Fungicide Management of Pasmo of Flax and Fungicide Sensitivity of Septoria linicola

Abstract

Pasmo of flax (Linum usitatissimum L.), caused by Septoria linicola (Speg.) Garassini, is commonly observed in western Canada every year. Severe pasmo infection can reduce both quality and quantity of flaxseed. As no flax cultivar is pasmo resistant, flax growers depend on fungicide application to manage this disease. However, continuous use of the same fungicide or fungicides may result in the development of fungicide insensitivity in the pathogen population. The first objective of this study was to determine the impact of three fungicide products, pyraclostrobin, fluxapyroxad and fluxapyroxad+pyraclostrobin; and application timing (early-flower, mid-flower and at both stages) on pasmo severity, seed yield and quality of flaxseed. Field studies were conducted at four locations in western Canada in 2014, 2015 and 2016. The results indicated that among the three fungicide products, fluxapyroxad+pyraclostrobin was the most effective. It decreased disease severity to 15% compared with 59% for the unsprayed control, followed by pyraclostrobin (20%) and fluxapyroxad (39%). Application of fluxapyroxad+pyraclostrobin also increased the seed yield to 2562 kg ha-1 compared with 1874 kg ha-1 for the unsprayed control, followed by pyraclostrobin (2391 kg ha-1) and fluxapyroxad (2340 kg ha-1). However, application of these products also delayed crop maturity. Fungicide application at mid-flowering stage was the most cost effective and decreased disease severity and improved seed yield. The second objective of this study was to determine the sensitivity of S. linicola isolates to pyraclostrobin and fluxapyroxad fungicides by spore germination and microtiter assay methods. In the spore germination assay, the EC50 of pyraclostrobin for most of the isolates was between 0.0005 to 0.007 µg mL–1 and fluxapyroxad between 0.001 to 0.05 µg mL–1. In the microtiter assay, the EC50 of pyraclostrobin was between 0.003 to 0.021 µg mL–1 and fluxapyroxad was between 1 and 4 µg mL–1. No fungicide insensitivity was observed among the isolates of S. linicola to these fungicide products. The findings of these studies will help to make effective pasmo management recommendations and provide a baseline against which to measure future S. linicola isolates suspected of insensitivity to pyraclostrobin and fluxapyroxad fungicides.