While coating the ends of branches when pruning generally is considered an outdated practice that actually does more harm than good in some circumstances, you can still do it. For example, when certain trees are wounded during a critical time of year for their type, it is important to give them extra protection. However, dressing tree wounds must be done on a case-by-case basis.
Latex Pruning Sealer
One of the advanced replacements for the old asphalt and tar pruning sealers, latex pruning sealers are an all-weather type that coats the end of the wound in an elastic barrier that will last over time. The latex prevents excess sap leakage and repels insects that are attracted to open wounds on trees, giving the tree time to heal itself naturally.
Asphalt sealers are some of the oldest in the industry and are considered a staple wound dressing for pruning. They are ideal for sealing trees that are cut or wounded in specific times of the year that are harmful to that type of plant, because they are the most hardy of the pruning sealers available. For example, when oak trees are cut or wounded in spring, using an asphalt sealer can prevent the spread of certain tree-specific pathogens, such as oak wilt.
Wax sealers are some of the best to use, given studies by such organisations as the United States Department of Agriculture and Forest Service, which agree that dressing tree wounds is more detrimental to the plant than helpful, except in cases of preventing the spread of certain tree diseases. As a result, wax sealers are a "green" alternative to latex and asphalt sealers because they contain no chemicals and do nothing more than create a thin barrier while the sap naturally heals the wound of the tree.
Nontoxic sealers, or all-natural sealers, are another type of prune sealer that can be helpful when coating tree wounds. They act in a similar fashion to asphalt and latex sealers, but they contain none of the chemicals. Instead, they are based out of natural products such as lanolin, beeswax and other types of natural oils that help to heal the plant rather than inhibit growth or block the sap from doing its job naturally.