One deadly plant disease found in wheat may have met its match. Yellow rust, also known as stripe rust, is one of three rust diseases found in wheat grown in cooler environments in northern latitudes. According to the USDA-ARS Cereal Disease Laboratory wheat stripe rust can cause losses of 40% with some wheat fields totally destroyed by the disease. It is a major disease of fall seeded wheat or spring wheat planted adjacent to infected fall seeded wheat. While stripe rust is lethal to wheat it does not produce toxins dangerous to human consumption like other fungi (e.g. Fusarium). It causes large yield losses and decreases the quality of wheat.
Fortunately, the development of large USDA Collaborative Agricultural Projects (CAP) in wheat and barley provided the support required to implement new breeding technologies and accelerate the development of resistant varieties. Dr. Jorge Dubcovsky, leader of the wheat breeding program at the University of California and of the Wheat-CAP (2005-2010) and Wheat-Barley CAP (2011-2016) and his team implemented these new technologies in California and were able to control the epidemic.
“Stripe rust is a devastating disease of wheat, and the appearance of more virulent races in the year 2000 resulted in large losses that reached 25% of the California wheat crop in 2003. These new races attacked the most productive wheat varieties from California so we had to look for new sources of resistance. The availability of more precise molecular methods of selection helped us to provide a fast answer to the stripe rust epidemics in the beginning of the 2000s so that today the epidemic in California is over and resistant varieties are available to the growers,” stated Dubcovsky.
Dubcovsky and his team of scientists and graduate students identified a gene with resistance to wheat stripe rust. “We use these genes to develop wheat varieties resistant to stripe rust that do not require the application of fungicides, which is beneficial to farmers, the environment, and the consumer. Today we have controlled the stripe rust epidemics and almost all the new varieties are resistant to these new races of stripe rust.” Dubcovsky has been working on stripe rust resistance projects for the last 15 years. This research was funded by the previous USDA’s National Institute of Food and Agriculture (NIFA) Coordinated Agricultural Project, WheatCAP, and the current Triticeae-CAP grants. 
Dubcovsky’s team first worked in mapping the location of these genes on the chromosomes and then in developing molecular markers from these chromosome regions to help select varieties carrying multiple resistance genes. This technology, called marker assisted selection, accelerates the traditional breeding process. The resulting varieties are not considered genetically modified organisms, since they are obtained by normal crosses.
“Marker assisted selection makes the selection process faster and more precise. Another advantage of this technology is that we can pyramid multiple resistance genes to extend the durability of the resistance,” said Dubcovsky. Resistance genes are normal wheat genes and can be introduced in any wheat variety by simple crosses. Then molecular markers are used to select the progeny of these crosses carrying the genes. Without this technology, once a breeder introduces one resistance gene, he doesn’t know when additional genes are being introgressed since the plants are all resistant. With the molecular markers scientists can follow the genes without the need to test for resistance.
In 2008 the first resistant wheat variety (Patwin) developed by marker assisted selection was released and today several resistant varieties developed by this technology are starting to be grown in California both from public breeding programs (e.g. Lassik, and Desert King-High Protein) and from collaboration between the UC-Davis program and private wheat breeding programs operating in California (e.g. Expresso, Blanca Grande 515, Summit 515, Westmore, New Dirkwin, etc.). “We have also mapped new resistance genes and we are starting to introgress those in our varieties as a second line of defense, in case new races manage to overcome the first genes we put in,” Dubcovsky said.
Today the stripe rust epidemic is well controlled in California and the new varieties are resistant to the new virulent races of stripe rust eliminating the need of costly fungicide applications. “Fungicides may be effective in controlling stripe rust but they can be expensive, increasing the cost of production. Also the application of fungicides is not the best for the environment if they can be avoided,” Dubcovsky continued. “Resistant wheat and barley varieties have enabled growers to avoid costly fungicide applications, reduce yield losses caused by the pathogen and produce a higher quality crop.”
