Cobweb mould

[LindaG]

Hi everyone - Happy Friday! Two of my phals have developed a cobweb mould. When I spotted it last weekend I repotted them in sterilized Leca after dunking both of the plants in bleach. Today the stuff is back! I thought I had given them a new start but apparently not. Plan to increase to strength of the bleach solution - is 2 TBL per 1/2 litre (2 c) of water too strong for the plants? And now I'll have to bleach the Leca before I reuse it. Any other ideas? I put these 2 on a heat mat for the week - their roots aren't as developed as I would like. Where does this mould come from anyway? I sure don't want to see it on anything else.