impossible without a picture...

but even treated dead mildew does not go away ...

watch for new growth .. if any .... after what you did to the very young seedlings ..

and if that comes out clean.. then you solved the problem.. but the old leaves will never look fresh again ...

also .... not knowing where you are.. or where the plants are.. figure out how this happened ... its usually wet leaves .. at night .. with too much night heat .... [thats when mildews grow ... and why this is usually not a spring time problem] ...

so to avoid it.. dry up you media a bit.. never wet the leaves in the evening ... increase air movement .. and reduce night temps.. if possible .... yeah i know.. easier said than done... lol ...

ken

ps: soap/oil and soda on a babe ... thats a bit overkill ... try not to react so heavily next time .... i am surprised they didnt get chemical burn .... its bordering on trying to kill them.. with too much love ...

thx 4 the reply ken.

sadly I don't have any good quality camera that can take picture very good, I have tried but it seems not view the dust that covered on my seedlings, because the dust will only seen when the leaves get direct sunlight and I have to watch it very near.

I live at south east asia, the temperature is very hot, it can reach about 27 C during the night and can reach 40 C during the noon.

u're right about soap and oil.
at the first time I sprayed that formula in the morning (about 7 am) a little number of leaves were burned within 2-3 hours, but when I apllyed at the evening after sunset it doesn't seem bring bad effect to my seedlings.

that baking soda formula doing good on my crape myrthle, just only 2 times treatment the powdery mildew goes at all, but when I applyed to the my lavender seedlings it doesn't affect at all.
I've never give water directly on the leaves.
the growth seem decreasing now, what I have to do?
is it better to continue baking soda treatment or using diluted milk or anything better than all of those treatments?

More than likely you have a friend, neighbor or relative who could help you with posting a picture. Ask around, please.