Fungicide Management of Pasmo of Flax and Sensitivity of Septoria linicola to Pyraclostrobin and Fluxapyroxad.

Among the diseases that have the potential to cause damage to flax every year, 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 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 at most of the site-years. Disease severity in the fungicide-free control was 70%, 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 2562 kg ha-1 compared with 1874 kg ha-1 for the fungicide-free control, followed by pyraclostrobin (2391 kg ha-1) and fluxapyroxad (2340 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 were 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.