Any idea what this is?

This looks to me like drought , then suddenly rain, but maybe someone with more experience can comment on it. Not sure if it’s relevant but it is either Ashmead’s or Golden Russet.

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That would be my guess too… tomatoes will sure crack like that… if you let them dry out some and then get a big rain.

A deep layer of mulch, compost, wood chips, straw, hay etc should help.