Pasmo disease of flax

Mycosphaerella linicola (anamorph: Septoria linicola) is a destructive fungal pathogen belonging to the Ascomycota division. It is the causal agent of Pasmo, one of the most common and damaging diseases of flax worldwide. The pathogen targets the entire plant, leading to significant physiological stress and reduced yield.

The symptoms of Pasmo typically appear as brown, circular or irregular spots on the leaves and stems. As the disease progresses, these lesions merge, eventually encircling the stem and giving it a distinct banded or mottled appearance. In severe cases, the infected stems become brittle and may lodge prematurely, severely impacting the quality of the flax fiber.

The life cycle of Mycosphaerella linicola is complex and involves both asexual and sexual stages. The fungus survives the winter primarily on infected crop debris and as mycelium in the seeds. In the spring, spores are released from pycnidia or perithecia, which are then spread to neighboring plants via water splashing and wind, initiating primary infections.

The environmental conditions favoring the development and spread of the disease include high humidity and temperatures between 18°C and 25°C. Prolonged periods of rainfall facilitate the rapid germination of conidia, leading to rapid disease escalation within the crop canopy. High plant density often exacerbates these conditions by reducing airflow and maintaining leaf wetness.

Control measures are essential to minimize economic losses caused by this pathogen. Key strategies include the use of certified, disease-free seed, implementation of a robust crop rotation program (at least a 4-year break), and the application of fungicides during the growing season. Proper management of crop residues through tillage also helps in reducing the initial inoculum load in the field.