As you’ll see in the videos as the plant start to grow, I have a small fan I run constantly any time I’m sowing seed (or taking cuttings). I want the air around the plants leaves to dry out the leaves.
Dry leaves are less likely to be attacked by any pathogen – and a nice dry leaf is a good thing in the greenhouse.
I use a circulating fan that waves back and forth so every bit of the seedling rack has air moving across it. When all the racks are filled, I’ll have two fans moving air (a bigger one does two levels and the small one is aimed at the top)
Too many people ignore this simple step.
It was one of the most important lessons I learned in the greenhouses and our air circulation fans ran 24-7 when there was any kind of crop in the greenhouse.
Note that it also keeps the air temperature in the room at a more constant level between the bottom (potentially cooler) and top (potentially hotter) racks. This even temperature in the room from top to bottom also helps seed germination.