Cosmos Seedlings Dying
I’m looking for some help with my cosmos seedlings. I’m tending to my joint studio’s container garden but I’m a very novice gardener this is the first time I’ve grown anything from seed.
My cosmos that were looking very healthy and strong and were responding well to hardening off have suddenly gone down hill rapidly. Their leaves are turning silvery and brown and then shivering up and dying. The bigger seedlings are holding their own but the smaller ones have been decimated and I’m worried the bigger ones won’t last much longer.
I was wondering if it could be powdery mildew and if it was what the best way to treat it is?
Other possible problems I can think of is that they’re in miracle grow compost which I bought as wasn’t sure what to get but it’s horrible stuff with white mould appearing in the soil. I switched to Canna 50L Terra Professional Soil Mix Bag for my sweet peas but cosmos still in the miracle grow.
My studio room that I’ve been taking them in and out of is very hot and humid as I don’t have control of the studio heating and they’re being very slow to turn it off. It seems worse after leaving them inside my room for a day when I was out.
Been keeping plants in separate pots but in trays together for ease of bringing them in and out. Maybe not enough air circulation.
I’ve been wondering if keeping them outside now would help, I’m in London and been hardening them off for 10 days.
Any advice much appreciated!
P.S Could anyone tell me when it’s ok to plant out Black Eyed Susan Vine?