Survival instinct

A fungus breeds sexually to infect resistant wheat crops  

By Sugandh Juneja
Published: Thursday 15 July 2010

imageAGRICULTURE experts from various countries gathered at St Petersburg in Russia on May 30-31 to strategize how to combat wheat rust diseases threatening the world’s food supply. Syria said 80 per cent of its wheat fields are infected by stripe rust.

Three of the world’s top 10 wheat producing countries— Pakistan, China and Australia—and Afghanistan sounded a similar alarm.

The disease, caused by Puccinia striiformis, begins in small patches that are difficult to detect and spreads rapidly, wiping out an entire crop. First detected as an infection on barley in USA in 1991, the fungus has since appeared as a major wheat rust, affecting America, East Asia, South Asia, East Africa and Oceania.

Scientists have failed to check the spread because every time they develop a resistant variety and fungicide, the fungus manages to outwit them. “Two features of the fungus underpin its success,” said Colin Wellings, agriculture scientist at the University of Sydney in Australia. “Survival between cropping seasons and adaptation to resistant wheat varieties.”

Pathologists from the Agricultural Research Service at the University of Minnesota, USA, recently unravelled how the pathogen is so adaptable and virulent. Contrary to the understanding that P striiformis spreads through asexually produced spores, they showed the fungus uses a wild evergreen shrub, barberry, as surrogate host to reproduce sexually and mould into a virulent variety.

Barberry also ensures the fungus survives even after the infected wheat plants are removed from the field and infects the next crop, they said. To understand the lifecycle of P striiformis, the researchers infected barberry plants by exposing them to infected wheat straw. After a few days, they examined the infection on barberry.

The fungal colony developed structures depicting sexual reproduction between different strains of the fungus. When wheat plants were exposed to the infected barberry, they caught the infection in 10 days. This shows the barberry served as a host for sexual reproduction of the fungus, the researchers noted in the June issue of Phytopathology.

Lead author of the study Yue Jin said in wheat fields surrounded by barberry plants, reshuffling of virulence genes during sexual reproduction likely contributes to the variability of the pathogen by generating new races that can infect resistant wheat varieties and survive fungicides.

The findings show controlling barberry shrubs in wheat-growing regions can help contain the rust. Barberry is already controlled in areas where stem rust Ug99 threatens wheat crop.

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