| 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 droughtstressed
and may die quickly as the weather warms
in late spring or early summer. Trees that are
especially susceptible include oak and dogwood.
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.
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.
Integrated Pest Management Strategies
1. Improve water drainage. Prior to planting, 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. Never plant trees deeper than they were
planted at the nursery.
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.
Consider replanting with tolerant plants after
Phytophthora root rot has been diagnosed. Trees
that are tolerant to Phytophthora root rot include
white cedar and American arborvitae. | 
