When tomato plants get too much water too fast. The interior grows quickly as it absorbs the extra water from rain or disproportionate watering, The skin can’t stretch to accommodate the extra fluid. so it start Cracking.
How to prevent cracks:
- Plant crack-resistant tomato varieties that have elastic skin, including Jet Star, Early Girl, Earl of Edgecombe, Heinz 1350, Juliet, Daybreak, Valley Girl, Mountain Pride, and Mountain Delight.
- Don’t over-fertilize : Excess nutrients can cause a growth spurt. Tomatoes can have a hard time compensating and may crack.
- Mulch the plants (wait until they’re established – about 3-5 weeks after planting) to help retain moisture in the soil.
- Keep water supply even throughout the season. While you can’t control the weather conditions, you can make sure that plants are on a regular watering schedule. Source.