In article dbc13f5f-0b1d-4459-afdd-590a47300d05
@c16g2000vbp.googlegroups.com, says...
Not enought potassium/potash or too much heat.
Tomatoes need tomatoe fertilizer on a regular basis (high in potash).



Possibly, heat is not really controlled much, no shading in greenhouse,
but then I've never had the problem in previous years.

Feeding is a possibilit, as this year I invested in a drip feed watering
system (cheap from Aldi), and realised that I've only been feeding once
a week, with a 10 litre can doing four grow bags (3 bags and 3 pots
actually!)

Thing that confi=uses is that some fruit are fine and others effected on
the same plant! Also fruit size is very large this year!

Roger T