Hemlock hydrangea rust is caused by the fungus Pucciniastrum hydrangeae. This rust is leaf disease of smooth hydrangea Hydrangea arborescens, in landscapes and nurseries. The fungus can also cause a minor browning and defoliation hemlocks Hemlock spp.

Some rust fungi need two hosts to complete their life cycle. Apple cedar rust, Hawthorne cedar rust,  White pine gooseberry rust, and Pine oak fusiform rust all need two hosts. Some rusts such as Rose rust do not need two hosts.

The first signs of rust disease in smooth hydrangea are brown/orange pustules on the underside of leaves and yellow spots on the upper side. On hemlocks, the fungus forms white to beige tendrils. The closer the plants, the easier for the fungus to complete its life cycle.

How do gardeners control hemlock hydrangea rust? Try to avoid planting these plants in close proximity. Prune shrubs to thin the plants and increase air circulation. Clean up fallen plant debris around both hemlock and hydrangea to reduce spores. Pine Sol can be used to disinfect cutting tools.