Its uneven watering or so they say, mine havn't been so badly affected this year which could be the varieties but as well as keeping the grobags as moist as possible ---on really sunny days --- but only when the greenhouse is shady and not later at night-- I spray the plants with a fine hose until it drips all over the leaves But you must be careful when you do it as they won't like to sit cold & wet

personally i keep my ground damp all the time, particuarly in hot and dry weather. I used to grow mine in grobags and had the same problem, but now I plant them straight into the greenhouse soil, which I change every 2 to 3 years with compost from the compost bin mixed with soil from the garden.
Touch wood I have had no problems since.Hope that is some help to you. Diadon

Watering more often almost always cures them, Alex. In the few occasions where it doesn't, applying limestone or gypsum to the soil helps.

Mines a concrete floor so grobags work best but an answer on Goy recently made me think that next year i'll still have the grobags but on each hole put a large pot and plant into that first

Tug ( sorry for hijacking your question Alex) I'm growing 2 varieties this year both as cordons the normal size toms are not a problem but this is the first time I've grown cherry type ( called cherriola) as a cordon and although the trusses have 20-30 toms on each one nearly every one splits although very few have black bottom---- any ideas?

Sounds odd to me, Pam. Drought, just one or two episodes, is usually what I think of when I see splitting fruit, but that doesn't sound like you! Did it happen shortly after a holiday?

no-- its happened all the time , in the past I've grown 'balconi' or 'tumbler'-- all bush tomatoes and no problem but these are cordons huge plants and so vigorous I've never had that many on a truss before-- its quite incredible--- but they split! ( and taste good too )

Well, outside of bigger plants needing water more often, it may just be the nature of that variety's response to the greenhouse conditions.

Tug, good as they are I think its back to the bushes next year but thanks for your speedy advice

I never could get 'San Marzano' to stop cracking, so I went back to 'Roma', too.
Ask ten gardeners what the best tomato is, and you'll get ten different answers!

true-- my others are a new variety to me called ' stupice' a potato leaved sort and these are robust plants that have been easy to grow, they havn't the sweet taste of the cherries but roasted with olive oil --- everything you could want -- its a good crop and am freezing those we can't eat or give away for soups & sauces they are so good