Phoma stem rot of chrysanthemum

Phoma stem rot, caused by the fungus Plenodomus chrysanthemi (previously known as Phoma chrysanthemicola), is a destructive disease affecting chrysanthemum crops. This fungal pathogen specifically targets the roots and the base of the stem, interfering with the plant's vascular integrity and leading to systemic decline and, in severe cases, the death of the entire plant.

The primary host for this pathogen is the Chrysanthemum genus. It poses a significant threat in commercial greenhouse production, where monoculture practices and recurring environmental conditions create an ideal habitat for the pathogen. It is often introduced into cultivation systems via infested soil, contaminated water sources, or through the movement of infected vegetative cuttings.

Symptoms of the disease initially appear as darkened, necrotic lesions at the base of the stem, often near the soil line. As the infection progresses, the tissues soften and rot, and characteristic tiny black pycnidia (fruiting bodies) become visible on the surface. Consequently, the foliage wilts, chlorosis develops, and the plant eventually collapses due to the inability to transport water to the upper canopy.

The life cycle and spread of Plenodomus chrysanthemi are heavily influenced by moisture and temperature. The fungus thrives in cool, damp conditions and persists for long periods in soil or decaying plant debris. It spreads primarily through water splashing during irrigation, the movement of contaminated soil particles on tools, and the continuous cultivation of susceptible hosts in the same substrate without proper sanitation.

Effective management requires a rigorous integrated pest management strategy. Cultural practices, such as soil pasteurization and ensuring proper drainage to prevent waterlogging, are crucial. Sanitation of tools and greenhouse surfaces is essential to break the cycle of infection. In cases of active outbreaks, the application of systemic fungicides directed at the root zone can help suppress the spread, though infected plants should be promptly rogued and destroyed.

• Steam pasteurization of potting media.
• Improved air circulation to reduce canopy humidity.
• Routine sanitation of irrigation equipment.
• Use of certified pathogen-free starting material.