You may use vinegar available in the market. This needs to be mixed with 4 litres of water using one tablespoon of vinegar. preferably apple cider vinegar. Shake the mixture thoroughly before transferring the mixture to a spray bottle. You may treat the fungus infection with the application of this mixture regularly.
The result will be visible within a week and in case you get a poor response, continue for a week further. Hope, this treatment will work.
The other alternative is cinnamon powder. Take half a teaspoon of cinnamon powder, dissolve the same in a glass of water thoroughly and pour the same inside the root of the plant. If such a treatment is done for a couple of days, you will notice the result within a week. Depending upon the condition, this can be extended a few more days. It does not harm the seedings or the soil while killing the fungus growth.