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