Phoma black stem

The disease known as Phoma black stem is caused by the fungus Plenodomus lindquistii (syn. Phoma macdonaldii). It is a significant fungal pathogen that primarily affects members of the Asteraceae family. In agricultural settings, it is recognized as a major limiting factor in sunflower production worldwide.

The pathogen hosts include both the annual sunflower and the Jerusalem artichoke (topinambur). The fungus can infect the plant at various growth stages, colonizing roots, stems, leaves, and reproductive parts. Its ability to survive in soil debris makes it a persistent threat in regions with intensive sunflower cropping systems.

Symptoms typically begin as dark lesions on leaves, often associated with the leaf petiole. As the infection progresses, it spreads to the stem, forming distinct, large, black or dark brown lesions. These necrotic areas can girdle the stem, leading to internal pith decay, structural weakness, and eventual lodging of the mature crop.

Environmental conditions play a crucial role in disease epidemiology. High humidity, frequent rainfall, and moderate temperatures promote the production and dispersal of pycnidiospores. These spores are spread by water splash and wind, allowing the disease to propagate rapidly through a field during the growing season.

The impact of Phoma black stem on yield is severe, often resulting in reduced seed weight and lower oil content. Beyond direct yield losses, the structural damage to the stem often makes mechanical harvesting difficult or impossible due to lodging. Effective management is essential to prevent these economic losses and maintain crop health.

• Implementation of a diverse crop rotation cycle, avoiding sunflowers for at least several years.
• Use of certified, resistant sunflower hybrids developed by breeders.
• Thorough field sanitation to reduce inoculum load in crop residues.
• Timely application of systemic fungicides during the most susceptible growth stages.
• Maintaining optimal plant density to improve air circulation and canopy drying.