I found that ventilation usually takes care of damping off on brassicas (cabbages, brocoli, etc.) seedlings and other seedlings as well. I think it should prevent the problem on apple seedlings, as well.
To prevent damping off, it helps if you fill whatever container you are going to use to the brim with the soil. This favors air circulation better than if the surface of the soil is in a depression (i.e. the sides of the container are higher than the soil surface). It makes a difference, I have noted this several times.
Also, if the surface of the soil is very wet and the day is overcast, one is more likely to have damping off on the plants that are most susceptible. In a flat of hundreds of tiny lettuce plants, the damage can be devastating and extremely quick. In fact, by the time you notice the problem, it is usually too late. The plants shrivel to a thread in a few hours and fall over. End of story.
In case of doubt, use a fan to keep the air gently moving in the area where you are growing your seedlings and you should not have to worry too much about it.