Very close up shot of wheat crop ears in field

Tan Spot in the UK

The disease

Tan spot, also known as yellow spot, is a disease of wheat caused by the fungus Pyrenophora tritici-repentis. Infection results in dark brown lesions on leaves, these lesions are darker than the otherwise similar lesions caused by Septoria nodorum [1]. The lesions present on the leaves caused by tan spot reduce both the quantity and quality of the yield. The disease can be serious in itself but frequently contributes to leaf spotting complexes [7].

Tan spot symptoms on wheat leaves - Click on an image to see a larger version

Usually tan spot disease of wheat is associated with warm and wet conditions. Exceptionally wet weather was present in the first recorded UK finding in June 1987 and therefore under normal conditions it is considered that the pathogen is unlikely to pose a significant threat to wheat in England [1]. However, in countries where the climatic conditions favour the pathogen, yield losses can be severe. In the central plains of the United States and Canada yield losses ranged from 3 to 50% and tan spot was reported as the fastest spreading disease in the Southern Cone region of South America [8].

Changes in cultural practices may also cause an increase in disease incidence. Pyrenophora tritici-repentis can grow saprophytically on host debris, including retained stubble. The following spring, ascospores form on such material and are discharged during wet weather, this is followed by repeated cycles of conidial production on diseased leaf tissue. Conidia are thought to be associated with long distance dispersal by wind, but the pathogen can also be transported through the use of infected kernels (called red smudge). Increases in minimum tillage wheat farming with the subsequent retention of stubble (no stubble burning), coupled with shorter rotations and use of highly susceptible cultivars could all have contributed to the recent emergence of this disease worldwide.

References

  1. Cook RJ, Yarham DJ, 1989. Occurrence of tan spot of wheat caused by Pyrenophora tritici-repentis on wheat in England and Wales in 1987. Plant Pathology 38, 101-102.

  2. Ellis MB, 1971. Dematiaceous Hyphomycetes. CAB International Mycological Institute, Kew Surrey, 424-425.

  3. Sivanesan A, 1987. Graminicolous species of Bipolaris, Curvularia, Drechslera, Exserohilum and their teleomorphs. Mycological Papers No. 158, 261 pp.

  4. Krupinsky JM, 1991. Grass hosts of Pyrenophora tritici-repentis. Plant Disease 76, 92-95

  5. The British Mycological Society Fungal Records Database. <http://194.203.77.76/fieldmycology/BMSFRD/bmsfrd.asp>

  6. Ellis MB, Waller JM, 1976. Pyrenophora tritici-repentis. CMI Descriptions of Pathogenic Fungi and Bacteria. No. 494, 2pp.

  7. Weise MW, 1987. Compendium of Wheat Diseases. 2nd Ed. The American Phytopathological Society, St Paul, MN.

  8. Ciuffetti LM, Tuori RP, 1999. Advances in the characterisation of the Pyrenophora tritici-repentis -wheat interaction. Phytopathology 89, 444-449.