Disease or insect?

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Last year I had one clump of TBs that developed a stem problem at bloom time. A middle section of the stem, 3-4 inches long, collapsed in on itself and could no longer support the weight of the blooms. When I examined it more closely the section looked dried out, and had a hollow core. I thought it might be insect damage, but could find no entrance or exit hole.

This year that same plant is heavily damaged again. Plenty of normal sized blooms, but the withered sections of stems start bending over. I'm especially concerned because I see the same thing happening now to two more plants --one new, one well established-- about 10 feet away.

The plants adjacent to the first plant look OK, which makes me think its bugs, but I just don't know.

Obviously I should tear these plants out if it is disease. Does anyone know about this?

Terry