Yes, I had the exact problem you described when I was a post-doc at
Berkeley. It's not Pythium, best guess from a fungal plant pathologist at
Berkeley was Sclerotinia in that this was easily culturable, although
causality was not rigorously established. I, like you, also found
fungicides, soil sterilization, and seed sterilization to be useless
against this pathogen. What I did (we haven't had the problem since) is as
1. Make sure you have adequate drainage--i.e., don't try to sow seeds on
one big flat of soil, use 3'' pots of soil.
2. Don't overwater--i.e., make sure the soil is on the moist side when you
plant and then don't water at all until you remove the domes and spray for
the first time.
3. Make sure you don't overdo it on the amount of time under high humidity
conditions, especially under greenhouse conditions--i.e., only dome the
flats until you see the first set of true leaves starting to emerge. Then
remove the domes and spray with BASTA.
4. Limit airborne inoculum--i.e., separate your flats from others,
especially once you remove the domes, and clean the area first, especially
if previously used with plants which set seed.
This worked, and we never had the problem again, either in Berkeley or
subsequently in Delaware.
>>I'm sowing seedlings at high density on soil for herbicide (Basta)
>spraying and selection of transformants. We are consistently getting
>problems that in a fair proportion of flats, some kind of damping off
>fungus gets in, usually around or after the spraying, and this gradually
>spreads out, plants go brown and keel over, and we can lose large no.s
>of plants including transformants. I have tried sterilising seed before
>sowing, and this helps a little but certainly does not eliminate the
>problem. I am expreimenting with pre-soaking the soil in fungicides of
>various sorts, so far without great impact for good or bad. I imagine
>the fungus problem might be a Pythium, but am far from sure. Has anyone
>else encountered this and have they found any could preventatives...
>>Thanks a lot