Pittosporum is a popular and attractive evergreen shrub prized in landscapes for its dense foliage and adaptability. Despite its toughness, this plant has a major weakness: root rot. This soil-borne disease is responsible for the majority of Pittosporum failures and is often fatal if not caught early. The condition is primarily caused by water molds, most commonly Phytophthora species, which thrive in saturated, poorly aerated soil. The pathogen attacks the root system, preventing the plant from absorbing water and nutrients, which ultimately leads to its decline.
Recognizing the Signs of Root Rot
The first indications of root rot are often visible above ground, manifesting as symptoms that mimic drought stress. Symptoms include yellowing leaves (chlorosis) and wilting of the entire canopy, even when the soil is moist or wet. The plant may also exhibit sparse foliage, stunted new growth, and a general lack of vigor.
To confirm the diagnosis, inspect the root system. Healthy Pittosporum roots are firm, flexible, and typically white or light tan. Infected roots appear dark brown or black with a mushy or slimy texture, indicating decay. This rot often produces a foul, musty, or sulphurous odor due to the anaerobic environment.
Emergency Treatment and Salvage
Immediate intervention is necessary once root rot is confirmed, starting with a complete cessation of watering. Carefully remove the plant from the ground or container for a thorough inspection of the root ball. Improving air circulation around the root zone is the first step in drying the affected area.
The next step is physical root pruning using sterilized tools. All dark, soft, or mushy roots must be cleanly cut away, tracing the rot back to the healthy, firm, white tissue. The shrub’s canopy should also be pruned back proportionally to the root mass lost, reducing water demand on the compromised system.
Replanting must use fresh, well-draining, and sterile soil media, or the plant should be moved to a location with better drainage. Fungicides may be applied as a last resort before replanting, but they are generally ineffective unless cultural problems are resolved.
Addressing the Underlying Causes and Prevention
Preventing root rot centers on managing soil moisture and ensuring excellent drainage, since the Phytophthora water mold requires saturated conditions. Pittosporum should never be planted in low-lying areas where water collects or at the bottom of a slope. Choose locations that allow for rapid water run-off, preventing the root zone from remaining saturated for extended periods.
For heavy clay soils, significant amendment is required before planting. Incorporating coarse organic matter, such as composted bark or grit, helps create pore spaces for better aeration and drainage. Planting the Pittosporum in a raised bed or on a slight mound can also lift the root crown out of the wettest soil layers.
Proper watering technique is the most impactful preventative measure. Use a deep and infrequent schedule instead of frequent, shallow watering to encourage a healthy, deep root system. Check soil moisture by inserting a finger several inches deep, watering only when the top two inches of soil feel dry.
Avoid overhead watering, which can splash pathogens and spores from the soil. When applying mulch, maintain a layer over the root zone, but keep it several inches away from the main trunk. Piling mulch against the trunk (volcano mulching) traps moisture and significantly increases the risk of rot.
The plant should always be set at the proper depth. Ensure the root flare is visible just above the soil line, and never bury the plant deeper than it was growing in its original container.