Late winter or early spring is a good time to inspect trees and ornamentals for symptoms of disease and to prune out any diseased plant material you might find. This is one of the best ways to keep the disease from spreading to other parts of the tree or ornamental or from spreading to healthy plants. It is usually best to prune when woody plants are still in dormant stages to reduce possible problems from diseases and insects. Also, before leaves are formed it is easier to see symptoms of diseased wood.
Some of the more common problems you might find before leaves are black knot in plum and chokecherry; blight (including fire blight) which can occur in apple, crabapple, saskatoon berry, mountain ash, and raspberry; canker, which can be found in most trees and shrubs; and rusts, which can be found in apple, crabapple, juniper, cedar, and saskatoon berry.
The symptoms associated with these diseases include; swellings and knots that are black and hard (black knot); twigs that appear blackened as though scorched by fire, sometimes curled into a shepherd's crook' on the tip (blight or fire blight); purplish to chocolate brown spots or bands; tissues which crack open and expose the wood underneath, or sunken, elongated areas which are often cracked or discolored (canker). In the case of conifers, foliage can turn yellow-green then purple, then drop. Large amounts of resin can flow from infected conifer branches, coating the surface of the bark around cankers and dripping onto lower branches. Galls that are brown, warty or decayed can also indicate diseases, as can pronounced swellings, or large numbers of smaller branches called witches' brooms.
To prune the diseased material, cut off the infected area along with at least eight inches of healthy tissue. Sterilize your pruning tools between each cut by dipping them in a solution of household bleach diluted one part bleach to nine parts water, leaving the tool in the solution for 15 seconds. When you are finished pruning, disinfect the tools a final time, then coat the metal parts with a light oil. Be aware that your hands, gloves, clothing, and gardening tools can also spread the pathogens, especially when foliage is wet. Burn or bury the diseased plant material to destroy the pathogens as soon as possible and do your pruning in dry weather to minimize the spread of the diseases. Not all symptoms you find will be caused by pathogens; some can be caused by environmental or physical damage, such as sunscorch, winter injury, or dog urine. If you are not sure, or need help with the pruning process, contact your county Extension agent.
References:
Agrios, G.N. 1988. Plant Pathology. Third edition. Academic Press, San Diego, CA. 803 pp.