The causal organism of this disease is Septoria linicola, a fungus that attacks above-ground parts of flax and overwinters in the soil on infected flax stubble. Flax is most susceptible to Pasmo in the ripening stage. Epidemics, however, can occur early in the season when favourable, moist conditions prevail. Pasmo can cause defoliation, premature ripening and can weaken the infected pedicels (the stem that attaches single flowers to the main stem) resulting in heavy boll-drop by rain and wind. Depending on the earliness and severity of the infection, Pasmo reduces the yield as well as the quality of seed and fibre.
Appearance: Infected flax tissue is characterized by tiny black pycnidia which are the fruiting bodies of the fungus. The debris carries numerous pycnidia which overwinter and produce masses of spores that cause the initial infections on leaves and stems. Spores are dispersed by rain and wind. High moisture and warm temperatures favour the disease. Lodging favours the development of Pasmo, because of increased humidity within the crop canopy, and results in patches of dead plants completely covered with the fungus.
Pasmo is characterized by circular, brown lesions on the leaves and by brown to black infected bands that encircle the stem. These bands alternate with green, healthy bands, making Pasmo easy to identify. Flowers and young bolls are also blighted. Older bolls are discoloured and contain shrivelled or non-viable seed.
Disease Cycle: The Pasmo pathogen is seed-borne. It also overwinters in the soil on infected flax stubble. Spores, dispersed by wind and rain, cause the initial infections on leaves and stems. It attacks above-ground parts of the plant.
Disease Control: Most commercial varieties lack resistance to this fungus. Pasmo can be controlled by using a crop rotation of at least 3 years to non-host crops and using clean seed. Early seeding, at the recommended rates, can reduce infection by avoiding high moisture conditions in the fall.