Frequently Asked Questions (FAQs)

What are the Rusts?

Stem Rust

Stem rust

Stem Rust, Puccinia graminis f. sp. Tritici, also known as black rust, is a feared disease in most wheat regions of the world because an apparently healthy crop three weeks before harvest could be reduced to a black tangle of broken stems and shrivelled grain by harvest. The widespread use of resistant cultivars worldwide has reduced the disease as a significant factor in production. However, the emergence of Ug99 has renewed the threat of stem rust.

Primary Hosts: Bread and durum wheats, barley, triticale

Alternate Hosts: Berberis vulgaris

Stripe Rust

Stripe rustStripe rust, Puccinia striiformis f.sp. Tritici, also known as yellow rust, is principally a disease of wheat grown in cooler climates (2° to 15°C), which are generally associated with higher elevations, northern latitudes or cooler years. It takes its name from the characteristic stripe of uredinia that produce yellow-colored urediniospores. Because of the disease’s early attack, stunted and weakened plants often occur. Losses can be severe (50 percent) due to shriveled grain and damaged tillers. In extreme situations, stripe rust can cause 100 percent losses.

Primary Hosts: Bread and durum wheats, triticale, a few barley cultivars

Alternate Hosts: Unknown

Leaf Rust

Leaf rustLeaf rust, Puccinia recondite, is also known as brown rust. In temperate zones it is destructive on winter wheat because the pathogen overwinters. Infections can lead up to 20% yield loss - exacerbated by dying leaves which fertilize the fungus. It is the most prevalent of all the wheat rust diseases, occurring in most wheat growing regions.

Primary Hosts: Bread and durum wheats, triticale

Alternate Hosts: Thalictrum, Anchusa, Isopyrum, Clematis

What is Ug99?

Ug99 is a lineage of wheat stem rust (Puccinia graminis f.​ sp.​ tritici), that is currently present in wheat fields in several countries in Africa and the Middle East.​ Surveillance experts predict it could spread rapidly through these regions and possibly further afield, potentially causing a wheat production disaster that would affect food security worldwide.​ It can cause major crop crop losses and is virulent against resistance genes that have previously protected wheat against stem rust.​

Although Ug99-resistant varieties of wheat do exist, a screen of 200,000 wheat varieties used in 22 African and Asian countries found that only 5-10% of the wheat grown in these countries consisted of varieties with adequate resistance.​

There are now eight known races of Ug99.​ They are all closely related and are believed to have evolved from a common ancestor, but differ in their virulence/​avirulence profiles.​

Where is Ug99? Where is Ug99?

The original race of Ug99, which is designated as 'TTKSK' under the North American nomenclature system, was first identified in Uganda in 1998, and characterized in 1999 (hence the name Ug99).​ It has since been detected in Kenya, Ethiopia, Sudan, Eritrea, Yemen, Iran, Mozambique, Zimbabwe, Tanzania, and South Africa.​

For more information, please refer to the Pathotype Tracker at

Where might it go next?

The overall hypothesis is that the Ug99 pathogen will continue to push into the Middle East, and onward.​ Movement is predicted to follow predominant west to east airflows.​ The exact timing or nature of future events cannot be predicted with certainty, as disease outcomes will depend on prevailing environmental conditions, host susceptibility, and a range of other factors.​ Outputs from the HYSPLIT trajectory model, using the confirmed 2007 Ug99 Iranian sites as sources, support the idea that airflows would likely move toward the east, but also indicate the possibility of more northerly movements into the Caucasus and Central Asia.​

Do I need to worry about Ug99?

Yes, any country where wheat is grown is susceptible to the threat of Ug99.​ For example, wind models suggest that Ug99 has the potential to reach India from Yemen.​ The Ug99 spores can also travel via human movements, which are much more unpredictable than wind movements.​ However, at the same time, the BGRI states that there is no need to panic.​ If scientists and research groups continue to collaborate with government agencies, then the threat will continue to be mitigated.​

Have Ug99 resistant varieties been tested or released?

Yes.​ Here is the map that identifies Ug99 resistant varieties that have been released or are in advanced testing stages since the project’s inception in 2008.​ Each variety is also tested for stripe rust resistance which makes adaption by farmers more likely.​


View Ug99 resistant varieties in a larger map