Eyespot
Encyclopedia
Eyespot is an important fungal disease of wheat
caused by the necrotrophic fungus
Tapesia yallundae
(syn: Pseudocercosporella herpotrichoides; W-type [anamorph]; Oculimacula yallundae) and Tapesia acuformis
(syn: Pseudocercosporella herpotrichoides; R-type [anamorph]; Oculimacula acuformis). It is also called Strawbreaker. Eyespot is more severe where wheat is grown continuously and when the weather is cool and moist. Treating crops against eyespot with fungicide costs millions to farmers and is complicated by the pathogen becoming resistant to the more commonly used fungicide
s. Severe cases of the disease can reduce yield by up to 40%. It is most common in temperate regions such as North and South America, Europe, Australia, New Zealand and Africa.
which penetrates successive leaf sheaths throughout the growing season. High humidity, cool, and damp weather at the soil surface favours disease development. Whilst dry and hot weather causes the leaf sheaths of the plant to dry and fall off, taking the inoculum with it, thus lessening disease.
for breaking the plant cell wall
. A specific sequence of enzymes is employed; without these enzymes the fungus would not be able to invade the plant cell. The process begins with breaking of the leaf cuticle by formation of appressoria which provide the mechanical force to break into the cell. Then Pectin
, cellulase
, hemicellulase and protease
enzymes are released to enable invasion into the host cell. Now the fungus is present in the plant stem, it causes disease by affecting nutrient and water supply to upper parts of the plant, weakening the plant stem.
In some cases beta, 1-3 glycan synthase is also released to penetrate the callose matrix.
Wheat cells also release hydroxyproline glycoprotein (HRGP) in their cell walls. Secretion of HRGP is dependent on the signal induced by the fungal elicitors stimulating the transcription of genes encoding HRGP accumulation in the cell wall. In the case of wheat HRGP is less accumulated allowing for more easy invasion by the fungus.
Wheat also releases silicon when it is attacked by the fungus. This acts as a regulator of the plant defence mechanism. It can interfere with cationic co-factor of enzymes which influences pathogenesis related events.
As a further defence mechanism wheat releases WGA
which contains lectins and has a toxic effect on metabolism of fungi.
Wheat
Wheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...
caused by the necrotrophic fungus
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
Tapesia yallundae
Tapesia yallundae
Tapesia yallundae is the causal agent for a variety of cereal and forage grass diseases. The anamorph o f T. yallundae is the W-type strain of Pseudocercosporella herpotrichoides...
(syn: Pseudocercosporella herpotrichoides; W-type [anamorph]; Oculimacula yallundae) and Tapesia acuformis
Tapesia acuformis
Tapesia acuformis is the causal agent for a variety of cereal and forage grass diseases. The anamorph of T. acuformis was formerly known as the R-type strain of Pseudocercosporella herpotrichoides. The W-type strain of Pseudocercosporella herpotrichoides is now known as Tapesia yallundae....
(syn: Pseudocercosporella herpotrichoides; R-type [anamorph]; Oculimacula acuformis). It is also called Strawbreaker. Eyespot is more severe where wheat is grown continuously and when the weather is cool and moist. Treating crops against eyespot with fungicide costs millions to farmers and is complicated by the pathogen becoming resistant to the more commonly used fungicide
Fungicide
Fungicides are chemical compounds or biological organisms used to kill or inhibit fungi or fungal spores. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality and profit. Fungicides are used both in agriculture and to fight fungal infections in animals...
s. Severe cases of the disease can reduce yield by up to 40%. It is most common in temperate regions such as North and South America, Europe, Australia, New Zealand and Africa.
Symptoms
The eye-shaped elliptical lesions which give eyespot it's name form on lower stem bases near to the soil surface. The lesions are straw yellow, often with black pupil-like dots in the centre, and are bordered by greenish-brown to dark-brown rings. In cases of severe infection stems are weakened at the point of infection which makes the host susceptible to lodging. This symptom is associated with the W-type (Oculimacula yallundae). Alternatively in other cases of severe infection the nutrient and water supply to the plant is disturbed, resulting in low quality grain and whitehead production due to early maturation. This is associated with the R-type (Oculimacula acuformis)Development of infection
It is more severe if wheat is grown continuously in same field over the same period. The fungus grows as myceliumMycelium
thumb|right|Fungal myceliaMycelium is the vegetative part of a fungus, consisting of a mass of branching, thread-like hyphae. The mass of hyphae is sometimes called shiro, especially within the fairy ring fungi. Fungal colonies composed of mycelia are found in soil and on or within many other...
which penetrates successive leaf sheaths throughout the growing season. High humidity, cool, and damp weather at the soil surface favours disease development. Whilst dry and hot weather causes the leaf sheaths of the plant to dry and fall off, taking the inoculum with it, thus lessening disease.
Invasion of fungi
Invasion of Pseudocercosporella herpotrichoides in wheat initiates with release of enzymeEnzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
for breaking the plant cell wall
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
. A specific sequence of enzymes is employed; without these enzymes the fungus would not be able to invade the plant cell. The process begins with breaking of the leaf cuticle by formation of appressoria which provide the mechanical force to break into the cell. Then Pectin
Pectin
Pectin is a structural heteropolysaccharide contained in the primary cell walls of terrestrial plants. It was first isolated and described in 1825 by Henri Braconnot...
, cellulase
Cellulase
400px|thumb|right|alt = Colored dice with checkered background|Ribbon representation of the Streptomyces lividans beta-1,4-endoglucanase catalytic domain - an example from the family 12 glycoside hydrolases...
, hemicellulase and protease
Protease
A protease is any enzyme that conducts proteolysis, that is, begins protein catabolism by hydrolysis of the peptide bonds that link amino acids together in the polypeptide chain forming the protein....
enzymes are released to enable invasion into the host cell. Now the fungus is present in the plant stem, it causes disease by affecting nutrient and water supply to upper parts of the plant, weakening the plant stem.
In some cases beta, 1-3 glycan synthase is also released to penetrate the callose matrix.
Plant defences
Once the fungus penetrates the wheat cell wall, the fungal cell wall materials (i.e. chitin) act as elicitors which interact with plant receptors and induce the plant defence mechanism. One product of this is reinforcement of the cell wall in an attempt to stop the invasion. The plant starts forming the papillae - a callose matrix (cellulose, suberin, protein, gums, calcium and silicon) which provides extra resistance to penetration, however this barrier does remain effective for long if the level of micro-organisms present is high.Wheat cells also release hydroxyproline glycoprotein (HRGP) in their cell walls. Secretion of HRGP is dependent on the signal induced by the fungal elicitors stimulating the transcription of genes encoding HRGP accumulation in the cell wall. In the case of wheat HRGP is less accumulated allowing for more easy invasion by the fungus.
Wheat also releases silicon when it is attacked by the fungus. This acts as a regulator of the plant defence mechanism. It can interfere with cationic co-factor of enzymes which influences pathogenesis related events.
As a further defence mechanism wheat releases WGA
Wheat germ agglutinin
Wheat germ agglutinin or WGA is a lectin that protects wheat from insects, yeast and bacteria. An agglutinin protein, it binds to N-acetyl-D-glucosamine and Sialic acid. N-acetyl-D-glucosamine in the natural environment of wheat is found in the chitin of insects, and the cell membrane of yeast &...
which contains lectins and has a toxic effect on metabolism of fungi.
Method of control
- The best method of control for eyespot disease is breeding for resistance. Currently the gene conferring resistance to eyespot is the Pch1 gene. To generate resistant culitvars plants containing this gene are bred with others to pass the gene to their offspring.
- Crop rotationCrop rotationCrop rotation is the practice of growing a series of dissimilar types of crops in the same area in sequential seasons.Crop rotation confers various benefits to the soil. A traditional element of crop rotation is the replenishment of nitrogen through the use of green manure in sequence with cereals...
is also important in reducing the extent of disease because eyespot fungi live on debris of the previous crop. Cropping the wheat with alternate non-host crops and with set-aside periods of at least one year helps to lessen disease. - Use of fungicide can be effective in the short term but is not a long term solution as the pathogen can develop resistance to fungicides. Application of chemicals is also costly.