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Tan Spot in the UK
First UK case of Tan Spot?
It was reported by Farmers Weekly (July 8-14th 2005) that the first UK case of tan spot was found in Kent. However, this was not the case. Tan spot was first recorded on wheat in June 1987 in two crops in North Essex [1]. Later that year, the disease was found in South Lincolnshire, Northamptonshire, Suffolk, Gwynedd, Suffolk, North Yorkshire and County Durham. Crop disease surveys (now CropMonitor) of winter wheat also detected the presence of the disease in 1988, 1990, 1991 1994, 1995, 2002, 2003 and 2004 (although at very low incidence), with the 1995 record originating from Kent.

There are earlier records of the fungus occurring on other hosts in the UK, such as couch grass (Elymus repens) in England and on rye (Secale cereale) in Scotland [2,3]. The fungus is also known to infect barley, bromegrass (Bromus spp.) and numerous other grass species [4]. Entries on the BMS Fungal Records Database [5] indicate that the fungus was present in the UK for sometime prior to those reports, perhaps even before 1950. The disease is described as 'cosmopolitan' and 'common and widespread on couch grass and wheat' [6]. The fungus is known to be present on all continents except Antarctica.

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].

The particular report in Farmers Weekly was interesting as it found on a farm that has been 'incredibly dry'. 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.


 
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