Boron

It could be irregular watering - if they have been dry for a while and then receive lots of water the sudden influx can cause the fruit to swell after the skin has set so it splits.

Plant rhubarb in autumn after the leaves have died down. This is when you can buy the crowns. Prepare the ground well as its a hungry plant. And don't pull any stems the first year.

Yup, agree with Stera.

I know that erratic watering can cause split skins but this has also happened to my toms in a hanging basket. They are watered twice every day and so that made me look further in to the problem. Apparently, changes in temperature can cause skin splitting too and this made sense in my case because of the hot days and cold nights of recent weeks in Devon.

Don't know then. Sorry this happened and hope you were able to use them anyway.

It can be overwatering too. they are fussy...!