Dry rot is a form of fungus that generally develops on materials containing cellulose, such as wood and wood products, and on cotton. While true dry rot doesn't generally occur with rubber, a type of deterioration similar to dry rot does occur with rubber products and is sometimes referred to as dry rot.
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Excessive exposure to UV rays is a common cause of a dry rot type deterioration in rubber products. This is one of the reasons car tires are the rubber-based object most likely to experience dry rot. Unless a vehicle is a show car sequestered away in protective storage, the average automobile is outside and exposed to the elements--like sunshine--for the bulk of the day.
In order to deflect some of the UV rays that can eventually destroy car tires, they are all manufactured in black. Tire manufacturers utilize a UV stabilizer called a "competitive absorber," known as carbon black, which absorbs the more destructive UV light rays. Tires become grayer as time goes on because the carbon black is gradually depleted.
Deterioration of rubber due because of ozone exposure is a rare instance in which not using an object regularly leads to its breaking down. When tires are in operation, the flexing of the treads causes the protective waxes within the tire to come to the surface and form a shield between the tire and the elements (a process known as "blooming"). When tires are not being used, the blooming doesn't take place, and the surrounding ozone causes excessive drying and cracking.
Oxidation and microorganism infestation tends to occur after, and as a result of, deterioration due to UV rays and ozone. The most common microorganisms to infiltrate and weaken the double bonds of rubber are connected with carbonyl and aldehyde compounds. When the structure of rubber has weakened from drying and cracking--due to UV ray and ozone exposure--these compounds have easier access to the inner and more vulnerable recesses of the rubber-based product.
