Temporal variation in airborne spore concentration of Chrysomyxa rhododendri: correlation with weather conditions and consequences for Norway spruce infection

Abstract

SummaryThe fungal pathogen Chrysomyxa rhododendri undergoes a host shift between Rhododendron spp. and Picea abies and has a considerable negative impact on the latter by infecting the new sprouting needles and reducing photosynthesis, growth and seedling survival. Repeated high infection rates in the Central European Alps were reported in recent years, and although the life cycle of the pathogen is well understood, knowledge on temporal patterns of host infection, spore dispersal and influence of weather conditions is limited. We analysed the period of needle infection by a staggered application of fungicide on twigs of Norway spruce. Seasonal and diurnal patterns of spore dispersal were investigated using a Burkard volumetric spore trap, and airborne spore concentrations were correlated with several weather parameters. Needle infection occurred within 3 weeks (19 June–9 July). Basidiospores (infecting Norway spruce) were trapped from May to July, and airborne spore concentration was positively correlated with air temperature, global radiation and wind speed, and negatively with air humidity and precipitation. Aeciospores (infecting rhododendrons) appeared from August to October, and high concentrations were significantly associated with rain events. Needle infection started with bud sprouting and was limited either by decreasing spore concentrations or morphological and chemical differentiation of the needles, which prevented infection. Both spore types showed distinct periods of dispersal based on the life cycle of the pathogen, and the variability in concentration within these periods was explained by local weather conditions. The correlation analysis indicated different mechanisms of spore dispersal for both spore types and, with respect to global warming, more favourable conditions for the pathogen in future. The results can explain the high year‐ and site‐specific variation in Chrysomyxa infection intensities of P. abies and may be helpful for developing successful strategies to control the pathogen and protect alpine spruce forests.