Latest revision: 
March 2023

Cause Chrysomyxa piperiana is probably the main species on native R. macrophyllum in Oregon and has the alternate host, Sitka spruce, which is reported in Oregon. The rust of cultivated rhododendrons is C. reticulata, a native North American rust that spread to nurseries from Ledum. Chrysomyxa spp. form spermagonia and aecia on Picea (spruce) and uredinia, telia and basidia on Rhododendron. The fungus can overwinter on any indicated hosts. The disease is most often in plantings along the coast.

Aeciospores that form on Sitka spruce are distributed from current year needles to rhododendrons during summer and fall. Current year rhododendron leaves are infected but may not develop uredinia until the following spring. Urediniospores can reinfect healthy rhododendron leaves and keep the disease cycle going without the need for other spore stages. Urediniospores can cause new infections throughout the year under favorable environmental conditions. These spores are airborne and spread the fungus rapidly from plant to plant. Germination of these spores begins within several hours if the temperature is 15°C to 20°C, the relative humidity is high, and the light is low. If these spores are exposed to high temperature and high humidity, their survival time is shortened. Chrysomyxa rusts typically overwinter in leaves as dormant mycelium. Telia are produced in the early spring and germinate without a period of dormancy to produce basidiospores, which infect Sitka spruce.

Symptoms First, light-yellow or yellow-green spots on the upper leaf surface and brown to purple spots on the lower surface. Blisters form in this area of the lower leaf surface, and they rupture to expose typical yellow to orange-red pustules containing the urediniospores. This windborne spore reinfects only rhododendron. Several generations may develop during the growing season. Severe infections may cause defoliation. The disease usually appears in late summer and fall on the previous season's foliage. Secondary fungal invaders may opportunistically colonize these areas causing various necrotic leaf spots.

Cultural control

  • Collect and burn infected leaves.
  • Space plants for good air circulation and rapid drying.

Chemical control These fungicides should be applied on 7- to 14-day intervals in spring and/or summer to protect new foliage from urediniospores from infected rhododendrons or from aeciospores dispersed from conifer hosts.

  • BioAdvanced Disease Control at 0.75 fl oz/gal water. H
  • Broadform at 4 to 8 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
  • Heritage at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
  • ProStar 70 WG at 3 to 6 oz/100 gal water is registered for azalea. Group 7 fungicide. 12-hr reentry.
  • Torque at 4 to 10 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
  • Tourney EZ at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
  • Trigo at 3 to 9 oz/100 gal water. Group 3 + 11 fungicide. 12-hr reentry.
  • Materials such as Banner MAXX, or Eagle may have activity but are not labeled specifically for rust control.

Reference Crane, P.E. 2001. Morphology, taxonomy, and nomenclature of the Chrysomyxa ledi complex and related rust fungi on spruce and Ericaceae in North America and Europe. Canadian Journal of Botany 79: 957-982.