Phoma leaf and stem spot of sunflower

Pathogen and disease type
The disease is caused by the fungus Boeremia heteromorpha (syn. Phoma macdonaldi), which is a necrotrophic pathogen. It causes Phoma black stem and leaf spot, affecting various parts of the plant and leading to systemic damage that compromises the structural integrity and overall health of the sunflower crop.

Host range
The primary host for this fungus is the commercial sunflower (Helianthus annuus). The pathogen is highly host-specific, yet it survives efficiently in crop residues and soil, allowing it to re-infect emerging sunflower crops across successive seasons if proper agricultural sanitation is not maintained.

Symptoms and signs of infection
The disease initially manifests as small, dark brown or black lesions on the lower leaves. As the infection progresses, large necrotic spots appear on the stems, particularly at the leaf axils. In severe cases, these lesions can expand and encircle the stem, leading to pith decay, stem lodging, and the premature wilting of the entire plant.

Conditions for development and economic impact
Warm, humid conditions with frequent rainfall are ideal for spore production and secondary spread. The fungus significantly impacts crop yield by reducing the photosynthetic area and disrupting nutrient translocation to the seed head. This leads to lighter, poorly filled seeds with reduced oil content, resulting in substantial economic losses for farmers.

Control and prevention measures

• Implementation of a long-term crop rotation (at least 7 years between sunflower crops).
• Cultivation of resistant or tolerant sunflower hybrids.
• Deep tillage to bury infected stubble and accelerate residue decomposition.
• Application of prophylactic or therapeutic fungicides during critical growth stages.
• Strict weed management to remove alternative hosts for the pathogen.