Hi there,
if tubers rot, it is most likely caused by too much water in the soil.
And you mentioned a wet sprig, so that could be a factor.

What kind of soil do you have, loamy or sandy?

If they started growing already and started wilting later, that could mean that they were healthy at the beginning but struggled with the soil conditions.

The sheer number of your failed Dahlias isn`t an accident. I would look at the soil first of all.

I am not familiar wit your climate. was it mild weather when you planted in May?

Dahlia tubers hate cold wet soil, so that could be another angle.

Good luck with your Dahlias, bye, Lin

and perhaps you could post a pic of the Dahlia bed, that would be helpful

Its most likely wet soil. Dahlias are native to Mexico. I had to transplant most of my tubers just days after planting them this year because of excess moisture and the area holding water just below soil surface, so that the soil around the tubers was sopping wet and muddy. I had to build mounds out of some sand/soil mixture about 7-8" tall, then I replanted the dahlias about 5-6" deep in the mounds, so that they are all about 2" above the ground around them. That should protect against sitting water.

Its either that, or the dahlia tubers you were provided werent viable, and were already dead, or didnt have eyes where the stem grows. I have seen both. Some times the packs of dahlia tubers sold at stores will die due to drying out, etc. That, and many tubers dug up in the fall at the end of the growing season will not produce an eye. If they dont have eyes, they wont grow.

Thanks for your help Highlander and linaria Almost all the tubers came from Swan Island so I am assuming they were viable. The soil is water retentive but not soggy and I planted them when the soil was warm. It is just odd that about 50% - 60% came up in the beds and then about 20% of those died the rest look fine. If it is a soil issue wouldn't all the tubers be affected? I really want to keep the ones that are up and healthy that way. Could this be a fungal problem?

I bought 8 tubers from Swan Island a few years ago, and I am growing the descendants from 4 of those varieties now. One of the tubers I bought from Swan Island didnt grow. Actually, it came up, but then abruptly died even though it was planted in the same area, under the same conditions of the other 7. I also had planted a tuber I bought from a local store that was obviously viable when I first bought it because I could see the stem coming out of its eye in the bag. I planted it about 2 weeks before the other 8 mentioned above, and it came up. But then we had 4 days of intermittent rainfall, and that tuber rotted and the plant died.

I would recommend you dig down to the level of a few of the tubers you planted and see if the soil at tuber level is wet. The problem you may have is that the soil may have been wet before, but now it could have dried out significantly. Its odd that so many good tubers would die for no apparent reason. For so many to die, there must be a reason aside from just random chance. If it was just a few you could chalk it up as nature's version of Murphy's law, as often a few bulbs or tubers from a quantity will not grow. But for so many to start growing and then die is odd. Even if you planted lots of tubers and they just never came up, that would be unusual.

Its hard to say what else it could be without being able to see the soil, or without being able to see the planting area and the lay of the land. Maybe a soil test would be a good idea, but the best things to do are: 1. Ask your local cooperative extension service and give them a soil sample, show them pictures, etc. 2. Ask Swan Island for a credit for the tubers that failed to grow.

Thank you Highland for your advise and concern. I have decided that it is that the soil is too wet and I am going to move the remaining dahlias to higher ground. My local extension service has been called and a soil sample taken. I have to figure out how to get better drainage in those beds. I cannot build up the soil because the beds are contained in stone. First things first today I am on a rescue mission. I have never moved dahlias before but I believe they will die if I leave them where they are. Wish me luck. Thanks again.

I would suspect a ground fungus, as rotting of the tuber normally doesn't happen when roots and sprouts are well-established, unless it's standing water.

Better drainage is a good call, but a preventative fungicide would be a good thing, too.