Phytophthora Root Rot and its Control on Established Woody Ornamentals
OD13 - Ornamental Disease Note 13

D. M. Benson, Research Plant Pathologist
R. K. Jones, Extension Plant Pathologist

Figure 1. Phytophthora root rot on azalea. Note the overall chlorosis, sparseness, and small size of the leaves. Figure 2. Phytophthora root rot on rhododendron. Note the leaf droop which resembles drought stress.

On azalea the symptoms vary with the cultivar (Figure 1). On Kurume hybrid types, such as Coral Bells, Hinodegiri, and Hino Crimson, new leaves are smaller than normal with interveinal chlorosis, possibly some purple coloration and defoliation. This chlorosis is often confused with a deficiency of iron or other nutrients. At times light applications of iron and complete fertilizer can improve the green color of leaves but only for a short time. Excessive yellowing and loss of older leaves are the predominant symptoms on Snow azalea. Usually, large plants slowly decline in vigor and die branch by branch over a period of several months to years, but sometimes they can die rapidly. Roots are reddish-brown, brittle and often limited to the upper part of a container or soil. The reddish-brown discoloration advances to the larger roots and eventually to the main stem. On rhododendron the primary symptoms are a rapid wilting and death of leaves. Leaves droop but remain attached to the limb (Figure 2). Watering does not restore leaf turgor. Usually several to many shoots are affected. Root symptoms are similar to those on azalea.

Figure 3. Phytophthora root rot on Taxus.

On Camellia japonica the main symptoms are a gradual decline in vigor, loss of dark green color, curling of leaves and excessive loss of older leaves. Root symptoms are similar to those on azalea. Camellia sasanqua is tolerant to Phytophthora root rot and this species is often used as a root stock for C. japonica. Taxus dies rather suddenly with the foliage turning reddish-brown (Figure 3). Roots are also reddish-brown in color and the discoloration may extend into the main stem. White pines in the landscape often die suddenly during dry periods.


Factors Favoring Disease Occurrence

Phytophthora root rot is favored by high soil moisture and warm soil temperatures. The disease does not occur as frequently and may not be as severe on well-drained sandy soils as in heavy clays or poorly drained soils, etc. The disease is common and severe in areas where run-off water, rain water from roofs, etc. collects around plant roots. Shallow soils with underlying rock or compacted hard pans, setting woody plants deeper than the soil level in the nursery or container, over-watering plants, or long periods of heavy rain also favor disease development.

Prevention

Phytophthora root rot must be prevented as chemicals are often ineffective in controlling this disease after above-ground symptoms become obvious. The following suggestions may aid in the prevention of root rot:

1. Purchase disease-free plants from a reputable nursery. Avoid plants that lack normal green color, appear wilted in the morning, evergreen plants that have excessive winter defoliation or dark discolored roots.

2. Plant root rot susceptible plants in well drained areas. If excess water from any source collects in the planting site, avoid planting root rot susceptible plants. If soil is heavy clay or does not have good internal drainage, set root-rot susceptible plants in raised beds and thoroughly mix a porous material such as bark (not sawdust or peat) into the bed. The material should be incorporated to a depth of 8-12 inches. In some areas drain tile and gravel placed 6-12 inches below the surface may also help reduce excess soil moisture.

3. Do not set the new plant any deeper than the soil level in the container or the soil line in the nursery. Firm the soil beneath the soil ball so that the plant will not settle into the bed.

4. In areas where root rot susceptible plants have died, replant with plants that are not susceptible to root rot.

5. Resistance. Within cultivars of various types of plants, some cultivars are highly susceptible (very likely to be killed by the fungus) and others are resistant. The rhododendron hybrids: Caroline, Martha Isaacson, Professor Hugo de Vries and Red Head are considered resistant. Resistant azalea cultivars include Rhododendron poukhanese, Formosa, Fakir and Corrine Murrah. Camellia sasanqua is resistant to root rot while C. japonica is highly susceptible. Keep in mind that resistant hybrids are not immune. When planted in poorly drained soils where the fungus is present these plants may also become diseased.

6. Fungicides. The spread of Phytophthora into or among plants also can be reduced through the use of mefenoxam (Subdue MAXX) or fosetyl-Al (Aliette), but these chemicals may not kill the fungus in infected plants (see Table 1). For individual plants in the landscape, approximately 10 sq. ft. of soil around the plant should be treated. For large shrubs, 20-30 sq. ft. of soil should be treated.

For long-term control of root rot, the following control measures should be utilized in this order:

1. Avoid very poorly drained areas; plant in raised beds.
2. Use highly resistant or resistant cultivars.
3. Use chemical control.

Table 1. Fungicides for control of Phytophthora root rot.

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Revised November 2000 by Plant Disease and Insect Clinic