Cause Rust on the common garden ground cover (creeping St. John's Wort - H. calycinum) is caused by the fungus Melampsora hypericorum. The same rust can occur on the therapeutic herb (H. perforatum) as well as Uromyces hypericifrondosi (formerly Uromyces triquetrus) (which may also be called U. hyperici). The latter has been reported from Oregon and has been found twice by the OSU Plant Clinic in the early 1990s. These are both autoecious rust fungi that can complete their lifecycle on the same host. Plants that are wet for extended periods in the spring develop more disease as was observed in the spring of 2021.
Symptoms Yellow, red, or brown spots on leaf surfaces. In spring and early summer, powdery orange spores are produced from fruiting bodies on the lower leaf surface. Later, in fall and early winter, tissue on the lower leaf surface becomes necrotic where infected, though orange sporulation may still be observed.
Cultural control Has been very hard to control in the Pacific Northwest.
- Remove infected leaves and leaf litter in the planting site. This may require mowing severely infected plantings, raking all leaves, and destroying infected leaf litter.
- Mowing in the late spring or summer during dry weather will encourage the growth of new, rust-free foliage.
- Avoid overhead irrigation or apply such that plants are not wet for extended periods of time.
- Plant resistant cultivars.
Chemical control Prevention is the best approach. Short of that, use wetting agents with fungicides.
- Armada 50 WDG at 3 to 9 oz/100 gal water. St. Johnswort not on the label but can be used if not phytotoxic. Do not use a silicone-based surfactant. Not for nursery or greenhouse use. Group 3 + 11 fungicide. 12-hr reentry.
- Although not specifically registered for St. Johnswort, products such as Heritage are safe on this crop, according to IR-4 data. Trials in California found this to be an effective product especially when an adjuvant is used. Use a non-silicone-based wetter sticker. Group 11 fungicide.
- Pageant at 12.5 oz/100 gal water plus an adjuvant. Do not use with organosilicone-based adjuvants. Group 7 + 11 fungicide. 12-hr reentry.
Reference Ivić, D., and Novak, A. 2011. Melampsora hypericorum (DC.) J. Schröt, the causal agent of St. John's wort (Hypericum calycinum L.) defoliation in the city of Zagreb. Fragmenta Phytomedica et Herbologica 31:100-103.