Dying Plum Tree: Swales in a Maritime Climate

I'm revisiting the idea of using swales in a maritime climate. I put in a swale / mulch basin about off of one of my downspouts.

The first year, an established plum tree showed signs of being over-watered, even without a swale.

The next year, I put in two, small four-way plum trees, one about 15 feet downhill from the swale and the other not affected by the swale. The one by the swale also got mulched while the second one didn't. The first year, the one by the swale grew much, much better, so I was thinking "hey, good results!" The second year, it quit raining at the beginning of May, so the trees got an equal amount of water, provided my me and a five gallon bucket. This year, we had a normal but warm spring and both trees looked fantastic. Lots of fruit that I was going to let them bear this year. Then, it really started to rain at the end of May and has rained heavily all through June. My tree below the swale is nearly dead from being overwatered while the tree not by the swale continues to look great.

Between the original mulching and the water from the swale, I am thinking that it made its feet too wet to survive a rainy spring and early summer. My squash is also having a hard time getting off the ground with such wet feet this year. I'm OK losing squash but not so OK with losing trees. I'm thinking that swales might not be the best idea in my area.

Yeah. We've had 2 years in a row of heavy spring rains. The result has been my swale being full for 6 weeks. I had a fungus hit my plum tree but pear trees did ok.

I need the swales for the 6 months of little rain, not the 3 months of heavy rains.

You are experiencing the Plume Effect from your swale.
In your situation it might actually be better for the trees to be on the uphill side of the swale, that way the plume will go away from the roots but the trees can send roots under the swale and get all the water they need without being drowned.