Bacterial wilt of poinsettia

Pathogen and disease type. The disease is caused by the bacterium Curtobacterium poinsettiae, which is classified as a vascular pathogen. It primarily attacks the xylem vessels of the plant, obstructing the flow of water and essential nutrients. This systemic colonization leads to metabolic distress, wilting, and eventual necrosis of the vascular tissue.

Host plants and spread. The primary host is the poinsettia (Euphorbia pulcherrima). In commercial greenhouse production, the pathogen is frequently transmitted through contaminated propagation material (cuttings). It is also spread mechanically via contaminated pruning tools and through water splashes during irrigation, which facilitate the entry of bacteria into the plant's vascular system.

Symptoms and signs. Early symptoms include the appearance of small, water-soaked spots on the foliage. As the infection progresses, these spots turn yellow and necrotic, leading to leaf drop. Affected plants often exhibit drooping leaves. A diagnostic feature is the browning or blackening of the vascular bundles, which can be observed upon slicing the stems.

Development and economic impact. The pathogen thrives in warm, humid greenhouse conditions, with temperatures between 25°C and 30°C being highly conducive to rapid spread. The economic impact is severe, as the disease can lead to significant losses in marketable stock. Once an infection is established within a greenhouse block, it can spread rapidly, making eradication extremely difficult.

Protection and prevention strategies. Due to the systemic nature of the infection, chemical treatments are rarely effective, making strict sanitation the primary line of defense:

• Utilize only clean, disease-free starting material from certified suppliers.
• Implement strict sanitation protocols, including the disinfection of tools with alcohol or bleach after each use.
• Regulate humidity levels to prevent leaf wetness and ensure good air circulation within the canopy.
• Immediately rogue and dispose of infected plants to prevent further spread to healthy stock.
• Avoid over-watering to keep the growing medium from becoming a reservoir for the bacteria.