There are a number of things that can cause bark splitting or cracking. Extreme heat or fluctuating temperatures followed by a frost event can cause sunburn and sunscald. Mechanical injury (e.g. the trunk was hit with a string trimmer) can cause cracks in the bark. Cankers caused by plant pathogens can sometimes resemble splitting or cracking in their early stages. If the splitting is taking place on the south or southwest side of the tree, that is a good indication that it is sunburn or sunscald. The main risk is that a wood boring insect or pathogen may enter where the tree has split open. In this case, it looks like the trunk has turned black and there is some white growth on the bark. It is hard to tell based on the photos alone, but this could be a wood decaying fungus or canker, perhaps anthracnose, perennial canker, or fire blight. I recommend you monitor the tree for symptoms in the leaves and fruit, such as wilting or discolored flowers or leaves, shriveled or malformed fruit or spots on fruit. If any additional symptoms appear, then it is likely you are dealing with something other than sunburn/sunscald and wood decay. Click on these links for more information on sunscald and wood decay, including management practices.

In the future, if you plant a new tree, you can prevent sunburn or sunscald by painting the trunk with a 50:50 mixture of white latex paint and water, or place a white trunk guard around the trunk. However, now that the damage has occurred, it is best not to paint or cover it, since the paint or trunk guards can trap moisture causing further decay. You can use a knife to try to trim away excess bark. This Cornell University factsheet provides a few tips on how to manage bark splitting: http://plantclinic.cornell.edu/factsheets/barksplitting.pdf.