I would like to share my experience I had with Miracle-Gro potting soil this year. I usually use it every year for transplanting seedlings into cups with no problems. This year, as usual, I bought some again to use for my cool season crops such as cabbage, Brussels sprouts, broccoli, etc. Everything was going well until about the second week, we noticed our seedlings turning yellow, and before long our entire group of seedlings were infested with aphids. All these seedlings were inside under grow lights on a shelving unit. I know of no other way aphids could have gotten on the seedlings except from the Miracle-Gro soil. We had a few ladybugs that had got inside the house as they often do and they were flocking to the seedlings eating the aphids. I mixed a solution of Dawn soap, tobasco sauce, olive oil, and water and sprayed the seedlings, almost in a panic to save them. The solution killed the aphids but my seedlings were set back quite a bit, they're just now ready to set out, a little late for my zone. I started mixing a little plant food in their water that seems to have helped bring them out of being stunted.
I can't positively say the aphids came from the Miracle-Gro soil, but I can think of no other way we could have got an infestation inside the house like that. And like I previously stated, we have had no issues before and used them same soil for the past several years. The cool crop seedlings are now outdoors being hardened off, I hope to plant them this week (my mother-in-law was visiting from Kansas, became very ill, doctors discovered large tumors in her lungs and brain) so I don't know if I'll get a chance to even plant them at all. I switched to a Scott's brand soil for my tomatoes, they are in the same area, no pests noted so far.