Plenodomus lindquistii

Plenodomus lindquistii is a pathogenic fungus belonging to the kingdom Fungi, phylum Ascomycota, and genus Plenodomus. Formally recognized in literature as Phoma macdonaldii, it serves as the primary causative agent of Phoma black stem and leaf spot on sunflowers (Helianthus annuus). It is a globally significant pathogen that affects both commercial sunflower crops and wild relatives.

The fungus impacts all aerial parts of the sunflower plant. Symptoms typically appear on leaves as necrotic, dark brown to black spots with yellow halos. As the disease progresses, it infects the stem, specifically around the leaf petioles, forming large, blackened lesions. These lesions often cause the plant tissues to weaken, leading to stem breakage and premature senescence during the grain-filling stage.

The biological cycle of Plenodomus lindquistii is primarily saprophytic during the winter, surviving in infected crop debris via pycnidia and mycelia. When environmental conditions turn favorable—specifically with high humidity and temperatures between +20°C and +25°C—the fungus releases conidia. These spores are disseminated by rain splash and wind, initiating primary infections on newly developed plant leaves.

The economic impact of this pathogen is severe, as it directly restricts the plant's ability to transport water and nutrients. Infected plants show reduced seed yield, lower seed weight, and decreased oil content. In favorable disease conditions, severe outbreaks can cause losses exceeding 50% of the harvest, making it a priority target for integrated pest management programs.

Effective control of Plenodomus lindquistii requires a combination of strategies:

• Deploying resistant sunflower hybrids.
• Implementing long crop rotation cycles to reduce soil-borne inoculum.
• Conducting deep tillage to bury infected plant residues effectively.
• Applying fungicides during the critical growth stages (V6 to R1).
• Managing host weeds that act as secondary reservoirs for the fungus.