Wheat tan spot (yellow spot) - Pyrenophora tritici-repentis (Died.) Drechs.


P.trichostoma (Fr.), anamorph Drechslera tritici-repentis (Died.) Shoem. (synonym Helminthosporium tritici-repentis Died.).

Systematic position.

Class Ascomycetes, order Dothidiales, family Pleorosporaceae.

Biological group.

Hemibiotrophic parasite.

Morphology and biology.

Homothallic fungus Pyrenophora tritici-repentis (Died.) Drechs. is a wheat tan spot (yellow spot) pathogen. The tan spot fungus grows saprophytically on host debris. In winter it survives on infested straw and wheat stubble. In autumn pseudothecia are formed on wheat residue. Small black eminent fungal carposomes are often abundant. In early spring these carposomes produce asci with 8 ascospores. Ascospore is brown, 18-28 x 45-70 .m in size, oval, 3-septate, with median cell having 1 longitudinal septa. Mature ascospores are expelled at a short distance (less than 15 cm) from the carposomes during spring rains. They germinate and initiate primary infection on young wheat leaves. The infection requires a moist period ranging from 6 to 48 hours. The initial symptoms are small dark brown spots surrounded by a yellow halo. They appear within five to seven days after infection. Lesions are large (to 1.5 cm) on susceptible wheat cultivars, elliptical, and light tan with a yellow halo. Some varieties have lesions with very large halos, and others show almost no halo. A small dark brown spot in the center of the tan spot gives the lesions the appearance of an eye. When lesions increase in size, they tend to coalesce, producing larger dead tissue. Eventually, older infected leaves begin to die from the tip. The erect abundant olive-black conidiophores (7-8 x 100-300 .m in size) are formed on the surface of large spots. Conidia are sub-hyaline, cylindrical, mostly 4- to 7-septate and 12-21 x 45-200 .m in size. Basal cell has a form resembling a snake head. The conidia cause secondary cycles of infection during the vegetation season. They are transferred by wind approximately 75 m. The infection transfer by seeds is probably poor.


Within the territory of CIS the wheat pathogen Pyrenophora tritici-repentis occurs in Moldova, Ukraine, Byelorussia, Central Asia, and Kazakhstan. P. tritici-repentis is the main pathogen causing winter wheat leaf spots in the Stavropol and Krasnodar Territories, Rostov Region, and in the Republic of Adygea.


In addition to wheat, P. tritici-repentis affects more than 62 species of fodder and wild cereals of the genera Agropyron, Bromus, Agrostis, Alopecurus, Avena, Hordeum, Lollium, Festuca, Stipa, Androgon, Setaria, and Beckmannia. Tan spot develops with greater probability on fields keeping wheat stubble or wheat residues. Long-term wetting of the foliage from rain or dew results in development of severe tan spot epidemics. The tan spot fungus spores germinate and infect wheat over a wide range of temperatures provided that the leaves are wet for a long period.

Economic significance.

P. tritici-repentis dominates among fungi causing leaf spots of winter wheat in the northern Caucasus (82.5% in 1999). In this region an epidemic appears 3-4 times per decade; the yield losses owing to epidemics have been estimated at 15-30%.

Reference citations:

Andronova A.E. 2001. Tan spot of winter wheat in South-West Russia. Zashchita i karantin rastenii No. 5: 32. (In Russian)
Ciuffetti L.M., Tuori R.P. 1999. Advances in the characterization of Pyrenophora tritici-repentis - wheat interaction. Phytopathology 89(6): 444-449.
Khasanov B.A. 1988. Yellow leaf spot in cereals, caused by Pyrenophora tritici-repentis (Died.) Drechs. Mikologiya i Fitopatologiya 22(1):78-83 (In Russian).
Novozhilov K.V., Zakharenko V.A., eds. 2000. Levels and tendencies of variability of species composition and intrapopulation structure, areas of complexes of harmful and useful organisms and forecast of dangerous phytosanitary situations on zones of the country. St. Petersburg. 100 pp. (In Russian)
Mikhailova L. 2000. Yellow leaf spot of wheat-Pyrenophora tritici-repentis. Mikologiya i Fitopatologiya 34(1):7-16. (In Russian)
Pospekhov G.V. 1989. The growth and fruiting of Pyrenophora tritici-repentis (Died.) Drechs. in culture. Mikologiya i Fitopathologiya 23(2): 117-121. (In Russian)
Sanin S.S., Cherkashin V.I., Nazarova L.N., Sokolova E.A., Strizhekozin Yu.A., Ibragimov T.Z. & Neklesa N.P. 2002. Phytosanitary expertise of cereal crops (plant diseases): Recommendations. Moscow: Rosinformagrotech. 140 pp. (In Russian)

© Mikhailova L.A.


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