Phomopsis stem canker of sunflower

The fungus Diaporthe kochmanii is the primary causal agent of sunflower stem canker, also known as Phomopsis. This pathogenic fungus affects the integrity of sunflower stalks, leaves, and reproductive structures, causing severe necrotic lesions that severely limit the plant's ability to transport water and nutrients.

The disease primarily impacts sunflower crops. While it can attack at various growth stages, it is most aggressive when plants are entering the vegetative-to-reproductive transition. If conditions are favorable, the entire field can show signs of infection within a few weeks, leading to widespread plant senescence.

Symptoms typically begin as irregular, chlorotic spots on the leaf margins, which eventually necrotize and spread down the petiole to the main stem. The infection causes distinct grey-brown lesions on the stem, which soften the internal tissues and make the plants highly susceptible to lodging during windy weather.

The development of Diaporthe kochmanii is strongly influenced by environmental factors such as high humidity, rainfall, and moderate temperatures ranging from 20 °C to 26 °C. The fungus spreads via windborne ascospores or rain splashes, while the primary reservoir for the pathogen remains infested crop debris in the soil.

The economic impact of this disease is significant, often resulting in reduced seed yield, lower test weights, and poor oil quality. To mitigate the risks posed by this pathogen, producers must adopt an integrated approach to plant protection that focuses on minimizing inoculum load and enhancing plant vigor:

• Selection of genetically resistant sunflower hybrids.
• Implementation of long-term crop rotation cycles.
• Effective tillage practices to bury infected plant residues.
• Timely application of systemic fungicides during the early growth stages.