Phomopsis stem canker

Diaporthe gulyae is a pathogenic fungus that causes a destructive disease known as Phomopsis stem canker. This pathogen is a significant threat to sunflower production worldwide. It acts as a hemibiotrophic parasite, initially colonizing host tissues before causing tissue necrosis, leading to rapid deterioration of the plant's health and structural integrity during the critical stages of development.

The primary host for this pathogen is the cultivated sunflower. However, the fungus is highly adaptable and can survive on various plant residues left in the field. Infection usually begins on the lower leaves, then moves through the petioles into the main stem. Once it reaches the stem, it causes deep lesions that impede the transport of water and nutrients, effectively starving the upper parts of the plant and the developing seed head.

Symptoms are easily identifiable in mid-to-late season. The characteristic sign is the appearance of brown, necrotic lesions on the leaves that progress to the base of the petiole. The infected stem tissue turns dark and spongy, eventually becoming brittle and causing the plants to lodge or break under wind pressure. During periods of high humidity, tiny black fruiting bodies (pycnidia) become visible on the surface of the lesions.

Environmental conditions play a decisive role in the spread of Diaporthe gulyae. Prolonged rainfall, high humidity, and temperatures between 20°C and 25°C are optimal for spore production and dissemination. The fungus spreads via rain splash and wind, making dense plantings particularly vulnerable. In addition, nutrient imbalances, particularly excess nitrogen and low potassium, can exacerbate the plant's susceptibility to the infection.

The impact of this disease on agricultural productivity is severe. Infected plants show reduced seed set, lower oil content, and impaired seed quality. In high-pressure years, field losses can be total due to widespread lodging, which makes mechanical harvesting impossible. The pathogen's ability to persist in soil and crop debris for several seasons complicates long-term management strategies for sunflower growers.

Integrated pest management (IPM) is essential for controlling this disease:

• Selection of sunflower hybrids with documented resistance to Phomopsis.
• Implementation of long-term crop rotation schemes.
• Deep plowing to bury crop residues and promote decomposition.
• Timely fungicide applications during the vegetative and budding stages.
• Rigorous weed control to eliminate alternative hosts for the fungus.