Pasmo disease in flax
Summarized by Dr. Favaratto and Dr. Smyth based on Islam et al. (2021) research - Fungicide management of pasmo disease of flax and sensitivity of Septoria linicola to pyraclostrobin and fluxapyroxad: "Among the diseases that have the potential to cause damage to flax (Linum usitatissimum L.) every year, the fungal disease pasmo, caused by Septoria linicola, is the most important. Fungicide application and a diverse crop rotation are the most important strategies to control this disease because there is little variation in resistance among flax cultivars. However, few fungicide products are available to flax growers. Field studies were conducted at four locations in Western Canada in 2014, 2015, and 2016 to determine the effect of two fungicide active ingredients applied singly and in combination: pyraclostrobin, fluxapyroxad, and fluxapyroxad + pyraclostrobin; and two application timings (early-flower, mid-flower, and at both stages) on pasmo disease severity, seed yield, and quality of flaxseed. The results indicated that among the three fungicide treatments, both pyraclostrobin and fluxapyroxad + pyraclostrobin controlled pasmo effectively; however, fluxapyroxad + pyraclostrobin was the most beneficial to improve the quality and quantity of the seed for most of the site-years. Disease severity in the fungicide-free control was 70%, and application of fluxapyroxad + pyraclostrobin decreased disease severity to 18%, followed by pyraclostrobin (23%) and fluxapyroxad (48%). Application of fluxapyroxad + pyraclostrobin also improved seed yield to 2,562 kg ha-1 compared with 1,874 kg ha-1 for the fungicide-free control, followed by pyraclostrobin (2,391 kg ha-1) and fluxapyroxad (2,340 kg ha-1). Fungicide application at early and mid-flowering stage had the same effects on disease severity and seed yield; however, seed quality was improved more when fungicide was applied at mid-flowering stage. Continuous use of the same fungicide may result in the development of fungicide insensitivity in the pathogen population. Thus, sensitivity of S. linicola isolates to pyraclostrobin and fluxapyroxad fungicides was determined by the spore germination and microtiter assay methods. Fungicide insensitivity was not detected among the 73 isolates of S. linicola tested against either of these fungicides."
Fungicide management of pasmo disease of flax and sensitivity of Septoria linicola to pyraclostrobin and fluxapyroxad
Significance
Pasmo is a fungal disease caused by Septoria linicola that affects flax crops, leading to substantial losses in both yield and seed quality. In Canada, especially in the Prairie provinces, pasmo is often the most important disease of flax. The problem is worsened by the fact that all commercial flax varieties are susceptible, and options for disease control are limited. This study aimed to evaluate how well different fungicides and application timings could reduce disease severity and improve flax production.
Analysis
The researchers conducted field trials over three years (2014–2016) at four locations in Western Canada, including Brandon, Melfort, Saskatoon, and Vegreville. They tested three fungicide treatments: pyraclostrobin alone, fluxapyroxad alone, and a combination of both (sold as Priaxor). Fungicides were applied at different flowering stages: either early flower (when about 10% of the flowers had opened), mid-flower (7–10 days later), or both times. This design allowed the team to see not only which fungicide was most effective but also whether the timing or frequency of spraying made a difference. The researchers measured several indicators: how much disease developed on the plants (disease severity), the yield of seeds harvested per hectare, the weight of 1,000 seeds (an indicator of seed size), the test weight (density of the grain), and the oil content of the seeds.
One of the most important discoveries was that using the combination of fluxapyroxad and pyraclostrobin gave the strongest disease control and yield benefit. In fields where pasmo pressure was severe, untreated flax had disease severity as high as 70%. The combination fungicide dropped that down to 18%, a dramatic reduction. This led to an average seed yield increase of about 37% (from roughly 1,874 kg/ha to over 2,560 kg/ha). Pyraclostrobin alone also performed well, cutting disease to 23% and boosting yield by 28%. In contrast, fluxapyroxad alone was less effective, reducing disease to about 48% and increasing yield by 25%. This shows that while both active ingredients help, combining them produces a stronger and more consistent result.
The timing of application also played an important role. Spraying fungicide just once at early flowering or mid-flowering significantly reduced disease. However, applying it twice—both early and mid-flowering—lowered disease severity even further, reducing it by an extra 10–13%. Interestingly, although two sprays gave the cleanest plants, the yield benefit of spraying twice was often only slightly higher than spraying once, and sometimes the difference in yield was not statistically significant. This means that in many cases, a single spray at mid-flowering was enough to protect yield and improve quality, making it a practical compromise between effectiveness and cost.
Fungicide treatments also influenced seed quality. The combination of fluxapyroxad and pyraclostrobin not only increased yield but also improved thousand seed weight, test weight, and oil content compared to untreated crops. In particular, mid-flowering applications led to slightly higher test weights and oil content than early flowering sprays. These quality improvements are important for flax processors and end users, as they can increase the market value of the harvest.
An additional part of the study looked at whether Septoria linicola populations had begun to develop resistance to these fungicides. Researchers tested 73 samples collected from multiple sites and used laboratory methods to measure how sensitive the fungus was to pyraclostrobin and fluxapyroxad. No evidence of fungicide insensitivity was found, suggesting that these products are still effective across the region. However, the researchers cautioned that repeatedly using the same fungicide, season after season, increases the risk that resistance will eventually appear.
For this reason, the study strongly recommended that farmers rotate fungicides with different modes of action or use combination products such as Priaxor rather than relying on a single active ingredient. It also advised avoiding unnecessary spraying if disease pressure is low, since overuse of fungicides not only wastes money but speeds up the development of resistance in pathogen populations. In years with low rainfall and dry conditions, such as in Vegreville during 2014 and 2015, pasmo was not detected at all, and fungicide applications did not improve yield or quality—highlighting the importance of tailoring fungicide use to weather conditions and disease risk.
Conclusion
The research demonstrated that flax growers can significantly reduce pasmo and improve their harvest by using fungicide treatments when pasmo is severe, especially the combination of fluxapyroxad and pyraclostrobin. The most practical strategy appears to be spraying once at mid-flowering, which offers strong disease control without the added expense of multiple applications.