Phomopsis stem canker

Pathogen and disease type
Phomopsis stem canker, caused by the fungus Diaporthe novem (formerly Phomopsis helianthi), is a destructive fungal disease affecting sunflowers. The pathogen primarily targets the vascular system, leading to systemic infection and significant disruption of nutrient and water flow within the plant tissues.

Affected crops
Sunflower is the primary host for this pathogen. While it causes the most damage in commercial sunflower crops, Diaporthe novem is known to maintain its life cycle on various wild plants, which can act as secondary sources of inoculum for surrounding agricultural fields.

Symptoms and signs of infection
The infection often begins on the leaves as brown necrotic spots, progressing through the petioles into the stem. The stem tissue at the infection site becomes silvery-gray, brittle, and hollowed out, which drastically reduces the structural integrity of the plant and causes lodging.

Conditions for development and spread
The development of the disease is heavily dependent on moisture. Prolonged wet periods, high humidity, and moderate temperatures provide the ideal environment for spore production and infection. Spores are primarily disseminated by wind and rain splashes, allowing the disease to spread rapidly across fields.

Economic impact and control measures
The economic impact of Phomopsis includes severe yield losses, low thousand-kernel weight, and reduced oil quality. Effective management requires a combination of resistant hybrids, crop rotation practices that avoid sunflower production for several years, residue management, and precise timing of fungicide applications.

• Selecting resistant sunflower hybrids.
• Deep burial of crop residues to reduce primary inoculum.
• Effective weed management to eliminate alternative hosts.
• Fungicide applications during the vegetative stages to protect the stem.