24 July 2002

Tan spot hits more UK wheat fields

PLANT pathologists have again found UK wheat fields harbouring a disease that is causing increasing damage to crops in Germany and France.

Tan spot or Drechslera tritici-repentis is better known in Germany as DTR, where it has become the number one economically-damaging disease for many growers.

It is favoured by minimal cultivation systems coupled with prolonged wet periods, the fungus needing very long spells of leaf wetness during the summer to infect wheat leaves.

Although similar in appearance to septoria, the blotches can be larger and destroy more green leaf tissue.

Morley Research Centre agronomist Mike Nuttall has found it for the past three seasons at Morley in Norfolk and elsewhere in East Anglia. The latest confirmation is of a number of lesions in a treated crop of Soissons at the centre.

DTR was also present at very low levels in 2001. "We got some reasonable samples from the previous year," he says. "I dont know which actual weather conditions favour it, because it is not a disease that is found much in this country, even by experts. It seems to be favoured by the same weather conditions as Septoria tritici, namely wet and reasonably warm."

Tan spot symptoms appear similar to those of septoria, but lack the cluster of pycnidia in the lesions, which can be bigger and differ in shape from a typical septoria lesion.

"For our country, it is perhaps of academic interest at present, because the fungicides that are available to control Septoria tritici will also control tan spot, particularly Acanto which has it on the label."

ADAS plant pathologist Bill Clark accepts that tan spot is present in the UK at very low levels, but says he has never been able to confirm it in laboratory testing of any of the suspect samples sent to him. An increase in tan spot could be one result of any global warming effects on the UK, he concedes.

In the past few years tan spot has become a problem in Poland, Hungary, Russia and the Czech republic. "It could happen in this country – usually it is a varietal effect and a disease builds up slowly for two to three years and then it is everywhere."

Simon Oxley, of the Scottish Agricultural College, has not found the disease so far. But the adoption of strobilurin fungicides by Scottish farmers may be responsible, since fungicides applied against septoria will also be mopping up any tan spot. &#42

Septoria-like symptoms characterise tan spot, which is favoured by wet weather and min-till. So far it is rare in the UK, but common in Germany.

Pic courtesy of ADAS

German trouble

Tan spot is a severe problem in northern Germany, where it is now regarded as a prime target for strobilurin use. It is more frequent in continuous wheat systems and Heine Krîger, who grows around 350ha of wheat near Lubeck, is typical of many in his use of a robust spray programme of Acanto (picoxystrobin) followed by Opera (pyraclostrobin + epoxiconazole) to protect his 10t/ha class two milling wheats.