High growth

While at about 2000 metres altitude in Austria, I found lumps growing on the leaves of a plant (see photo). The smaller lumps were yellow, but they changed to orange, then red, as they got larger. They were growing directly from the leaves, not the stem. And they were also very cold – there was condensation on them. What are these fruit-like growths? ■  The plant looks like one of the Alpine rhododendron species, probably Rhododendron ferrugineum (the rusty-leaved alpenrose or the snow rose). This plant is subject to a number of fungal diseases, one of which causes a leaf gall like the one shown. It is caused by the fungus Exobasidium rhododendron. The fungus infects the plant and releases plant hormones which cause the galling. The gall forms a perfect place for the fungus to grow and reproduce. It is rarely a serious disease and does little damage to the plant. It is generally prevented by snapping the galls off the plant to prevent spore release by the fungus. Peter Scott Hove, East Sussex, UK