight rise in temperature and a relatively low level of fluorine pollution of the soil on relations between the pedunculate oak and oak powdery mildew. Lower values of the leaf infection rate in seedlings growing in elevated temperature and fluorine pollution suggest that in warmer years a lower level of infection by M. alphitoides may be expected in areas affected by fluorine pollution.