Symptoms of our more common yard and garden plant diseases in Montana fall into eight broad catagories: 1) blights, 2) cankers, 3) galls, 4) leaf blisters or curls, 5) mildews, 6) rots, 7) rusts, and 8) wilts.
BLIGHTS: 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.
CANKER: symptoms usually form on woody stems and may be sunken areas, cracks, or raised areas of dead or abnormal tissue. 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.
GALLS: are swollen masses of abnormal tissue that range in size from small to quite large. 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.
LEAF BLISTERS are yellow bumps on the upper surfaces of the leaves with gray depressions on the lower surfaces. LEAF CURL DISEASES cause new leaves to be pale or reddish with the midrib deformed. 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.
MILDEWS are usually one of two types. Downy mildew is usually a white to purple, fuzzy growth, usually on the undersides of leaves and along stems. It turns black with age. Powdery mildew is a white to grayish powdery growth on the upper surfaces of leaves.
ROT DISEASES cause decay of roots, stems, wood, flowers, and fruit. They can be soft and squishy or hard and dry, and color can be either light or dark.
RUST DISEASES typically produce symptoms that include a powdery tan to rust-colored coating or soft tentacles.
WILT DISEASES cause permanent wilting, often followed by death of part or all of the plant.
Most symptoms you find in your yard or garden will be caused by insects or environmental problems such as water stress or nutrient deficiencies, so try to rule out these possibilities first. If you have yard and garden reference books, you can often identify the disease based on the categories of symptoms listed above. The species of plant on which the symptoms occur will also help identify the problem as most diseases are quite specific in the kinds of plants they will attack. If you need additional assistance with identification, your county Extension agent can help.
Agrios, G.N. 1988. Plant Pathology. Third edition. Academic Press, San Diego, CA. 803 pp.