There are two basic types of splitting: concentric and radial. The type you're describing (cracking before the fruit is ripe) is usually radial splitting. The cracks start at the stem scar and split out toward the blossom end of the fruit. Radial splitting is most often an indication of uneven irrigation-usually from over watering or excessive amounts of rain. Crops that receive large amounts of water infrequently are prone to cracking. Other causes include fluctuations in growth rates (sudden periods of rapid growth causes splitting), fruits being subjected to temperature fluctuations (usually caused by plants being de-leafed too early and the resulting sun-over exposure), or when plants become overly succulent due to excessive nitrogen in combination with a lack of potassium.
Controls include properly balancing soil nutrients to avoid over succulent fruits, proper pruning and leaf removal and proper water management.
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By P. (Guest Post) 08/10/2005
The problem is most likely caused by irregular watering. A dry spell followed by a lot of rain can cause tomato cracking, or if you let the plants dry out quite a bit and then water them. I had this problem recently. The weather had been rather dry for a week or two, and then thundershowers occurred daily or more for a whole week. The cherry tomatoes developed lots of cracks.