This is a science to increase our knowledge about plant diseases. The study includes such factors as the biology of the pathogenic organisms, how the pathogens cause diseases, and the interactions between pathogens and host plants. The end result achieved is to devise better methods of preventing or controlling diseases and alleviating the damaged that they cause. Link

Anthracnose: A necrotic ulcer-like lesion caused by fungus.

Black spot: Black appearance, often surrounded by yellow appearance of foliage.

Bacterial leaf spot: Black or brown water-soaked appearance, often with yellow halo. Leaves yellow. Caused by wet foliage.

Bacterial blight

Bermuda grass Decline. Circular patches .5 to 3 ft. diameter; initially yellowish; gradually turning brown and thinning; roots darkened; chlorotic leaf blades may develop next to green shoots at margins of diseased area; roots brown and without feeder roots and root hairs.

Blossom end rot and early blight on tomatoes.

Brown patch is a circular patch of light brown on turfgrass.Fungus attacks the base of the leaf sheath.

Canker: Generally a dry, discolored, sunken, or cracked area on a stem, branch, or trunk. A canker generally has a distinct border between diseased and healthy tissue whereas a rot does not. Cankers may be caused by pathogens or by injuries.

Chlorosis: Yellowing or whitening of normally green leaves. May be due to a virus, nutrient deficiency, lack or oxygen in a waterlogged soil, alkaline soil, or other factors.

Cotton Root Rot is a fungal plant disease. The first symptoms are slight yellowing or bronzing of the leaves, wilting within 24 to 48 hours after bronzing, followed by wilting of the lower leaves within 72 hours. Permanent wilt occurs by the third day, followed by death. Roots are usually extensively invaded by the fungus.

Crown gall bacteria infect plants through wounds. Damage to infected plants results from interruption of water and nutrient movement up the stem. Galls also interfere with normal growth and development, therefore, infected plants may be stunted and unthrifty.

Curly top virus is widely distributed in Texas. It is most noted for destruction of tomatoes, peppers, and sugar beets, but watermelon, beans, spinach, and squash are also susceptible as well as many weeds and ornamentals. Transmission is by the sugar beet leafhopper.

Damping off: The rapid death and collapse of very young seedlings in the greenhouse or field. Common damping-off pathogens include the fungi Pythium, Rhizoctonia, Fusarium, and Phytophthora.

Die back: Extensive death of twigs beginning at the tips and advancing toward the bases (physiological - cold injury) often injured by early or late frosts that kill younger twigs and may serve as a site for secondary fungi to get started therefore all dead twigs and limbs should be pruned from the trees.

Dollar Spot appears as silver dollar size circular spots that are straw color and looks sunken-finer textured turf grasses with hour-glass shaped lesions on leaves.

Downy mildew.

Drought Symptoms on Plants. The most common symptom of drought injury is dying around margins of leaves with the dead leaf tissue between veins toward the midrib. A deficiency of water affects the marginal leaf tissue which results in a partial or complete collapse of the cells. Prolongation of this state results in the death of feeder roots. Root pruning will also cause drought symptoms to occur.

Dutch Elm Disease first visual symptoms are what is referred to as "flagging" within the crown of the tree, which is a branch of a tree that develops symptoms of wilting and/or yellowing of the leaves on a otherwise apparently healthy tree.

Fairy Ring is a circle or arc of stimulated grass or of toadstools may appear in lawns. These rings of lush growth are caused by the release of nutrients, in particular nitrogen, from the activity of the fungus living on organic matter in the soil.

Fire Blight is a bacterial disease that causes rapid blackening and desiccation of blooms and foliage. Affected shoots bend at the tip in a "shepherd's hook". Black, sunken twig and branch cankers develop later.The bacteria stays in the cankers over winter, and in the spring, oozes from the cankers.

Freeze damage. Trees die suddenly in late spring and early summer. Cut the bark at the crown line to see if the root system is still alive. If it shows greener wood than the above ground portion, then winter damage should be suspected. Freeze damage results in a darkening of the wood just under the bark of the trunk and main branches.

Fungal leaf spot. Circular to irregular leaf lesion with a dry, brown or black center. Leaves turn yellow, then brown and drop from the plant.

Gray Leaf Spot. Yellowish chlorosis, then turns brown; numerous oblong, gray to ash-brown leaf lesions with purple margins; grayish spore producing bodies may be noticable on these lesions; leaves sometimes appear twisted; death of infected leaves; yellowing turf that appears drought stricken.

Leaf Blister fungi overwinter as spores attached to buds. In the sprint they appear as firm, dry blisters on leaves. It is usually not life threatening, but if spreading it should be treated. Plants most affected are oaks, birches, buckeyes, elms, maples, poplars, and willows.

Leaf scorch can be caused by transplanting, soil compaction, nearby excavation, a nutrient deficiency, chemical injury, weather or poor soil. The roots cannot supply enough water to offset the water lost through the leaves in transpiration. Hot, dry winds will increase the severity.

Mosaic: Leaf symptoms characterized by intermingled patches of normal and light green or yellowish color; generally viral.

Necrosis is dead tissue or plant parts caused by Necrotic Spot Virus.

Necrotic Ring Spot. Circular patches less than a foot in diameter; above ground damage are result of root injury in the spring and in previous fall; roots and rhizomes turn black and dark surrounded by a ring of dead brown grass-“frog eyed” appearance; some plants survive and grow back after fungus stops feeding in the center of the patch.

Nematodes. When crops show slow decline, unthriftiness, chlorosis or slower than normal growth, nematodes, other than root knot, may be the cause. Plants will tend to wilt first.

Oak leaf blister. Caused by the fungus, common on many species of oaks, particularly Red Oaks, but seldom white oaks. Symptoms appear in early summer as yellow, blister­like, circular, raised areas. They turn from yellow to reddish brown with pale yellow margins.

Oak Wilt. A fungus that plugs water-conducting vessels, reducing flow of water up the stem of the tree. Often causes leaves to wilt and fall prematurely. Disease spread by beetles feeding on tree wounds. Live Oaks dies one area of the tree at a time. Red Oaks die in a flash of fall color in early summer.

Powdery Mildew is white to gray, dusty patches of fungal growth on leaves and stems and leaf sheaths; chlorotic lesions may develop; entire leaf turns yellow and eventually dies; surviving plants weakened. Powdery mildew will begin as discrete circular, powdery white spots. The fungus is favored by high relative humidity periods.

Ring spot: A circular area of chlorosis or necrosis with a green center; generally viral.

Rust is a fungus caused by moist conditions.

St. Augustine Decline (SAD). Called Take-all Patch. Circular patches .5 to 3 ft. diameter; initially yellowish; gradually turning brown and thinning; roots darkened; chlorotic leaf blades may develop next to green shoots at margins of diseased area; roots brown and without feeder roots and root hairs.

Scab. Localized lesions on host fruit, leaves, tubers, etc., usually slightly raised or sunken and cracked, giving a scabby appearance.

Scorch is the burning or drying and browning of leaf margins. Scorch usually results from unfavorable environmental conditions for the plant.

Sooty mold is a charcoal black fungus that appears as a black coating on the surface of leaves, fruits, twigs and branches of many deciduous and evergreen shrubs and trees. This fungus is not pathogenic to plants but obtains its nourishment from insect honeydew.

Wilt Diseases. Plants first show a wilted appearance. Individual branches or single leaves may be affected at first. Leaves develop a yellow color, often in V-shaped sectors between the major veins. The fungus can be readily isolated from the diseased stems. Diseased plants may die soon after first symptoms or they may sprout at the base after the top dies.

Witches' broom: Also called rose rosette. Broom-like fungi growth looks much like herbicide damage, but its effect soon becomes much more dramatic. Infected canes will sprout thick clusters (rosettes) of stubby, soft and brittle stems with elongated leaflets referred to as "witches broom". A plant disease characterized by yellowing and stunting of the host plant.