Wednesday May 7 2014

UG99 wheat disease spreads as Uganda prepares to release resistant varieties

Wheat at a trial site in Kisumu, Kenya. Ugandan scientists are

Wheat at a trial site in Kisumu, Kenya. Ugandan scientists are collaborating thier counterparts across the border to check wheat diseases. PHOTO BY LOMINDA AFEDRARU 

By Lominda Afedraru

It is always the goal of agricultural scientists globally to breed varieties with the capacity to resist pests and diseases and it is also their desire to see farmers reaping good yields out of what they sow.

However, this is not yet in sight in the case of scientists engaged in breeding wheat varieties that cannot be attacked by the deadly fungal wheat disease UG99, which was discovered in Uganda in 1999 by Dr William Wagoire. It is now spreading so fast in a number of wheat-growing countries globally especially in the Asian countries.

Crop scientists breeding various wheat varieties in countries such as Uganda, Kenya, South Africa, UK and US, among others, realised that UG99 had spread from Uganda to countries like Yemen, Iran, India, Bangladesh and Nepal
But the current trend of the disease spread indicates that it has reached Pakistan and Australia with fears that it may soon get into Europe and the US.

It was in 1998 that Dr William Wagoire, now director, Buginyanya Zonal Agricultural Research and Development Institute, having observed the stem rust, which was destroying farmers’ wheat, decided to collect samples of the virus, which appeared to be more dangerous.

He tested it on a gene called Sr 31, which scientists the world over were previously using to curb the stem rust disease. The experiment demonstrated that his sample represented a unique and dangerous pathogen patterns that could wipe the crop globally.

Wagoire compiled a report about his discovery in 1999, which was published in a peer-reviewed journal where it was named UG99.

Previously, stem rust was the most feared disease destroying various wheat varieties grown globally because it could turn healthy wheat crops into a mass of stems ending up producing no grains. With UG99, it destroys the crop completely with no success of regeneration.

Scientists in the UK at John Inns Centre, Norwich Research Park for the last four years have been breeding wheat varieties to counter UG99, using a gene found in wild grass that related to wheat.

Dr Brande Wulff, the lead researcher, explains: “We started conducting this research four years ago by making crosses between resistant wild relatives of wheat grass, which are genetically mapped to UG99 stem rust. At the laboratory level we are trying to define resistance and the next stage will be identifying the resistant gene which we shall eventually stack to our breeding wheat variety.”

Searching for a solution
The team, according to Dr Brande, is cloning DNA of the wild grass called Aegilos Sharoneusis, commonly known as Sharon goat grass, to their indigenous wheat varieties. This will result into a genetically modified product.

They started with stacking a gene called Sr 31 using the mutation where they realised increase in the yield rate by five per cent. Farmers in UK were willing to adopt it but it has since succumbed to wheat rust.

They further used Sr 24, which was distributed by the International Maize and Wheat Improvement Centre (CIMMYT) based in Mexico. Later, they also used Sr36 and all these genes according to the scientist have been overcome by UG99.

Currently, the researchers are using Sr 14 and Sr44, which they think has resistance to stem rust and UG99.

Dr Brande and his team will take approximately two years to clone these genes to the wheat variety. They will specifically study the DNA of the stacked wheat variety and see if it is not susceptible to the plant.

A recent report on the status of UG99 by UN’s Food and Agricultural Organisation indicated that wheat represents approximately 30 per cent of the world’s production of grain crops. For instance, a total of 598 million tonnes was harvested in 2011 on 220 million hectares of land.

The statistics indicates that once UG99 establishes itself in all wheat growing countries globally, the annual revenue loss will amount to over $3b (Shs7.5trillion) and the large population of rural farmers in Africa will be affected most.

The effort by scientists globally including Uganda in trying to breed varieties resistant to wheat rust and UG99 is ongoing.

Uganda is collaborating in the breeding programme done through mutation with scientists from Kenya. They are at an advanced level of breeding varieties, which are resistant to U99 wheat rust.

Dr Waigore said, his team at Buginyanya has been collaborating with scientists at the University of Eldoret in Kenya and International Atomic Energy Institute in Austria, Vienna who provided them with the genes.

Variety release
Mutation breeding is the process of exposing seeds to chemicals or radiation in order to generate mutants with desirable traits to be bred with other cultivars.

Through mutation, the team has been able to advance the genes into the local wheat varieties grown by farmers locally to come up with the resistant lines.

According to Dr Wagoire, his team is now in the process of preparing for the release of two or three varieties that are resistant to UG99. This is expected to be during the next planting season.

They are set to present these varieties to the plant release committee at the Ministry of Agriculture, Animal Industry and Fisheries. Once they are approved for release that is when the varieties will be given a name.

Wagoire said although they will be in a position to release the varieties this year, this will not stop the researchers from breeding other varieties because it is a continuous process as the crops may succumb to diseases in the future due to climate change effects.

Wheat in Uganda is grown in the districts of Kapchorwa and Sironko on the slopes of Mt Elgon, Mbale, Kisoro, Kabale and on hills in Mbarara and Bushenyi.

About the UG99 disease

Wheat stem rust (Puccinia graminis f. sp. tritici) is historically the most damaging disease of wheat. The disease has the capacity to turn a healthy looking crop, only weeks away from harvest, into nothing more than a tangle of black stems and shrivelled grains at harvest.

Under suitable conditions, yield losses of 70% or more are possible. Wheat stem rust is highly mobile, spreading rapidly over large distances by wind or via accidental human transmission (infected clothing or plant material).

Wheat stem rust has largely been under control for over three decades due to the widespread use of resistant cultivars. In 1999, a new virulent race of stem rust was identified from wheat fields in Uganda – popularly known as Ug99 after the year and country of discovery.

Using North American scientific nomenclature, Ug99 is known as race TTKSK. Ug99 (Race TTKSK) is a cause for concern as it exhibits unique virulence patterns. No other race of stem rust has been observed to overcome so many wheat resistance genes, including the very important gene Sr31. By 2007, Ug99 (Race TTKSK) had spread via wind movements out of East Africa, into Yemen and as far as Iran.

Rust pathogens change rapidly, often by mutation. Six additional variants are now recognized in the Ug99 lineage. All exhibit an identical DNA fingerprint, but differ in virulence patterns. Additional important resistance genes e.g., Sr24 & Sr36 have now been defeated by variants of Ug99. Ug99 or variants are considered a major threat to wheat production with an estimated 80-90% of global wheat cultivars susceptible.