Symptoms include sudden withering and death of leaves and branches, or in the case of blossoms, wilting and discoloration. Conspicuous spots or irregular dead areas on leaves and twigs which cause foliage to distort and drop prematurely could be a blight. Damage from blights can be minor, as in the case of some anthracnose blights, to serious, as in the case of fireblight. Sometimes the cankers ooze conspicuously, or in the case of evergreens, drip sap onto the branches beneath.
Cankers can sometimes be one of the symptoms manifested by another disease.
Certain insects can also cause galls. Cut a gall open and search for signs of an insect inside- if you find none, the gall is probably caused by a disease. The leaves pucker and curl as they expand. Certain insects cause similar symptoms, but on closer inspection, if insects are the cause the insects themselves or other signs of their presence can be seen. Downy mildew is usually a white to purple, fuzzy growth, usually on the undersides of leaves and along stems.