University of Maryland Extension

Phytophthora Root Rot - Trees and Shrubs

symptoms of Phytophthora root rot

Phytophthora root and crown rots. Photo: Joseph O'Brien, USDA Forest Service, Bugwood.org

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This soil pathogen exists in low levels throughout Maryland and becomes a problem in wet sites. As roots are killed the leaves begin to turn a lighter green and eventually yellow. Infected plants initially appear wilted. As symptoms progress leaves roll inward towards the midrib and turn brown. Highly susceptible cultivars can die within two weeks, where as more resistant plants may not die until many weeks after the plants have developed the initial wilt symptoms. The entire root system may become diseased or portions may escape infection and support the plant until other stress factors cause death. On older plants, symptoms of root rot may be present a season or more before death. In such cases, plants often exhibit symptoms on part of the plant. The plants also decline in vigor and suffer additional damage from other pathogens or insect pests.

Phytophthora dieback, although uncommon in the landscape, is a distinct phase of the Phytophthora disease syndrome on rhododendrons, azaleas, leucothoe and Japanese andromeda. It can be brought into the landscape on infected plants and can be severe on plants grown under overhead sprinkler irrigation. The disease occurs when the pathogen is splashed onto the foliage. Thus, infected plants may show symptoms on leaves and shoots, but may have healthy root systems. Plants with dieback develop symptoms on the current season growth. Mature leaves are often resistant, however, if they become infected, they usually fall prematurely. Infected leaves show chocolate brown lesions that often expand and cause dieback of the shoot tips. Infected leaves droop and curl towards the stem. Diseased leaves remain attached to the stem. Growth of the pathogen through the midrib tissue often produces a V shaped lesion that extends along the leaf midrib into the stem.

Rhododendron with Phytophthora

Rhododendron infected with Phytophthora

infected root system

Phytophthora infected roots

Control: Although chemical controls can be used in nursery production these measures are often too expensive and impractical in a landscape. Several soil and spray applications are required throughout the summer season to control both of these diseases. In addition, no chemical treatments will cure plants that show symptoms. The best disease prevention options are to avoid poorly drained compacted soils, low areas that collect water runoff and locations near downspouts. Construction of raised beds or grade changes may be needed to ensure proper drainage. All newly planted rhododendrons should be watched closely for symptoms, and infected plants or prunings should be removed promptly. Symptoms on older plants can be caused by introducing infected plant material into the same planting bed, changes in water drainage patterns, and low plant vigor. Resistant varieties are available (see publication (PDF) HG 51a Azalea and Rhododendrons: Disease & Insect Resistant Plants). However, if they are flooded for 48 hours or longer, or are drought stressed to the point of wilting, resistance is temporarily lost and the fungus can invade.

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