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Common Symptoms of Diseased Plants

    Symptoms of diseased plants vary with different plant pathogens, different host plants, plant parts, infection stages and environmental conditions. Generally, it is very difficult to identify the pathogens from symptoms alone. However, there are a few common symptoms that can help you sample the correct parts of the plants.

    Chlorosis is a common symptom of infection by viruses and viroids, and it is also an early symptom of infection by some bacteria or fungi.  Chlorosis is produced by the loss of chlorophyll in infected plant tissues.  Chlorotic areas may be pale green, light yellow, reddish, purple or white, depending on the predominate pigments present in the particular plant other than chlorophyll. Chlorosis can spread throughout the whole leaf surface, part of the leaf or around leaf edges, or it can form specific patterns such as ringspots, spots, streaks or stripes.

    Compared with healthy plants of the same variety, infected plants are much shorter or smaller.  Pathogens cause the general reduction in growth.

    Flower Breaking
    Viral infection induces loss of pigment in flower tissues.  The effected flower petals show patterns of small light or dark breaks.

    Line Pattern
    Line pattern is a symptom of vein tissue infection in plant leaves.  The symptom shows up as a green or yellow pattern of single or multiple irregular lines or bands on leaf surface.

    Mosaic is a very common symptom in plants with viral infections.  The pattern is seen as alternative areas of light and dark, or yellow and green on leaves and the areas can be angular, sharply bordered, or divided by leaf veins. Mosaic is caused by reduction of chlorophyll in infected leaf tissues.  The symptom is best seen by holding the leaf up to light or viewing if with light underneath.

    This is a common symptom of viral infection, but sometimes is very difficult to see clearly. Mottle is a variegated pattern of yellow and green, or light and dark areas with less clear or diffuse boundaries on leaves. Again, this symptom is best seen by holding the leaf up to light or viewing it with light under it.

    Necrosis is the symptom of death of plant tissues or organs that results from pathogen infection.  This symptom can occur in any plant tissues, such as the leaves, petioles, stems, branches, roots, flowers, fruits or tubers. Necrosis may appear in small areas or cover a large areas of the plant tissues.

    Necrotic Spotting
    Necrotic spotting can be caused by all different plant pathogens such as bacteria, fungi and viruses. This symptom is shown as areas of dead tissues in leaves, and can be induced by systemic or local infections of pathogens. The necrotic spots vary in sizes (small, middle or large) and vary in shapes, from round to irregular.

    Necrotic Flecking
Necrotic flecking is similar to necrotic spotting, but it is usually seen as pinhole-sized spots on leaves.

    Plant Wilting and Death
   Infection of pathogens in root system, vascular system or main stem may cause wilting or death of the whole plant. Generally, the infected plant starts to wilt during the early stage of infection, and then the whole plant slowly dies. In a few cases, the wilted plant may be recovered partially or completely.

    Plant Multiform
    There are many different multiforms of plants such as enation, blistering, mopping, or narrowing an shoestring induced by pathogenic infection.  Plant multiform is caused by uneven growth of different parts of the plant tissues. 

    Typical ringspot is a spot with many circular patterns (much like a target-bullseye pattern). The ringspots can be chlorotic (areas of reduction of chlorophyll) with yellow, red or purple colors, or necrotic (areas of dead tissues) with brown or black colors. 

    Streaks and Stripes
Streaks and stripes mainly occur in monocotyledonous plants infected by plant pathogens, especially by viruses. Mosaic or mottle areas caused by reduction of chlorophyll in infected leaf tissues become elongated along the veins to form streaking or striping. Generally, the streaks or stripes are chlorotic with yellow, red or purple colors. Streaks or stripes may become necrotic with brown or black colors especially in later stages of infection.

    Similar to dwarfing, the reduction in growth of plants by pathogenic infection results in plant stunting.   Stunting may occur in the whole plant or in parts of the plant.  In most cases, stunting is accompanied by other symptoms such as mosaic, mottle or streaks.   

    Tumors are tissue swellings stimulated by pathogen infections. This symptom generally appears in stems, branches or roots of woody plants and some herbaceous plants.  Sometimes the tumors show as roughened or cracked surface. 

    Vein Banding, Vein Clearing and Vein Yellowing
    Infection in vascular tissues results in yellow, clear or translucent tissues around leaf veins.  Appearance of the affected areas vary in size, color and shape depending on host plants, the pathogens and environmental conditions.
Therefore, they are given different names, such as vein banding, vein clearing or vein yellowing.   

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