Wilting Buddleia.

The most likely reason for the base to rot is poorly drained soil. Water moulds are found in all soils, but need it to be very wet in order to infect and damage a plant. Heavy soils are the worst for this.

Buddlejas, despite their reputation, are surprisingly prone to the roots rotting off. And, so I've recently found, this is even more likely when a wet winter follows a very dry summer. Their natural habitat is wet in summer and dry in winter.

Preparing the soil well before planting always helps, as does regular mulching - which both keeps moisture in during dry spells and improves drainage by encouraging worms to burrow around the roots. I also use a little lime as a both a clay breaker and to promote biological activity. Buddleja tolerate all soils, but do prefer slightly limey conditions so it's a win-win.

 

Today we lifted the rest of the Buddleja roots and they were very healthy looking. The soil it was planted in was very loose. It was a mixture of soil and compost and was not waterlogged in the slightest, despite all the water I had put into the ground over the past 10 days. It was just the two main stems that had rotted just above ground level.
I didn't get every last piece of root out so perhaps next year or the year after it may reappear. One can only hope.

 

Is it just regular buddleia davidii?

I happen to know it was one of the easiest plants to start from cuttings if you see any nearby.

 

It was a cultivar, Buddleja Berries and Cream.