Virus symptoms :
Virus infected plants can display a wide range of different symptoms. The whole plant can be stunted (dwarf-like) and infected leaves can show abnormal colorations like mosaics and yellowing.
Very often infected leaves have a rolling or curly appearance and also the fruits can display various symptoms dependent on the virus.
The yield of crop plants can be severely reduced by a virus infection. However, the age of the plants, the viral strain and many other factors are affecting both yield and symptoms of infected plants. Virus symptoms are important in order to discover a viral disease. Symptoms may also be helpful in identifying which virus is causing disease.
Symptom localisation -
* * * * Symptoms are in general expressed where virus replicates.
Virus is transported in the phloem following the source-to-sink route for photoassimilates. Therefore, virus will tend to accumulate in young (sink) tissues and virus replication is usually high here. Symptoms are therefore often strongest in upper leaves of the plant. Symptoms around the site of virus inoculation are denoted local symptoms. When virus spreads from the site of inoculation and causes symptoms in other parts of the plant, this is referred to as systemic symptoms. Some symptoms takes some time to develop - for example phloem necrosis causing yellowing (See "leaf color").
In this case older leaves can show more severe symptoms than young leaves. Flowers, fruits, seeds and tubers are also sink tissues and symptoms are often found here.
Symptom terminology : Leaf Color
* * *
* * * * Mottling -
If leaves display a pattern of abnormal coloration this is referred to as a mottle. Mottling can be both light and dark. A mottle can be necrotic (dead tissue) and when spread to many leaves it is referred to as systemic necrosis
* * * * * Mosaic -
If a mottle is light and creates a mosaic- like pattern it is often referred to as a mosaic. Mosaic symptoms are common and often very characteristic - but not all virus infections give rises to mosaic symptoms. In monocots mosaics usually consist of elongated light areas referred to as "streaks" or "stripes". A mosaic is a kind of mottle, and often mottling and mosaic are used synonymously.
* * * * * Chlorosis -
The term, chlorosis, is used to describe reduced amount of chorophyll resulting in light color. If the leaf is light green it is referred to as "mild chlorosis" wheras "severe chlorosis" or "yellowing" is used to describe a total depletion of chorophyl giving a yellow color. Note that veins sometimes resist the yellowing process resulting in veins having a dark green appearence.
Chlorosis often spreads as a smooth gradient across the leaf starting from the tip. In field crops chlorosis may be recognized as patches of light crops.
* * * * * Vein clearing -
In some viral diseases veins become light and more distinct. It may not always be the veins them selves but rather the tissue surrounding the veins which becomes subject to a localized chlorosis and thereby make the veins appear more distinct. The term vein banding can also be used.
* * * * * Leaf spots -
Some infections are associated with leaf spots which can be either chlorotic or necrotic. Often some of the spots are ring formed - these are simply referred to as ring spots. After some time nercrotic spots can appear as wholes in the leaves. Leaf spots are very common, as local symptoms following virus inoculation. If the inoculated plant initiates a local defence reaction this may be observed as a spot or a ringspot. Local reactions are often necrotic.
* * * Leaf morphology
Leaf rolling and curling -
Leaf rolling is used to describe folding of leaves along their mid axes resulting in a more or less tube-like structure. When the folding is more irregular or does not result in a tube-like structure it is usually referred to as leaf curling. Often leaves show a mixture of rolling and curling and the two terms are used synonymously by many virologists.
* * * Leaf distortion
Leaf distortion means that the growth and development of the leaf has been disturbed resulting in changes ranging from small deviations from normal leaf shape to severe deformations.
* * * * Rugose
Rugose means : "rough leaves". This term covers both crinkling (leaves looking edged or wrinkled) and leaf puckering (blister-like irregularities formed on the leaf)
* * * * Enation
Abnormal outgrowth of vascular tissue in leaves or on the stem
* * * Other symptoms
* * * Stunting
Stunting (or dwarfism ) is the most common symptom associated with a virus infection and sometimes it is the only observable sign of infection. All parts of the plant can show shortening (roots, leaves and stem) and particular in dicots shortening of internodes can cause a tight and "bushy" look
* * Reduced yield
Most virus infections cause yield reductions. However, the level of reduction is highly variable dependent on the virus strain, time of infection and environmental factors. The figure to the right shows typical yield reduction observed in winther barley on soil infected with the virus Barley mild mosaic virus (BaMMV). This virus causes hardly other symptoms than reduced yield. Crop yield can be reduced due to the virus directly. However, virus infections also make the plants more susceptible to abiotic stress (Cold, draught, nutrition deficiency) and virus can thereby also indirectly cause crop losses.
* * Fruits and seeds
In addition to reduced yield virus infection can cause changes in color and deformation of fruits and seeds. Abnormal color on fruits can be referred to by different names such as miscoloration, mottling, mosaic, marbling and blotching.
* * * * Tubers
In tubers several viruses cause necrotic browning in various ring patterns both inside and outside the tuber. This symptom is referred to as spraing . Cracking of the tuber surface can also be a result of virus infection.
* * * * Flowers
Abnormal coloration of flowers can be the result of a virus infection.
If streaks or patches of abnormal coloration is observed it is referred to as color breaking. Yellow streaks on tulips represent a classical example caused by the so named Tulip breaking virus. Today, however, streaks on many tulips have also been obtained by mutagenesis and breeding and is not always due to a virus infection.
* * Stem pitting
Stem pitting means that small wholes ("pits") and groovings occurs on the trunk below the cork of woody species. Fruit trees are often grafted for vegetative propagation. If a grafted tree becomes infected with a virus which is tolerated by one part of the tree - fx the root stock, but not tolerated by the other part - a hypersensitive reaction will kill the phloem tissue in the non tolerant part of the stem. This will cause "pits" to be formed in the trunk either above or below the grafting line dependent on which part of the grafted tree is hypersensitive to the virus. Later the tree will die slowly "decline" due to destruction of the transportation system.
Virus infected plants can display a wide range of different symptoms. The whole plant can be stunted (dwarf-like) and infected leaves can show abnormal colorations like mosaics and yellowing.
Very often infected leaves have a rolling or curly appearance and also the fruits can display various symptoms dependent on the virus.
The yield of crop plants can be severely reduced by a virus infection. However, the age of the plants, the viral strain and many other factors are affecting both yield and symptoms of infected plants. Virus symptoms are important in order to discover a viral disease. Symptoms may also be helpful in identifying which virus is causing disease.
Symptom localisation -
* * * * Symptoms are in general expressed where virus replicates.
Virus is transported in the phloem following the source-to-sink route for photoassimilates. Therefore, virus will tend to accumulate in young (sink) tissues and virus replication is usually high here. Symptoms are therefore often strongest in upper leaves of the plant. Symptoms around the site of virus inoculation are denoted local symptoms. When virus spreads from the site of inoculation and causes symptoms in other parts of the plant, this is referred to as systemic symptoms. Some symptoms takes some time to develop - for example phloem necrosis causing yellowing (See "leaf color").
In this case older leaves can show more severe symptoms than young leaves. Flowers, fruits, seeds and tubers are also sink tissues and symptoms are often found here.
Symptom terminology : Leaf Color
* * *
* * * * Mottling -
If leaves display a pattern of abnormal coloration this is referred to as a mottle. Mottling can be both light and dark. A mottle can be necrotic (dead tissue) and when spread to many leaves it is referred to as systemic necrosis
* * * * * Mosaic -
If a mottle is light and creates a mosaic- like pattern it is often referred to as a mosaic. Mosaic symptoms are common and often very characteristic - but not all virus infections give rises to mosaic symptoms. In monocots mosaics usually consist of elongated light areas referred to as "streaks" or "stripes". A mosaic is a kind of mottle, and often mottling and mosaic are used synonymously.
* * * * * Chlorosis -
The term, chlorosis, is used to describe reduced amount of chorophyll resulting in light color. If the leaf is light green it is referred to as "mild chlorosis" wheras "severe chlorosis" or "yellowing" is used to describe a total depletion of chorophyl giving a yellow color. Note that veins sometimes resist the yellowing process resulting in veins having a dark green appearence.
Chlorosis often spreads as a smooth gradient across the leaf starting from the tip. In field crops chlorosis may be recognized as patches of light crops.
* * * * * Vein clearing -
In some viral diseases veins become light and more distinct. It may not always be the veins them selves but rather the tissue surrounding the veins which becomes subject to a localized chlorosis and thereby make the veins appear more distinct. The term vein banding can also be used.
* * * * * Leaf spots -
Some infections are associated with leaf spots which can be either chlorotic or necrotic. Often some of the spots are ring formed - these are simply referred to as ring spots. After some time nercrotic spots can appear as wholes in the leaves. Leaf spots are very common, as local symptoms following virus inoculation. If the inoculated plant initiates a local defence reaction this may be observed as a spot or a ringspot. Local reactions are often necrotic.
* * * Leaf morphology
Leaf rolling and curling -
Leaf rolling is used to describe folding of leaves along their mid axes resulting in a more or less tube-like structure. When the folding is more irregular or does not result in a tube-like structure it is usually referred to as leaf curling. Often leaves show a mixture of rolling and curling and the two terms are used synonymously by many virologists.
* * * Leaf distortion
Leaf distortion means that the growth and development of the leaf has been disturbed resulting in changes ranging from small deviations from normal leaf shape to severe deformations.
* * * * Rugose
Rugose means : "rough leaves". This term covers both crinkling (leaves looking edged or wrinkled) and leaf puckering (blister-like irregularities formed on the leaf)
* * * * Enation
Abnormal outgrowth of vascular tissue in leaves or on the stem
* * * Other symptoms
* * * Stunting
Stunting (or dwarfism ) is the most common symptom associated with a virus infection and sometimes it is the only observable sign of infection. All parts of the plant can show shortening (roots, leaves and stem) and particular in dicots shortening of internodes can cause a tight and "bushy" look
* * Reduced yield
Most virus infections cause yield reductions. However, the level of reduction is highly variable dependent on the virus strain, time of infection and environmental factors. The figure to the right shows typical yield reduction observed in winther barley on soil infected with the virus Barley mild mosaic virus (BaMMV). This virus causes hardly other symptoms than reduced yield. Crop yield can be reduced due to the virus directly. However, virus infections also make the plants more susceptible to abiotic stress (Cold, draught, nutrition deficiency) and virus can thereby also indirectly cause crop losses.
* * Fruits and seeds
In addition to reduced yield virus infection can cause changes in color and deformation of fruits and seeds. Abnormal color on fruits can be referred to by different names such as miscoloration, mottling, mosaic, marbling and blotching.
* * * * Tubers
In tubers several viruses cause necrotic browning in various ring patterns both inside and outside the tuber. This symptom is referred to as spraing . Cracking of the tuber surface can also be a result of virus infection.
* * * * Flowers
Abnormal coloration of flowers can be the result of a virus infection.
If streaks or patches of abnormal coloration is observed it is referred to as color breaking. Yellow streaks on tulips represent a classical example caused by the so named Tulip breaking virus. Today, however, streaks on many tulips have also been obtained by mutagenesis and breeding and is not always due to a virus infection.
* * Stem pitting
Stem pitting means that small wholes ("pits") and groovings occurs on the trunk below the cork of woody species. Fruit trees are often grafted for vegetative propagation. If a grafted tree becomes infected with a virus which is tolerated by one part of the tree - fx the root stock, but not tolerated by the other part - a hypersensitive reaction will kill the phloem tissue in the non tolerant part of the stem. This will cause "pits" to be formed in the trunk either above or below the grafting line dependent on which part of the grafted tree is hypersensitive to the virus. Later the tree will die slowly "decline" due to destruction of the transportation system.
