Photosynthesis in rust-infected adult Norway spruce in the field.

•  Norway spruce (Picea abies) has been infected by the needle rust Chrysomyxa rhododendri in many regions of the Central Alps with increasing intensities in recent years. Basidiospores formed on leaves of the main host, Rhododendron sp., only penetrate into developing current-year needles of Norway spruce, which leads to shedding of infected needles in the autumn. •  Net photosynthesis at ambient CO , net photosynthesis vs internal CO , pigment and nitrogen concentration were compared, as well as annual radial growth of severely infected and healthy adult Norway spruce under field conditions. •  Net photosynthesis at ambient CO was significantly decreased (-47%) in infected current-year needles. However, net photosynethesis in uninfected, older (1-yr-old) needles increased, expressed per needle area, per gram of chlorophyll a + b as well as per gram of nitrogen. Annual ring width analysis revealed growth reductions (-25%) of the infected tree in periods of repeated C. rhododendri infections. •  Although adult Norway spruce trees dispose of compensatory photosynthetic potentials of older needle age classes, C. rhododendri infections cause a substantial decrease in annual radial growth. If high infection intensities persist we predict serious damage to the subalpine spruce forest.