Phoma black stem

Pathogen and disease type. Phoma black stem is caused by the fungus Phoma haematocycla (also identified in scientific literature as Phoma macdonaldi). It is a significant fungal infection that affects sunflowers worldwide, attacking stems, leaves, and occasionally the roots of the crop during its growing season.

Host range. The primary host for this pathogen is the cultivated sunflower (Helianthus annuus). The fungus is a necrotrophic parasite capable of surviving as mycelium or pycnidia in crop debris left on or in the soil, serving as a primary source of inoculum for subsequent seasons.

Symptoms and signs. Initial symptoms appear as brown or black lesions on the lower leaves. As the disease progresses, large, distinctive black lesions develop on the stem, typically at the node where the leaf petiole attaches. These lesions can expand significantly, sometimes encircling the stem and leading to tissue decay and weakening of the stalk.

Conditions for development and spread. The fungus thrives in warm, humid environments, especially when rainfall is frequent during the growing period. Spores are dispersed through rain splashing and wind, allowing the infection to move rapidly from lower leaves to the upper parts of the plant and to neighboring stems.

Economic impact. The damage caused by Phoma black stem is severe, including reduced seed filling, lower oil concentration in the kernels, and premature plant death. In cases of severe infection, the weakened stems are highly susceptible to lodging, which significantly complicates harvesting and causes major yield losses.

Management and prevention measures:

• Implementing long-term crop rotation to reduce soil inoculum levels.
• Selecting resistant or tolerant sunflower hybrids.
• Deep tillage practices to bury infected plant debris after harvest.
• Applying fungicides preventively during the early stages of rapid vegetative growth.
• Managing planting density to ensure adequate airflow and reduce micro-humidity around the stems.