Black spots on tomato stems - diagnosis?

[Templeton]

Hi Folks, asking for a friend in Barcelona, Spain. I've had a look through the stickys, but can't really find anything appropriate.
His tomatoes have these large black necrotic areas on the stems. He grows in a community garden, and many of the plots are suffering. The locals are confused, haven't really seen it before.
It doesn't seem the leaves are affected (yet). the stems have discolouration in cross section. Hoping someone can help an Aussie asking from Oz for an Aussie to help the Catalans in Barcelona!