Sunscald and frost cracking are the result of the interaction of light and temperature. They are caused by thawing and freezing due to a rapid fluctuation in stem temperatures. Exposure to afternoon sun causes an increase in stem temperature and the subsequent thawing of stem/trunk moisture. As the sun sets, temperatures drop rapidly. If the temperature drops below freezing, ice crystals rupture internal tissue. Sunscald and frost cracking occur from the same causal conditions. In sunscald, the cambium, phloem and xylem are damaged, a sunken area appears on the trunk, but the bark is not split. Frost cracking exhibits the same internal damage with vertical splits in the bark. The splits can reopen and close with changes in air temperature. As temperatures increase in spring, the bark tissue dries and the cracks remain open. Thinbarked plants are prone to sunscald and frost cracking, especially when subjected to southwest exposures. Tree wraps can minimize sunscald and frost cracking, but thin-barked plants may always be susceptible to this problem if environmental conditions favor temperature fluctuations.
Information and photo from a section of "Abiotic Plant Disorders - Symptoms, Signs and Solutions A Diagnostic Guide to Problem Solving" by Robert E. Schutzki and Bert Cregg, Departments of Horticulture and Forestry, Michigan State University Michigan State University. Go to http://www.ipm.msu.edu/cat08land/pdf/9-19abiotic.pdf for the full factsheet with photos.
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