Cankers caused by Phytophthora cinnamomi show red-brown discoloration.
Joseph O’Brien, USDA Forest Service, Bugwood.org;
Smaller photo courtesy NCSU

Many ornamental trees and shrubs are susceptible to Phytophthora root rot and can develop root and crown rot, particularly if the soil around the base of the plant remains wet for long periods of time.

Typical symptoms of a root disease are apparent on infected plants. The leaves will appear drought-stressed and may die quickly as the weather warms in late spring or early summer. Trees that are especially susceptible include oak and dogwood.

Source: Missouri Botanical Garden

The facts about Phytophthora root rot

Symptoms and diagnosis: Leaves will appear drought stressed, sometimes turning dull green, yellow, red, or purple as they wilt. Infected trees may survive a few years before the disease kills the whole plant. The bark around the soil-line may appear darkened. Cutting away some bark should reveal red-brown discoloration in the wood underneath it. Disease symptoms are distinguishable from Armillaria root rot because mycelial mats do not develop in tissues infected with Phytophthora root rot.

Leaf symptoms caused by Phytophtora spp. on rhododendron

Life cycle: Root rot-causing Phytophthora species can survive in the soil for years, as long as moist conditions persist. It can spread through splashing rain, irrigation water, and runoff water. Disease fungi can spread through contaminated soil and garden equipment as well. Rot is more likely to spread in early spring and late fall during cool, rainy weather. But symptoms are more likely during stress periods of low rainfall. Flooded and saturated soil conditions for 6–8 hours are especially conducive to the spread of root rots. Wounds are not required for infection.

Cultural control: 1. Improve water drainage. Make amendments to the soil composition to help manage drainage away from tree roots. Don’t allow water to pool around the collar or root system. Core aerate to improve drainage and lessen compaction. 2. Raise the planting site to avoid poor drainage and prevent pools of standing water from forming around valuable ornamentals. Plant on mounds of soil. 3. Separate plants, shrubs, and trees according to their irrigation needs, and avoid overwatering. 4. Remove soil. If soil has been piled up over the crown, remove it carefully to expose root flare. This is where fungal infection is likely to occur. 5. Plant resistant or less susceptible varieties. Trees that are tolerant to Phytophthora root rot include white cedar and American arborvitae.

Downward rolling of leaves is also an early symptom on rhododendron.

Chemical control: Chemical control of Phytophthora root rot is successful only when integrated with cultural best practices. Fungicides work better as preventative treatments. Incorporated fungicides are usually more uniformly distributed throughout the substrate or soil and may provide better protection. If a root rot fungicide was not incorporated into the substrate, begin drenches or foliar sprays immediately after plants have been transplanted. Soil drenches usually provide better protection from root rot than foliar sprays. Treatment schedules and rates will depend on the plant, the level of disease pressure and the fungicide used.