I had 2 dozen very healthy dahlia seedlings that were potted on and growing well but over the last 10 days the true leaves are first curling up then wilting and dying. The seed leaves remain fine. They have good light, warmth and sufficient water. One plant had aphids but the others were all thoroughly checked out and appear to be aphid-less. Some are growing in modules and some in 3" pots; some of each have been affected but not all the ones that are sickly were in the same greenhouse. I'm going to pot all the healthy ones on again in case it's a compost issue and keep my fingers crossed but any suggestions as to what the problem might be would be helpful.