Tackling Wheat’s Arch Enemy
The stem rust fungus ‘Ug99’ is one of the greatest threats to global wheat production today. It has been called the “polio of agriculture” due to its virulence and far and fast reach. This wind-traveling mutant strain produces millions of spores – all of which can cause infection. Fungicides are very effective in controlling Ug99 but plant scientists are also working on resistant wheat varieties. Dr. Sridhar Bhavani, wheat breeder, pathologist and international coordinator for stem rust research in East Africa at the International Maize and Wheat Improvement Center’s (CIMMYT’s) regional office in Nairobi, Kenya explains his work in combatting Ug99.
What is Ug99 and how does it affect wheat?
“Ug” stands for “Uganda” and “99” refers to 1999, the year this race of wheat stem rust disease was characterized. This race/strain affects mainly stems but also leaves and spikes holding grain. It stops and/or disrupts the movement of water and nutrients from soil roots to other plant parts, which makes a crop fragile and causes stems to break and prevents grain production. Losses can be devastating if the disease attacks early where susceptible varieties are grown. The warmer the weather, the earlier the disease attacks and the more damage is done. Ug99 adapts well to warm climates, such as in East Africa, and with climate change, stem rust could become even more prevalent.
Where is Ug99 the most prevalent?
Ug99 and its related races are predominantly found in 13 countries: Egypt, Eritrea, Ethiopia Kenya, Iran, Mozambique, Rwanda, South Africa, Sudan, Tanzania, Uganda, Yemen and Zimbabwe. However, Ug99 is a significant threat to wheat farmers worldwide because it does not have any geographical boundaries, making farms in other countries potential victims. Susceptible varieties can produce billions of spores (about 100 billion spores per hectare) that are tiny, light and easily transported by wind from farm to farm and from one wheat-growing region to another.
How did you succeed in breeding Ug99-resistant varieties?
Ug99 was first identified in Kenya in 2001. To combat it, new resistant wheat varieties were released about every four years with different resistance genes. But each time, the disease mutated and overcame the genes, causing “boom and bust” cycles. The last time this happened was in 2014 in Kenya. Our strategy now is gene pyramiding (stacking with either 2-3 race-specific genes, 5-6 race-non-specific genes or a combination of both types of resistances) to manage Ug99. Such stacking of multiple resistance genes offers long-term durability even if one gene becomes ineffective. Moreover, combining several race non-specific genes confers resistance to yellow and leaf rusts and powdery mildews in addition to Ug99.
How long does it take to develop a new Ug99-resistant variety?
At CIMMYT, it usually takes 5-6 years – about half the time of traditional plant breeding because of “shuttle breeding” in two locations for two growing seasons per year. This concept was developed by Dr. Norman Borlaug in Mexico, then extended to Kenya. CIMMYT is the only organization in the world with wheat crosses and population sizes large enough to identify multiple gene combinations for various traits, including resistance to Ug99. CIMMYT also currently uses marker-assisted selection to accelerate breeding with improved traits.
How else can Ug99 be combatted in the future?
Using improved monitoring and surveillance to understand Ug99’s diversity, spread and virulence along with modern breeding tools will be the strategy. One future way to enhance the durability of resistance to rust diseases is by stacking multiple genes as “gene cassettes.” This allows for genes to be transferred as one block rather than singly at a time.