Crown/collar Rot & Shoot Blight

Phytophthora citricola, Phythophthora cactorum

Hosts

Red elder (crown rot), lilac (crown rot, shoot blights), dogwood (collar rot)

Distribution and Disease Cycle

Description of this image follows
Necrosis on the roots and root collar caused by Phytophthora infection.
Photo credit: Andrej Kunca, National Forest Centre - Slovakia, Bugwood.org

Several species of Phytophthora cause crown or collar rot in elder and dogwood and crown rot or shoot blight in lilac. This fungal pathogen overwinters in infected plants, on fallen plant debris or in the soil surrounding infected plants. Poorly drained soil or excessively wet soil conditions can favour infections because the fungus can move within the soil to susceptible roots or can be spread by rain splash. Healthy leaves or tissue must be wet for infection to occur, so the incidence of disease often increases following rainy weather where tissue remains wet for extended periods of time.

Symptoms and Signs

In most cases, Phytophthora infects the roots or lower trunk of susceptible ornamentals leading to shoot dieback or wilt, plant stunting, underized, chlorotic leaves, premature fall colour, dead branches, and eventual plant death. The inner bark or wood at the base of infected plants will be discoloured or have necrotic streaks that are a cinnamon brown or dark brown colour, often with a distinct margin between infected and healthy tissue. Phytophthora can also infect shoot tissue causing trunk and branch cankers or shoot blights. Because other root problems can cause similar symptoms, laboratory diagnosis is usually necessary to confirm Phytophthora infections.

Control

The incidence of crown rot in elder or lilacs can be reduced by using disease free stock and planting into clean soil that has not previously grown either elders or lilacs. Collar rot or crown canker in dogwood can be reduced by avoiding soils known to be contaminated with Phytophthora, avoiding damage to the crown areas and removing any small cankers. Correcting soil drainage in areas with excessively wet soils and the incidence of crown/collar rot, may reduce the incidence of disease. There are no chemical control options for crown/collar rot or shoot blight caused by Phytophthora spp.

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