Wheat stem rust (Puccinia graminis f. sp. tritici) has caused devastating epidemics of wheat in the history of mankind. It can destroy entire wheat harvests – leaving nothing but black stems and shrivelled grains. Currently, a new variety, Ug99, is spreading from Africa towards Central and South Asia, where 20% of the world’s wheat is produced.
Last MEMF blog’s entry was about Norman Borlaug, who saved millions of lives by developing high-yielding and stem-rust resistant wheat cultivars that spurred the Green Revolution. This blog will focus on the pathogen that he was fighting: stem rust.
Stem rust plagues wheat and threatens our daily bread
Stem rust is such a devastating disease, because it attacks the crop that is most intimately connected with human civilization: wheat. Without the cultivation of cereals by hunter-gatherers in the Fertile Crescent, there would have been no civilization. Today, wheat is cultivated on 25% of the global arable land. After rice, it is the second most important grain crop of the world and accounts for one fifth of humanity’s calorie intake (rice has a similar share, all other foods combined account for the rest). From South Asia through to Central Asia across the Middle East and on to North Africa, wheat is a staple food (meaning the main dietary component). Can you imagine the Italians without pasta, North Africans without couscous, Indians without Chapatis or the Lederhosen-wearing inhabitants of Bavaria without Weissbier?
Next to these traditional wheat-eating (and drinking) regions, some 40 million tons of wheat are imported every year to Sub-Saharan countries. Wheat used to be a minor crop there, but nowadays it is important crop for Sub-Saharan food security.
The over-complicated life cycle of stem rust fungi
The first symptoms of stem rust are beautiful -from my very personal point of view. Bright-orange, “rusty” pustules occur on the wheat leaf surface and stems containing up to 350.000 urediniospores. Urediniospores are asexual spores. Asexual means that the spores are genetically identical clones of one another. If the conditions are favourable (Stem rust likes it hot and humid.), the fungus produces so many urediniospores that they form huge orange clouds above the fields and quickly infect every single plant. The spores are dispersed by the wind and can infect wheat plants that are grown thousands of miles away.
Currently, this asexual life cycle (read: No Sex!) of stem rust is the most important. This is due to large-scale eradication of a horny bush named barberry. Wondering what barberry has to do with a disease of wheat?
This question leads us to the complicated life cycle of rust fungi – one of the most complicated ones on earth. If you think that your (sex) life is complicated, you don’t want to be a rust fungus. Next to the clonal urediniospores, stem rust also mates on wheat and produces black teliospores. Teliospores are sexual spores. In other words, they are new varieties emerged from the recombination of their parents’ genetic material. The stem rust’s children, if you want.
Teliospores cannot infect wheat. They infect barberry – as an alternative host. On the barberry plant, the stem rust fungus celebrates a bacchanal, producing at first small clusters of pycnia that excude pycnidiospores a sticky honeydew. The honeydew attracts insects, which will carry the pycnidiospores to other parts of the barberry plant where they mate again and grow through to the lower leave surface to release dikaryotic aeciospores. Like the teliospores, aecsiopores are not able to infect the plant on which they are produced. Instead they are disseminated by wind and rain to young wheat plants and start the infection cycle as new genetic varieties all over again.
Yes, you counted correctly! Stem rust produces four different kinds of spores on two different plants. It took scientists decades to put together the pieces of this complicated lifecycle (If you got lost, there is a cute animation about the rust lifecycle.). Anton de Bary finally succeeded in 1860. The sexual life cycle of stem rust involving the telio -, pycnio- and aeciospores does not play a big role in stem rust infections in Europe and the US. Farmers early on recognized the vicious connection between barberry and stem rust infections leading to large-scale barberry eradication program in Europe and the US in the early 20th century. However, the new variety from Africa, Ug99, might have evolved on a barberry plant.
Rust never sleeps – Ug99 is on a deadly trail towards Europe and Asia
So, what is Ug99?
Ug99 – Ug for its country of origin, 99 for the year it was confirmed – is a highly aggressive strain from Uganda that overcame most wheat resistance genes. It was Norman Borlaug, the Father of the Green Revolution, who had created most of the stem-rust resistant wheat varieties. His masterpiece was a complicated cross-hybridization of a segment from a rye chromosome into wheat. This chromosome segment not only contained resistance genes to stem rust – among them the durable Sr31 gene -, but also genes that increased grain yields. This made the new cultivars quickly popular in the whole world. They were so effective that stem rust declined to almost insignificant levels everywhere by the mid-1990s.
Until Ug99 entered the stage! Ug99 can overcome more wheat resistance genes, including Sr31, than any other stem rust variety before. The fungus changes rapidly by single step mutations and already consists of seven varieties with different virulence patterns. It spreads quickly through Africa and the Arabian Peninsula. In 2004, it spread from Uganda to Kenya, and then to Ethiopia and southern Sudan causing up to 80% yield loss in Kenya and Uganda. In 2007, Ug99 jumped the Red Sea and is now widespread in Yemen and Iran. So far, Ug99 did not arrive in one the major wheat producing countries: China, India, the US and Russia. If it does, our daily bread is threatened: 80-90% of the global wheat cultivars are susceptible to Ug99.
What do scientists do? Concerted effort – the Borlaug Global Rust Initiative
Norman Borlaug was immediately alarmed when he learned about Ug99. At the age of 91, he took up the fight against stem rust once again. By 2005, he had drummed up an international consortium of scientists from CIMMYT, ICARDA, FAO and the ARS of the US Department of Agriculture – the Borlaug Global Rust Initiative. Coordinated by Cornell University the BGRI concentrated efforts on developing and deploying new effective resistance of wheat stem rust by stacking four or five resistant genes. So far, the researchers produced 60 experimental wheat varieties. The new wheats have only one drawback: lower yield (the opposite of Borlaug’s wheat varieties).
Lower yields make their use unpopular in countries where Ug99 has not yet arrived – including the major wheat producers. Thus, the threat of wheat stem rust prevails! As Borlaug said: “Rust never sleeps.”
Sources: FAOstats; wheat.org; Schumann, G.L. and K.J. Leonard. 2000. Stem rust of wheat (black rust). The Plant Health Instructor. DOI: 10.1094/PHI-I-2000-0721-01. Updated 2011.