Rhododendron necrosis

Rhododendron necrosis is a complex of diseases primarily caused by pathogenic fungi such as Phytophthora cinnamomi and Botryosphaeria dothidea. These pathogens attack the vascular system and tissues of the plant, causing severe stress and leading to the eventual death of branches or the entire shrub if left untreated.

Symptoms often manifest as dark, sunken lesions on stems, leaves turning brown or wilting, and the eventual dieback of entire branches. When the root system is affected, the plant suffers from an inability to transport water and nutrients, leading to rapid yellowing and wilting even when soil moisture appears adequate.

The biology of these pathogens is closely linked to moisture levels. They often overwinter in soil or infected plant debris. Under conditions of high humidity and moderate temperatures, the pathogens produce spores that spread through splashing water, contaminated tools, or infested planting medium, effectively invading healthy tissue through small wounds.

Favorable conditions for the disease include poorly drained soils, prolonged periods of leaf wetness, and mechanical damage to the bark or roots. Rhododendrons planted in inappropriate soil pH or environments with poor air circulation are significantly more susceptible to infection compared to plants maintained under optimal horticultural practices.

Management and control require an integrated approach. Prevention starts with selecting disease-free nursery stock and providing excellent drainage. If necrosis is detected, immediate pruning of affected branches is necessary, followed by the application of systemic fungicides to manage soil-borne inoculum. Proper sanitation of tools is critical to prevent the spread to adjacent healthy plants.

• Improve soil drainage and aeration.
• Remove and destroy symptomatic plant material.
• Disinfect pruning tools with alcohol or bleach solutions.
• Use systemic fungicides according to local recommendations.