Light and scanning electron microscopy of aecia and aeciospores of Cronartium ribicola on Pinus koraiensis branch tissues.

Morphological characteristics of aecia and aeciospores of Cronartium ribicola on Pinus koraiensis branch tissues were investigated using light and field emission scanning electron microscopy (FESEM). Mature P. koraiensis trees in Jeongseon, Korea, showed yellowish aecia on stems and branches. Aecia and surrounding tissues were excised from the lesions and vapor-fixed for FESEM imaging, which revealed morphology including intact blister-shaped, flattened, and burst forms. Light microscopy revealed yellowish aeciospores having surface projections. Aeciospores were mostly ovoid and measured approximately 20 µm long. The FESEM showed irregularly shaped cracks on the aecia that had erupted through the bark of P. koraiensis. Some aeciospores had germinated, producing two germ tubes from a spore in a burst aecium. Aeciospores had both smooth and verrucose regions on the surface, and some had concave or convex regions. Aeciospore layers and underlying fungal matrices including aecial columns were obvious in the cross-sections of aecia. Approximately 1 µm-high wart-like surface projections could be resolved and comprised less than 10 angular platelets stacked in vertical rows. Remains of the primary spore wall were present between surface projections. These results provide insights into the morphology of the heteroecious rust fungus with the help of vapor fixation and high-resolution surface imaging.