See:
Rhododendron - Phytophthora Blight and Dieback
The fungus-like organism Phytophthora taxon morella on California or Pacific wax myrtle (formerly Myrica californica). This disease has been found in the central Oregon coast from Florence to Lincoln City and was first noticed in a few plants in 2007. In 2009 it was found to be more severe and widespread; little is currently known about the disease but it is expected to be similar to other foliar blights due to this pathogen. The pathogen is favored by low temperature and is more active during the winter and early spring than at other times. It overwinters in infected leaves on the plant and on the ground. During these periods, sporangia produced in the leaves will release zoospores that are spread by driving rain and wind to new tissues where they initiate new infections. New leaves produced in late fall or winter are very susceptible. Leaves produced during the spring are less so, and new infections fall off as the weather warms. Plants that are in shady conditions, where the leaves remain moist for much of the day, are more severely affected than those in more open sites.
Symptoms This is primarily a leaf blight, but branches experiencing repeated defoliation will eventually die. Cankers have not been observed in natural settings. Lower leaves are affected first, with the foliage turning a dark or charcoal gray with no associated chlorosis or other discoloration. Infection sometimes resembles other Phytophthora infections (V-shaped areas of necrosis following the mid-rib), but not all leaves with these symptoms are due to Phytophthora. Leaves in the interior part of the plant, or those in shade will be more severely affected. Diseased leaves turn completely brown and dry with age or with hot weather, and infected landscape plants may be completely bare of lower leaves. Roots are not directly affected.
Cultural control
- Prune off infected leaves and branches.
- Remove and destroy fallen leaves from around the base of plants.
- Do not over-fertilize, and do not apply fertilizer late in the season, which stimulates succulent new growth which is likely to be more susceptible to infection.
- Avoid overhead irrigation or apply such that plants are not wet for extended periods of time.
- Keep weeds away from the base of the plant to increase air circulation.
- Avoid reusing pots from a previous crop for propagation. If pots must be reused then wash off all debris and soak in a sanitizing solution or treat with aerated steam for 30 min.
- Prevent potted plants in nurseries from falling over allowing foliage to touch the ground.
Chemical control Apply fungicides in the fall to protect new leaf growth just before fall rains begin. No fungicides are specifically registered for this crop; however, the following may be effective. Test materials for safeness on this crop prior to wide scale use. Rotate fungicides from different groups that have a different mode of action for resistance management.
- Aliette at 2.5 to 5 lb/100 gal water. Do not use with adjuvants. Group P7 fungicide. 24-hr reentry.
- Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
- Stature SC at 12.25 fl oz/100 gal water. Do not make more than two (2) sequential applications. Group 40 fungicide. 12-hr reentry.
Reference Putnam, M.L., Serdani M., Curtis, M., and Angima, S. 2011. Phytophthora leaf blight - a new disease of California wax-myrtle (Morella californica) in Oregon, USA caused by a Phytophthora species. New Zealand Journal of Forestry Science 41 Suppl.:S57-S63.