Fortunately, cracking is not caused by bugs or viruses and is not contagious, but there is very little you can do to prevent Mother Nature from releasing a gusher. Keeping tomatoes too damp can inhibit the uptake of calcium (leading to blossom end rot). There are some tomato varieties that are more resistant to cracking than others, but popular beefstake tomatoes are notorious crackers.
Of course, not every agrees on the causes of cracking. The University of Illinois Extension Office blames severe pruning: “Cracking varies with the variety. Many of the newer varieties are resistant to cracking. Severe pruning increases cracking. Keep soil moisture uniform as the tomatoes develop and plant resistant varieties to minimize this problem.”
But I like the explanation of Texas A&M:
Cracking is a physiological disorder caused by soil moisture fluctuations. When the tomato reaches the mature green stage and the water supply to the plant is reduced or cut off, the tomato will begin to ripen. At this time a cellophane-like wrapper round the outer surface of the tomato becomes thicker and more rigid to protect the tomato during and after harvest. If the water supply is restored after ripening begins, the plant will resume translocation of nutrients and moisture into the fruit. This will cause the fruit to enlarge; which in turn splits the wrapper around the fruit and results in cracking. The single best control for cracking is a constant and regular water supply. Apply a layer of organic mulch to the base of the plant. This serves as a buffer and prevents soil moisture fluctuation. Water plants thoroughly every week. This is especially important when the fruits are maturing. Some varieties are resistant to cracking, but their skin is tougher.
Iowa State agrees:
Fruit cracking is a common problem on tomatoes. Cracks usually appear at the top or stem end of the fruit. Cracks radiate out from the stem (radial cracks) or circle the fruit in concentric rings (concentric cracks). Fruit cracking is associated with wide fluctuations in soil moisture levels. A heavy rain or deep watering after a long, dry period results in rapid water uptake by the plant. The sudden uptake of water results in cracking of ripening fruit. Generally, fruit cracking is most common on the large, beefsteak-type tomatoes.
Fruit cracking can be prevented by supplying the tomato plants with a consistent supply of moisture during the summer months. During dry periods, a thorough soaking once every seven days should be adequate for most tomato plants. Conserve soil moisture by mulching the area around tomato plants with dried grass clippings, straw, shredded leaves or other materials. Also, plant tomato varieties that possess good crack resistance. Tomato varieties that possess good to excellent crack resistance include Jetstar, Mountain Spring and Mountain Fresh.
Montana State University takes a hybrid approach:
Tomato harvest is a highly anticipated event in our gardens. But after tending to these plants for an entire season, it's sure disappointing to find the fruit cracked and rotted. Or fruit that doesn't ripen at all. There are two kinds of fruit cracking in tomato - radial and concentric.
Radial cracks are the more common, start near the fruit stem, and develop down the sides of the fruit wall. Concentric cracking appears as several circular cracks around the stem end of the fruit. So is there anything you can do to prevent tomato fruit cracking?
Cracking of tomatoes is most common during hot, rainy periods when temperatures are in the 90s, and particularly following long dry spells. It is most severe on fruit that is ripening in full sun. The high light intensity and warm summer temperatures in our area make the situation worse. Here's what you can do:
Mulch the soil around your plants to keep it consistently moist. Use drip irrigation instead of overhead impact sprinklers. Fertilize your plants judiciously and encourage good foliage cover. Plants that have been heavily pruned, or those that have lost their foliage to insects, disease or weather issues will have their fruit exposed to the bright sunlight. These ripening fruits will heat up and most likely crack. If this has been a problem for you, next year, keep in mind that there are tomato cultivars that are resistant to cracking. "Beafsteak-type" tomatoes are known for cracking. Cultivars like 'Early Girl', 'Daybreak' and 'Valley Girl' resist cracking. And just as there are some cultivars resistant to cracking, there are others that have the tendency to crack, like 'Sungold' and 'Sun cherry' cherry tomatoes.
In any event, generously water your tomatoes at least once a week when it is hot and dry or you risk a gusher rain storm blowing up your tomatoes when you least expect it.