Fungal proliferation and calcium accumulation in the orange slime of Cornus controversa

A giant dogwood (Cornus controversa) was bleeding transparent sap that turned into gummy orange slime with a pungent odor on the bark in early April. The tree exudation originated from a branch wound that occurred during the most recent spring pruning. Scanning electron microscopy revealed the intact bark surface was irregularly undulating and covered with at least three types of epicuticular waxes: polygonal rodlets; polygonal tubules; and threads, whereas the dried orange plaques were covered with fungi, possibly the co-occurrence of yeast and filamentous forms. The yeast forms were ovoid, lemon- or club-shaped, and approximately 10 μm in length. Obviously constricted septa and clamp connections were rarely detected on the filamentous forms. No bacterial proliferation was detected on the orange slime. X-ray microanalysis revealed a steep increase in calcium concentration in the orange plaques compared with that in the intact bark. These results suggest that bark-dwelling dimorphic red yeasts may constitute one of the dominant microbial consortia of the orange slime of giant dogwood. Given the high amount of sugars and calcium in the sap, and tolerance to diurnal temperature fluctuations in early spring, the red yeasts may be specialist species that thrive under a limited range of environmental conditions.