Root rot is the common name for Phytophthora; P. cinnamomi is usually credited with causing the greatest damage to container and field-grown woody ornamentals. Several other Phytophthora species are known to attack rhododendron, azaleas and junipers and disease outbreaks can occur almost anytime of year.
Phytophthora greatly reduces the volume of the roots, which become brittle and brown to reddish-brown in color. A network of fine, discolored feeder roots may be confined to one area or include the entire root system.
The fungi often colonize the crown of the plants, and girdle the stem at or just above the soil line. A brown to reddish-brown discoloration of the tissues occurs just below the bark and may extend up the stem above the soil line (hence the name P. cinnamomi.) Brown, water-soaked cankers oozing a dark-colored fluid or gum may develop at the soil line of some large shrubs and trees.
These symptoms can easily be confused with nutritional disorders, overwatering, drought stress, and a number of other factors and might be overlooked by the grower. Established plants may show symptoms of general decline for one or more years before succumbing to root rot, while newly planted azaleas will quickly die.
Foliage of azaleas and rhododendrons may also be infected. Olive-colored blotches turn brown sometimes with a red margin. Diseased leaves are usually shed by the plant. Damage usually appears on limbs near the base of the plant.
Spore production and release, as well as infection of the roots by P. cinnamomi generally occurs in warm (77F - 82F) water-saturated soils and potting media. Other Phytophthora fungi may be more active in cooler soils. Splashing and runoff water is often the primary means of pot-to-pot spread of Phytophthora spores.
Low soil pH (3.5 to 4.5) will suppress spore release, but root rot activity is not slowed at the soil pH range most conducive to plant growth. Excessive use of nitrogen can increase the risk of disease because new plant growth is especially vulnerable.
Prevention is the key to controlling root rot. Cuttings should be taken from only disease-free plants. Pruning shears or knives should be cleaned with rubbing alcohol or a similar disinfectant. Place cuttings in well-drained, soilless media in sterilized or new pots or cell packs.
When watering, do not allow water to stand around plants, especially those in containers. Azaleas and rhododendrons should be planted in raised beds, with pine bark or a similar coarse organic amendment worked in to a depth of 8 to 12 inches to improve drainage.
When rooting azalea and rhododendron cuttings, use composted hardwood bark (red oak) and aged pine bark, which have been shown to reduce root rot. (Composting hardwood bark releases chemicals that are toxic to the fungi.)
Among hardy azalea hybrid groups, the Indian, Glenn Dale, and Satsuki hybrids are more root rot-resistant than Kurume azaleas. Among the Rhododendron species found in the nursery trade, R. davidsoninaum cv. Serenade, R. delavayi, and R. poukhansnse are resistant to root rot.
Resistant junipers include J. virginiana cv. Prostrata, J. chinensis cv. Prostrata, and J. horizontalis cv. Bar Harbour and Prince of Wales.
For chemical control measures, soil drenches applied shortly before or after transplanting will also give good protection. One to four months later, a program of regular fungicide drenches or foliar sprays should be started. Soil drenches usually give better protection from root rot than foliar sprays.
Spray schedules and application rates for a given fungicide will depend on the plant being grown; consult your county extension agency for fungicides and recommended application schedule.