The saprophytic growth of fungi around the surface of soil is a product of temporary sterilization by some means and a biological vacuum allows for rampant saprophytic sugar fungi development. Normally the growth micro organism is checked by competitive lack of nutrient. After a sterilization effect occurs the field lays open and nutrient is release and sugar degrading fungi rapidly cover the soil. The disequilibrium this represents is remedied when pasteurization rather for full microbial die off is used for soil treatments. Iprodion might or might not be effective for eliminating the fungus. We might be able to have better idea if you could obtain the fungal identification and we could check out  the literature on it. If we can get an identification we could search for fungicide efficacy data and make some recommendation but we probably cannot predict how this might effect Nitrogen factors such as Nitrogen transformations of micro organisms as nifification or denitrification cycles.

Thank you for the reply. I haven't sterilized the soil by any means. The fungus is cottony white over the surface of soil. Not only Arabidopsis, but close-by ryegrass plants are also affected. The humidity level in the room is high (60 - 70%). The point about identifying the fungus is good. However since Iprodione is a broad spectrum fungicide, it should eliminate the fungus with ease. I want to try it. But my main concern is while trying it on my experiment it might interfere with the experiment itself. As in my experiment I am providing my Arabidopsis plants a very controlled and known N supply. I think I might test some extra plants first with Iprodione before I use it on the experiment. Thank you