My daughter has a small tomato patch. The skin of her tomatoes start splitting before they fully ripen. Why is this, and what is the cure? I know cherries will split if they get wet. Is the same true for some varieties of tomato? - Jinx
When mine start doing that, it is almost always after we’ve had some fairly heavy rains over a period of a few days. One day of rain doesn’t seem to do much, but two or three days, and they start splitting.
I assume this is because the fruits are absorbing more water than the skins can accomodate in a short period of time, but you know what they say about assumptions around here, so I wait for a more knowledgeable source as to the reason(s).
Some diseases cause cracks. Some varieties are crack resistant, but none are crack proof. Water available just as they reach the ripe stage makes it more likely they will split. About all you can do is pick them early to avoid the cracks or live with it.
Tomatoes ripen in the dark, so if you do pick them early you can ripen them in a paper bag placed in a warm spot, like the top of your refrigerator.
Inconsistent watering will cause splitting. Put your lovelies on a schedule and mulch them heavily.
In addition to notes above, some strains or more predisposed to splitting than others. If you think the splitting is a problem try and find a different strain next year.
Over or improper fertilizing can cause fluctuations in growth and encourage splits.
What is actually happening is that tomatos put off ethylene, which is a gaseuos plant hormone. That is what is ripening your tomatos when you put them into a paper bag, not the dark.
Gassing them with ethylene is how green tomatos are ripened for sale in the store.
Let 'em split, and eat 'em anyway. They’re just as good. 
Seriously, just be glad you have tomatoes. The blight is in the process of stealing mine, and we had such incredibly horrible weather this summer that none of them even ripened before the fungus hit.
None of mine were ripening, and the blight finally killed a bunch, and now some explosion of rodents have been gorging on the remaining ones that have just started to ripen. They gutted about a dozen 5 inch tomatoes today, and uncounted small ones. They’re actually worse than rabbits because they can get through the cages and climb the vines to eat the ones five feet in the air. I’ve actually have a couple vines that reached 9 foot this year. This is a first for me. I guess it’s the 50% horse manure soil mix in that part of the garden I redid. I’ll be ripping out some vines tomorrow as there is no reason to keep some of the plants.
My worst splitting tomato this year are the pear. I either pick them before fully ripe or they split.
I have to laught that, for all the years my folks had a tomato patch, I never recall splitting to be a problem. Yet, from this thread, it seems almost everything can cuase it. Thanks all for the advice. We’ll have to try several of these ideas.
as mentioned ethylene promotes ripening.
if you allow the gas to concentrate it will promote ripening.
if you want to not promote ripening then keep the fruit ventilated and separated from each other. cooler temperatures will slow it down as well.