Patterns of damage distribution and time patterns in development of damage have been valuable in making the gross distinction between damage caused by living factors and damage caused by nonliving factors. Additional clues must be obtained to distinguish among factors within the living and nonliving categories.

DISTINGUISHING AMONG LIVING FACTORS:

To further identify which subcategory of living factor caused the damage, requires a close examination of the symptoms and signs.

Symptoms are the modified appearance of the affected plant, for example necrotic tissues, chlorosis, cankers, galls, leaf distortion.

Signs are presence of the actual organism or evidence directly related to it. Visual observation of the insect on the leaf, presence of fungal mycelium, spores, insect egg masses, insect frass, mite webbing, etc. Signs can be used as clues in identifying the specific living organism that produced the plant damage.

A combination of clues from both symptoms and signs are required for preliminary distinction between pathogen and insect-mite damages.

SYMPTOMS AND SIGNS OF PATHOGENS

Differentiating between bacterial and fungal pathogens is not always clear cut, but certain symptoms are distinctive (Figures 7 and 8; Table 1).

Fungal Diseases (Figure 7). Fungal leaf spots and stem rots are characterized by various symptoms: Dry texture, concentric rings, discoloration and fruiting structures. Fungal leaf spots and stem rots are usually dry or papery. This is especially true in dry climates. The most distinguishing clue of a fungal disease is the presence of signs: Mycelium and fruiting bodies of the fungus itself. The fruiting bodies range in size from microscopic to those easily detected with the naked eye. They are found within the leaf spot or stem rot area.

Spots usually vary in size, generally round, occasionally elongated on stems.

Zones of different color or texture may develop giving the spot a bull's eye effect: the deadest tissue (tan) is in the center of the spot where the fungal spore germinated. Then as the fungal mycelium front moves outward from that point of dead tissue to healthy, not yet infected tissue, on the perimeter, the foliage color changes from dead tan in the center to healthy green on the perimeter.

Spots are not limited by leaf veins since mycelium grows on leaf surface.

Bacterial leaf spots are often angular because they are initially limited by leaf veins.

Color of the bacterial spots is usually uniform. Bacteria are one-celled organisms that kill as they go. Tissue may first appear oily or water-soaked when fresh, but on drying becomes translucent and papery tan.

Each type of fungus has its own characteristic structures which enable plant pathologists to identify them.

Foliar Pathogens: The leaf spots caused by fungi generally have distinct margins (Figure 7). Many times they are circular with concentric rings resulting from growth of the mycelium from the center point of initial infection outward (much like crocheting a doily): The condition of the leaf tissue and associated color ranges from dead (necrotic tan) in the center, to recently dead (darker brown ring), to dying (darker ring with possible light yellow, chlorotic edge indicating the advancing edge of the fungal infection). The margins of fungal leaf spots (Figure 7) and stem rots (Figure 3) can be brightly discolored, such as purple (Fusarium stem rot) or yellow (Helminthosporium leaf spot), making these symptoms quite striking.

Root and Stem Pathogens: Root rot and vascular wilt result from fungal infection and destruction of root and stem tissues. The most common visual symptom is gradual wilting of the above ground shoots.

Bacterial Diseases (Figure 8). Bacteria do not actively penetrate healthy plant tissue like fungi. They enter through wounds or natural openings such as leaf stomata or twig lenticels. Once bacteria enter the plant, they reproduce rapidly, killing the plant cells.

Bacterial galls: In some cases, toxic materials are produced that cause plant tissues of roots, stems or leaves to grow abnormally as in crown gall.

Vascular wilt: In some cases the bacteria poison or plug the vascular water conducting tissue and cause yellowing, wilting, browning and dieback of leaves, stems and roots.

Bacterial leaf spot disease: The bacteria usually enter through leaf stomata. Symptoms include water-soaking, slimy texture, fishy or rotten odor, confined initially between leaf veins resulting in discrete spots that have straight sides and appear angular. Many bacterial leaf spots, such as Xanthomonas leaf spot on Philodendron (also called red edge disease), expand until they reach a large leaf vein. This vein frequently acts as a barrier and inhibits the bacteria from spreading further. A chlorotic halo frequently surrounds a lesion. Lesions may enlarge through coalescence to develop blight lesions. Some lesions exude fluid containing bacteria. Water-soaking frequently occurs in bacterial leaf spot diseases, such as Erwinia blight of Dieffenbachia. Holding the leaf to light usually reveals the water-soaking. The ability of bacteria (usually Erwinia species) to dissolve the material holding plant cells together results in a complete destruction of leaf or stem integrity. Some fungi also produce this symptom but not usually as extensively as Erwinia. In general, bacterial infections show this characteristic more than fungal infections. In final stages, cracks form in the tissue and disintegration follows.

Viral Diseases (Figure 9). Viruses are "submicroscopic" entities that infect individual host plant cells. Once inside a plant cell, they are able to infect other cells. Viruses are obligate parasites: They can only replicate themselves within a host’s cell. Because the virus commandeers the host cell to manufacture viruses identical to itself, the plant cell is unable to function and grow normally. In the virus infected plant, production of chlorophyll may cease (chlorosis, necrosis); cells may either grow and divide rapidly or may grow very slowly and be unable to divide (distortion, stunting). The symptoms of most virus diseases can be put into four categories:

Left side of leaf: Vein clearing (chlorosis) with interveinal tissue remaining green usually indicates a virus disease or uptake and xylem translocation of a herbicide such as diuron. This is in contrast to the leaf veins remaining green with surrounding chlorotic tissue associated with nutrient deficiencies such as iron deficiency.

Right side of leaf: Mosaic is a patchwork of green and yellow areas over surface of leaf. The leaf may also be puckered and distorted. These symptoms usually indicate a virus disease, especially if yellow areas blend gradually into green areas. If margins are distinct, mottling may indicate a nutritional problem or genetic variegation.

1) Lack of chlorophyll formation in normally green organs.

Foliage may be mottled green and yellow, mosaic, or ringed (yellow or other pigmented ring patterns), or be a rather uniform yellow (virus yellows).

Veins : Vein clearing is a common first symptom of some viral diseases. The veins have a somewhat translucent or transparent appearance. In vein banding there is a darker green, lighter green or yellow band of tissue along the veins.

2) Stunting or other growth inhibition: The reduction in photosynthesis, because of less chlorophyll, leads to shorter internodes, smaller leaves and blossoms and reduced yield.

NEMATODES: Plant nematodes are microscopic roundworms that damage plant tissues as they feed on them. Many feed on or in root tissues. A few feed on foliage or other above-ground organs.

Shoot Nematodes (Aphelenchoides spp.) - Foliar nematodes feed inside leaves between major veins causing chlorosis and necrosis. Injury is most often seen at the base of older foliage. When plants with a net-like pattern of veins become infested with foliar nematodes, the tissues collapse in wedge-shaped areas and then change color.

Root Nematodes - The most common above-ground symptoms caused by root-infesting nematodes result from damaged root systems: Moisture and nutrient stress symptoms and general stunting are common. The root lesion nematodes (Pratylenchus spp.) and burrowing nematodes (Radopholus similis) destroy the root cortex tissues as they feed. The root-knot nematodes (Meloidogyne spp.) inject growth-regulating substances into root tissues as they feed, stimulating growth of large tender cells to provide themselves a permanent feeding site, and causing overgrowth of root tissues around them to form visible, swollen "galls" or "knots". Other root nematodes stunt growth, apparently by killing root meristems.

3) Distortions of leaves and flowers, witches’ brooms or rosettes result from nonuniform growth within a tissue or uncontrolled growth.

4) Necrotic areas or lesions: Being obligate parasites, viruses require the survival of their host plant for their own procreation. Hence, viruses rarely cause death. Necrosis that does occur is usually confined to discrete areas of the plant; necrosis rarely occurs to such an extent that the entire plant is killed.

Viruses typically discolor, deform or stunt plants rather than induce necrosis or cause death. Expressed symptoms (chlorosis, stunting, distortions) can be valuable clues for virus identification, but can be easily confused with symptoms induced by other problems such as nutritional disorders, spray injuries, or certain feeding damage induced by mites or insects. In addition, because of their extremely small size, the virus or signs of the virus are not visible to the unaided eye: The virus particles are detectable within the plant cell through the electron microscope.