[galen]

A bit of a strange one this...

Always clean my pots and seed trays with recommended Jeyes fluid dilutions over winter before reuse

In theory compost I buy, reputable and a well liked make, is also "sterilised"

As soon as I add Perite to the mix I seem to get a problem. When I sow seeds I use an anti damping off product to spray surface, same when pot on but as soon as I water anything, I get a white, quite vigorous, mold on the surface. It only seems to occur when I've either added Perite to the original compost or used it as a kind of mulch with the seedlings. Should also say I water from the bottom.

I'd assumed that Perite for horticultural use is also sterile but am I wrong on this? Apart from some basil grown as a winter windowsill crop haven't lost anything yet but am getting concerned, especially as it's now hit my chilli and tomato seedlings 

[solway cropper]

It could be that the fungal spores are airborne and not actually in the compost/pearlite.

If you give them ideal growing conditions they'll grow!!

just a thought.

[galen]

That's a good point which I never thought of even knowing some basics on the subject - Well 20 years ago I did. Newly converted property, seems to be hermatically sealed at times and yes I know I can open windows...

Started some brassicas using same compost mix (will I ever learn) in unheated plastic greenhouse last week, I'll see how they fair 

[New shoot]

Perlite is expanded rock and is totally sterile Galen.  It does have a lot of water holding capacity, so maybe it is sitting too wet and that encourages the mould you are seeing?

Just a thought, but where are you doing your potting up?  Are there existing seed trays or plants nearby that have this mould on the compost surface, because as Solway Cropper says, the spores would be airbourne.  This would include houseplants.  I would also check and clean any watering equipment you use