|I just reread your initial post and realized, you don't have an antenna at either end. It looks to me like this "yagi" thing is an extender, not an antenna. If so, this could definitely mean the leaves are causing interference.|
If this setup ran OK without any leaves on the trees but isn't now that there are leaves, then it could very well be the foliage causing your issue. Did the issue start about the time the leaves came out?
This could be easily fixed by using two antenna's in a point-to-point shot from house to shop.
If you have the extra $$$, you may want to look at buying two SOHO level antenna's and making a direct point-to-point shot from house to shop. One brand name I'm familiar with is "EnGenius". Google them and have a look.
A point-to-point shot like this would negate the trees as being part of the issue.
If however, your setup did run OK with leaves on the trees then please say so now so we can rule that out as an issue
Just an FYI folks, I have a point-to-point wireless shot running between two buildings here in town (about 1/2 mile apart) and there's a hill, thickly covered with trees in between. There's no degradation of signal in the summer after the leaves come out. Mind you, this is enterprise level equipment we're talking about and not mickey mouse SOHO stuff so that could be why.
Also, changing from G or N to A/B won't make any difference.