Crop disease surveys are performed for several reasons. MCA looks to this project as being useful for extension purposes, as well as providing data to support research priorities in Manitoba.

The disease survey is conducted each August – September, depending on maturity. Eight to ten fields are scouted, primarily for pasmo incidence and severity, but also for Alternaria, aster yellows, fusarium, powdery mildew, rust and sclerotinia. These are all uncommon in Manitoba, and considering only 100 plants are being rated for the above diseases, it is rare to have incidence of anything other than pasmo and aster yellows reported. The last time aster yellows were reported in the flax disease survey was 2023, when incidence was relatively high in a small number of tested fields. Since then, there has been low incidence but it can still be found on the odd plant. Mainly because it is easy to identify and the infected plants stick out among “normal” plants.

In the 2025 flax disease survey, 10 fields were surveilled, which equivalates to one field for every approximately 2,800 acres of flax. For larger acreage commodities, there would typically be one field surveyed for every 5,000 acres in a Rural Municipality.

Due to dry weather conditions in 2025, disease incidence was very low to zero for every pathogen looked at. In fact, only three of the 10 fields scouted showed signs of pasmo in the 100 sampled plants. In those three fields, incidence was 10%, 14% and 2%, which is to say that only 10, 14 and two plants in the 100 plants sampled in each field had pasmo present. Severity refers to the total area on the plant that is infected with pasmo, expressed as a percentage. In the three fields with pasmo present, severity ranged from 1% to 30% on the plants affected. Overall, incidence and severity were both lower than normal; likely an effect of dry conditions early in the season. Pasmo, like several other pathogens, spreads rapidly in high temperature, damp conditions.

Pasmo is 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. Pasmo can cause defoliation, premature ripening and can weaken the infected pedicels 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. Most commercial varieties lack resistance to this fungus. Pasmo is characterized by circular and brown lesions on the leaves and by brown to black infected bands that alternate with green and healthy bands on the stem. The best control is achieved by early seeding at the recommended rates to avoid high moisture conditions in the fall, using clean seed, treating seed with a fungicide, controlling weeds, and following a rotation of at least three years between flax crops.

Pasmo-infected flax stalk.

Aster Yellows is a very random disease, infecting plants that have been fed on by aster leafhoppers that are specifically infected by the aster yellow phytoplasma. Aster yellows affect many different crops, with canola being the most economically significant in Manitoba. Aster leafhoppers are sucking insects that transmit aster yellow phytoplasma directly into the phloem of a healthy flax plant, thereby infecting that plant. Damage is evident in misshapen or unproductive bolls.

Flax aster yellows.

In preparation for the 2026 flax crop, farmers will want to follow some key tips for disease prevention:

• Use clean seed
• Diversify crop rotation
• Use seed treatments
• Keep fields weed-free

For more information on:

• Importance of Crop Disease Surveys
• Diseases Affecting Flax