It is that time of year when Phytophthora root rot starts showing up in rhododendron and azalea plantings. The fungus is favored under extremely wet conditions and in heavy, poorly drained soils. The wet spring may have aggravated the condition more this year.
Phytophthora root rot of rhododendrons is caused by several species of Phytophthora. The pathogens, primarily P. connamoni, P. citriocola and P. cactorum, are soilborne and invade roots under wet conditions. Most cultivars of rhododendrons are highly susceptible to attack by Phytophthora.
Symptoms of Phytophthora root rot range from mild to extreme. Plants may show reduced growth but no other obvious above-ground symptoms. Reddish-brown internal discoloration of some roots and lower stem may indicate that the pathogens are reducing the ability of the root system to absorb water and nutrients. More severe symptoms, including drooping, rolled and dull-colored leaves, permanent wilting, and defoliation, can occur when soil and media conditions are persistently wet.
Homeowners can check for Phytophthora root rot. In infected plants, the tissue under the bark at ground level is darkly discolored. Peel back some of the bark at the base of the plant. If the plant is infected, a distinct boundary between healthy, white tissue and diseased, brown tissue will exist.
Management of Phytophthora root rot consists primarily of prevention. The key is to manage the water status of the root environment to avoid prolonged periods of saturation. The most effective way to do this is to ensure that the container medium or soil has excellent drainage. Container media with high proportions of bark or sand will drain better than media that are primarily peat. To maximize container drainage, keep the containers on pallets or on a layer of gravel 3 or more inches deep. In addition, a black plastic ground cover can help to prevent splashing of Phytophthora spores from the soil onto pots. Avoid areas where standing water occurs. Monitor overhead watering to avoid long periods of saturation.
A number of fungicides will help to suppress disease development. However, they will be effective only in conjunction with the cultural techniques above. In the nursery, products such as Subdue, Aliette, Banol, Banrot, Truban, Terrazole, and Captan can be effective against Phytophthora in the soil. Because of their expense, they are usually used to prevent spread of a problem once it is detected, rather than as a preventative. This means that plants must be inspected regularly for symptoms. These chemicals can be effective in landscape settings as well. Again, cultural techniques aimed at prevention are the key to control. Chemical treatments will not cure plants already infected with root rot.
Once the disease is detected, infected branches should be pruned out; severely infected plants should be removed from the planting along with the surrounding soil and destroyed. If plants have died, dry the soil out. Improve the drainage before replanting azaleas or rhododendrons in the same location. If drainage cannot be improved, plant in beds raised 12 inches or more above ground level or plant shrubs that are resistant to wilt and root rot.
Sometimes Phytophthora will only cause dieback in several twigs. If this is the case, these twigs can be pruned out well below the cankered (dying) area.
This article originally appeared in the August 25, 1995 issue, p. 130.