Fluctuating growth rates cause tomatoes to suddenly expand. This might happen after a period of drought followed by generous watering, or low temperatures followed by a sudden sunny spell. The split might then become infected, causing the tomatoes to rot.
Tomatoes split open while still on the vine. This can lead to secondary infections and the tomatoes often start to rot.
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Water plants regularly and consistently, so it's not feast then famine for the plants. Ventilate greenhouse tomatoes during very hot spells and provide shading. Promptly pick off any split tomatoes before they start to rot.