Damage in needle tissues after infection with Chrysomyxa rhododendri increases cuticular conductance of Picea abies in winter.

Chrysomyxa rhododendri is a rust which infects Picea abies growing near the alpine timberline. Attacked needles are normally shed, but few remain on shoots. We hypothesised that these needles increase transpiration of Picea during winter. Partly damaged, completely damaged and healthy needles of an infected tree as well as healthy needles of a resistant tree were compared in a microscopy analysis, and needle conductance of shoots was measured gravimetrically. Despite needle shedding, more than 6% of needles remaining on infected tree shoots were damaged. Partly damaged needles showed local brownish areas in the periphery and completely damaged needles necrotic parenchyma and epidermal tissues. Cuticular conductance of affected shoots was up to 25.23 +/- 2.75 mmol m(-2) s(-1) at moderate water potential and thus twofold higher than in the resistant tree. Needle shedding reduces negative effects of Chrysomyxa infections during summer, but remaining damaged needles impair tree water relations in winter.