Importance of cereal rust disease in American agriculture
Rusts are among the most damaging diseases of wheat and other small grain crops. In the Great Plains of the United States, stem rust and leaf rust epidemics often have caused yield losses in wheat far exceeding 20 million bushels. A stem rust epidemic in 1916 destroyed almost 300 million bushels of wheat in the United States and Canada. The 1935 wheat stem rust epidemic destroyed at least 135 million bushels, mostly in the Dakotas and Minnesota. In 1953 and 1954, stem rust caused $365,000,000 losses in the U.S. wheat crop, including more than 75% of the durum wheat, our source of pasta. While losses due to wheat stem rust have been abated since the 1960's by effective resistance breeding, that is not the case for wheat leaf rust. As recently as 1993, leaf rust destroyed over 40 million bushels of wheat in Kansas and Nebraska. In 1985, Texas and Oklahoma lost 95 million bushels of wheat to leaf rust. The country can ill afford such losses, especially for wheat, a major export commodity.
The need for a national cereal rust research
Wheat is grown in nearly every state of the U.S., but the greatest concentration is in the Great Plains. The high concentration of wheat from central Texas to Minnesota and the Dakotas makes this region especially vulnerable to stem rust and leaf rust epidemics. The cereal rust fungi are superbly adapted for long distance spread. Rust epidemics start in fall-sown crops of winter wheat in the southern plains. Each rust infection in a wheat leaf or stem produces tens of thousands of spores that are released in the wind like pollen to produce new infections wherever they land on other susceptible wheat plants. Some spores establish new infections after traveling hundreds of miles in high altitude air currents. Within one to two weeks, each new infection begins releasing spores to initiate the next generation of infections. The prevailing winds during spring and summer in the Great Plains are from south to north. This allows epidemics of cereal rusts to sweep north all the way from Texas to Manitoba. During harvest of winter wheat in Texas and Oklahoma, the rust moves north through the maturing crops of Kansas and Nebraska and into the fields young spring wheat in the Dakotas and Minnesota. Clearly, no individual state along this cereal rust path can solve its cereal rust problems by itself.
Spread of cereal rusts in the regions east of the Mississippi River does not follow such an obvious south to north pattern. However, the movement of new rust races from one region to another is a major concern. Cereal crops grown in one region can serve as a reservoir for races that may be able to overcome the resistance of cereal varieties in other parts of the country. For example, Cereal Disease Lab scientists determined that stem rust race Pgt-QCCJ, which overcomes the resistance of all current American barley varieties, originated in the Pacific Northwest. In the late 1980's it became established in the Great Plains where it threatens the major barley producing areas of Minnesota and North Dakota. Wheat leaf rust race MBGL was rarely found in the Great Plains until 1990, one year after it had become the second most common leaf rust race in the Southeast.
Why developing rust resistance in cereals requires a major research program
Breeding small grain crops for rust resistance is not like breeding for better grain quality, higher yield, or even tolerance to physical stresses such as drought or heat. Breeding gains made in those traits remain for as long as a variety is grown. This is not true for gains in rust resistance. Resistance built into a wheat variety over 15 years of breeding work can be totally wiped out by a shift in pathogenic races in the rust fungus population. There are more than 60 known genes for stem rust resistance and more than 45 genes for leaf rust resistance in wheat. Each of these resistance genes will provide excellent resistance against some rust races, but none of them can be expected to work against all races. Genetic studies with rust fungi have shown that for every resistance gene in the host plant can be matched by a corresponding virulence gene in the rust fungus that will allow the fungus to overcome that resistance. Pathogenic races in rust fungi are determined by which combinations of virulence genes are present in the fungus. The combination of virulence genes, of course, determines which resistant varieties the rust race can attack and which varieties are safely resistant to it. Because new rust races can and do arise, the cereal breeder's work is never done. New races may require new resistant varieties. Knowing why and how often new rust races appear is an important part of breeding for rust resistance.
Cereal Rusts in History
Early records indicate that wheat was affected by blight, blasting, and mildew, which are now assumed to be at least, in part, due to the rust fungi. Aristotle (384-322 B.C.) writes of rust being produced by the "warm vapors" and mentions the devastation of rust and years when rust epidemics took place. Theophrastus reported that rust was more severe on cereals than legumes. Excavations in Israel have revealed urediniospores of stem rust that have been dated at about 1300 B.C. "Stern Robigo, spare the herbage of the cereals, ...withhold we pray thy roughening hand..." was part of the official prayer at a Robigala ceremony as given by Ovid (43 B.C.-17 A.D.) and gives the impression that stem rust was a serious disease in Italy during that time.
The Italians Fontana and Tozzetti independently provided the first unequivocal and detailed reports of wheat stem rust in 1767. In 1797 Persoon named the causal organism of wheat stem rust Puccinia graminis. In 1946 Chester provided one of the first detailed histories of published literature on the rusts.
In the early records, wheat leaf rust is not distinguished from stem rust. However, by 1815 de Candolle had shown that wheat leaf rust was caused by a distinct fungus and described it as Uredo rubigo-vera. The pathogen underwent a number of name changes until 1956 when Cummins and Caldwell suggested P. recondita, P. triticina is the accepted nomenclature today.
Although Gadd first described stripe rust of wheat in l777, it wasn't until l896 that Eriksson and Henning showed that stripe rust resulted from a separate pathogen, which they named P. glumarum. In l953, Hylander et al. revived the name P. striiformis for the stripe rust fungus.