A "new and virulent" strain of yellow stripe rust is threatening to wipe out wheat crops on some Middle Eastern farms, and has dashed hopes of record wheat yields in Syria and Turkey.

The warm and rainy conditions which have placed many parts of the region on course for bumper harvests have incubated a previously unknown type of the fungal disease which causes "substantial vegetative damage" and the premature ripening of crops.

"Crops are maturing an average of four to six weeks earlier than normal in the worst affected areas," US Department of Agriculture officials said.

"Crop scientists familiar with the disease indicate that yield reductions of 35-50% are typical in seriously infected fields, while in the worst instances, nearly total crop loss is possible."

Disease spreads 

The outbreak, which has been declared a regional epidemic by scientists in both Syria and the at the US-based Borlaug Global Rust Initiative, has so far reached Iran, Iraq, Lebanon, Syria and Turkey, with the last two countries expected to suffer the most serious crop losses.

The USDA cut its estimate of the Turkish crop by 1.0m tonnes to 17.5m tonnes, and slashed its forecast for the Syrian crop by 25% to 3.75m tonnes.

The revisions came as the department, in its latest benchmark report on global crop supply and demand, reduced its estimate for the world wheat crop in 2010-11 by 3.7m tonnes to 668.5m tonnes.

US rust

And it co-incides with growing concerns about levels of conventional rust in America's winter wheat crops.

Arlan Suderman at Farm Futures said: "Fears are mounting that stripe rust will produce disappointing yields in Kansas and Colorado and points to the north.

"Stripe rust is also creating threats in the Pacific Northwest as well."

However, the USDA flagged better prospects for Pacific Northwest crops as it raised by 0.5 bushels per acre to 43.9 bushels per acre, its forecast for the average US wheat yield in 2010-11, with the production estimate raised by 24m bushels to 2.07bn bushels.