Eyespot (wheat)

Eyespot is an important fungal disease of wheat caused by the necrotrophic fungus Tapesia yallundae ("Pseudocercosporella herpotrichoides" W-type [anamorph] ) and Tapesia acuformis ("Pseudocercosporella herpotrichoides" R-type [anamorph] ). It is also called Strawbreaker. It is more severe where the wheat is grown continuously and the weather is cool and moist. It costs millions of dollars in fungicide to farmers and is now becoming resistant to fungicides. It can reduce the yield up to 40%. It is most common in North and South America, Europe, Australia, New Zealand and Africa.Hollmann, M. (n.d.). Eyespot of Wheat, Retrieved October 27, 2007, from [http://www.dasc.vt.edu/graduate/marcusi/webpage/graphics/graphics/EYESPOT%20in%20WHEAT.htm] ]

ymptoms

The eye shape elliptical lesions forms on lower base of stem and lower leaf sheath. The eyespots are straw yellow in colour from the middle and are surrounded by the greenish brown to dark brown rings. But in case of severe infection stem gets weaken at the point of lesion or sometime broke down and easily lodge. Ultimately which disturb the nutrient and water supply to plant result in low quality grain by turning its head white due to early maturation and reduced the yield by decrease in number and lodging of wheat.Prescott, J.M., Burnett, P.A., Saari, E.E., Ranson, J., Bowman, J., Milliano, W., Singh, R.P., & Bekele, B. (n.d.). Wheat Disease and Pests: a guide for field identification. Retrieved November 2, 2007, from [http://wheat.pw.usda.gov/ggpages/wheatpests.html#eyespot] ]

The Link below shows the eyespot on a wheat stem:

http://grain.jouy.inra.fr/cgi-bin/graingenes/report.cgi?class=image;name=Eyespot+lesion+typical+of...Psuedocercosporella+herpotrichoides...

The Link below shows infection of disease:

http://grain.jouy.inra.fr/cgi-bin/graingenes/report.cgi?class=image;name=Eyespot+lesions+occur+at+or+near+the+soil+line...

Development of infection

It is more severe if wheat is grown continuously in same field over the same period. The fungus grows as "mycelium" which comes in contact of growing stem. High humidity, cool, and damp weather at soil surface favours the disease development.

Invasion of fungi

Invasion of Pseudocercosporella herpotrichoides in wheat starts with release of enzyme for breaking the plant cell wall. It needs a proper sequence of enzyme otherwise fungus would not be able to invade the plant cell. It starts with breaking of cuticle by melanin which provides the mechanical force to break it. Then Pectin, cellulases, hemicellulase and protease enzyme is released to completely invade the host cell. Now the fungus enters in the plant, it damages it by affecting its nutrient and water supply to upper parts of plants which weakens the plant stem.But sometime beta, 1-3 glycan synthase is also released to penetrate the Callose matrix. Vidhyasekaran, P. (n.d.). Fungal Pathogenesis in Plant and Crop: Molecular Biology and Host Defence Mechanisms. Retrieved November 3, 2007, from [http://books.google.com/books?id=TF3SjmtWowMC&pg=PA161&lpg=PA161&dq=pseudocercosporella+herpotrichoides+defense&source=web&ots=oBc5ymhZlc&sig=W1ymkjtgT1g6BE7cHqVuVGVBxWw#PPP1,M1] ]

Plant defences

Once the fungus penetrates the wheat cell wall, the fungal cell wall material act as a elicitors such as chitin which induce the plant defence mechanism by reinforcement of cell wall to stop the invasion. Plant starts forming the papillae a callose matrix (cellulose, suberin, protein, gums, calcium and silicon) which is a cell wall apposition. It provides extra resistance to penetration. But this barrier of wheat does not last long if the concentration of micrograms is high.Wheat cell also release hydroxyproline glycoprotein (HRGP) in its cell wall. Secretion of the HRGP depends on the signal induced by fungal elicitor which stimulates the transcription of genes encoding HRGP accumulation in cell wall.But in case of wheat, HRGP is less accumulated; hence the fungus invades it easily.Wheat also releases silicon when it is attacked by the fungus, which acts as a regulator of plant defence mechanism. It can interfere with cationic co factor of enzymes which is influencing the pathogenesis related events.

Wheat releases the WGA (wheat germ agglutinin) which has lectins in it and has toxic effect on metabolism of fungus. [Mishkind, M., Raikhel, N. V., Palvitz, B. A., Keegstra, k. Immunocytochemical Localization of Wheat Germ Agglutinin in Wheat. The Journal of cell Biology, Vol 92, No.3 Retrieved November 2, 2007, from [http://links.jstor.org/sici?sici=0021-9525%28198203%2992%3A3%3C753%3AILOWGA%3E2.0.CO%3B2-X&size=LARGE&origin=JSTOR-enlargePage] ]

Method of control

1) The best method of control this disease is making a breed which is resistant against the Eyespot. It can be done by selecting the gene of healthy plant and the gene is Pch1. [Santra, D.K., Kidwell, K., & Campbell, K. (n.d.). Disease Resistance - Eyespot. Retrieved October 28, 2007, from [http://maswheat.ucdavis.edu/protocols/Eyespot/index.htm] ]
2) Crop rotation is also the good idea to reduce the extent of disease as the organism live on last crop debris. Cropping the wheat with alternate period of time helps to get rid of disease. [K-State Research and Extension. (2000) Rotating Crops Best Defence Against Wheat Strawbreaker Retrieved November 1, 2007, from from [http://www.oznet.k-state.edu/news/sty/2000/strawbreak.htm] ]
3) Use of fungicide is not an effective idea for long period of time as it makes fungi resistant to fungicides and is expensive as well.

References