Botryosphaeria stem rot

The disease is caused by the fungal pathogen Botryosphaeria ribis. It is a significant threat to many plant species, acting as a vascular pathogen that disrupts the transport of water and nutrients, eventually leading to systemic necrosis and the death of the infected stem segments.

This fungus primarily affects a variety of ornamental plants, with geraniums and specific species such as scented-leaved pelargonium and ivy-leaved pelargonium being highly susceptible. It often behaves as an opportunistic pathogen, taking advantage of wounds or physiological stress in the host plant to gain entry and establish an infection.

Symptoms typically include dark, sunken lesions on the stems, often located near the soil line or at pruning sites. As the fungus progresses, the internal tissues turn brown or black, and the stem loses its structural integrity. Foliage above the infection site becomes chlorotic and eventually wilts as the pathogen chokes the vascular system of the plant.

Development and spread are favored by warm, humid conditions and poor airflow in greenhouse environments. Water splash from irrigation often distributes spores to healthy tissues, while contaminated tools used for propagation or maintenance are the most common vectors for the pathogen. High nitrogen levels and overwatering can further increase plant susceptibility.

• Sanitize pruning tools with alcohol or bleach solutions between plants.
• Use sterilized, well-draining growing media for all propagation activities.
• Improve air circulation to reduce humidity on stem surfaces.
• Promptly remove and destroy all infected plant parts to prevent spore release.
• Apply systemic fungicides as a preventative measure during high-risk periods.

The impact of Botryosphaeria stem rot is severe, as infected plants rarely recover and often serve as a reservoir for further contamination within a nursery or collection. Management relies heavily on strict sanitation practices and environmental controls, as curative treatments are often ineffective once the vascular tissue is extensively colonized by the fungus.