Elderberry rust ... causes growth distortions and swellings (i.e., galls) on leaves and stems [of Sambucus]. Galls are often very large, bright yellow and powdery ... In extreme cases, galls can resemble banana slugs that have attached themselves to branches. Infected flowers become thick, swollen and green-tinged rather than white. Affected plant parts are covered with a network of small (approximately 1/16 inch in diameter) ring-like spots ...

The fungus overwinters in sedge debris [Carex is the alternate host], and spores ... blow to elderberry plants in the spring,