Hi: Most the seed fungicide treatment is systemic fungicide, I don recommend to use this source of seed for pathogenicity test. It will still have remained fungicide inside the seed tissue.
I recommend you to avoid usind seeds treated with fungicides, in particular the systemic one. It is not easy to remove internal fungicide residual. but as Aqleem Abbas said , you can reduce the proportion of fungicides by washing intermittently with water.
I do not think you can get rid of fungicide from seeds, especially if they are systemic fungicide as Dr. Jawad explained. However, if your fungicide is not systemic (on surface) , you could get rid of it by utilizing distilled water through washing many times...