It is through this haustorium that nutrients pass from the plant to the fungus, allowing it to grow and reproduce. Rust fungi life cycles are complex. Most of these stages start and end with a type of spore, hence rusts typically produce four or five different spore types and often have a spore produced on one plant species e.
These fungi are heterotrophs that feed upon material produced by other living organisms. They are unusual because they can only be fed by living cells of their host.
Even though the nutrients that they need e. Pictured above is a juniper branch with dormant gall caused by a rust fungus. The leaf on the right is a crab apple showing galls on the underside of the leaves. Often the top of the leaf shows orange spots. Spores produced by the galls infect juniper, completing the life cycle. Depending upon the rust species, the impact on the host can vary from negligible to devastating. Rust diseases are very significant to several crop species including wheat, corn, coffee and white pine, where they can have serious economic impact.
Previous examples of long distance dispersal of rust fungi include spread of a unique race of wheat stem rust from South Africa to Australia, spread of coffee rust from Africa to South America, and spread of southern corn rust from Central America to Africa.
To make matters worse, the Ug99 lineage of the stem rust fungus has expanded its virulence through mutations that allow it to overcome the resistance of at least two other vertical resistance genes that wheat breeders have relied on for protection from stem rust in North America and many other parts of the world. In response to the threat of impending wheat stem rust epidemics around the world, an international effort was organized in to reduce the vulnerability of the world's wheat crops to rust diseases.
Department of Agriculture. Primary efforts are concentrated on developing and deploying new effective resistance to wheat stem rust globally. Vertical resistance must be considered in the short term even though the durability of the resistance may be questionable. Combining two or more effective vertical resistance genes will provide a better chance for longer lasting resistance. For the long term, however, wheat breeders may rely more on minor genes with additive effects of partial resistance expressed primarily in adult plants i.
These advanced lines are being intercrossed to produce improved wheat varieties combining as many as four or five horizontal resistance genes to effectively suppress stem rust epidemics. To preserve these effective combinations of horizontal resistance genes even when the Ug99 epidemics subside, it will be necessary to identify genetic markers for each of the genes so that breeders can continue to select for their presence even in the absence of disease.
Chemical control In some areas where disease pressure is high, fungicides are applied to wheat to control rust diseases. Fungicides that inhibit the synthesis of sterols [i. Potential approaches to management Urediniospores infect wheat only through stomata. Scientists have studied how germinating urediniospores locate stomata on leaf surfaces Figure Although several factors are involved, the germ tube is able to detect the guard cells by their physical dimensions relative to the epidermal cells.
Once a stoma is found, an appressorium is produced and infection begins. In the future, it may be possible to breed wheat resistant that is resistant to urediniospore infection because it has epidermal patterns that are not recognized by the fungus. Stem rust is one of the major diseases of wheat and barley and, therefore, a potential threat to the world food supply.
Wheat is the largest food crop in the world, and barley is the sixth largest. Cereal rusts have probably been a problem since the first cereal crops were grown in the Fertile Crescent. Spores of P. Wheat, barley, and barberry all originated in the Fertile Crescent, so this complex relationship in the stem rust life cycle has an ancient history.
Wheat stem rust was a serious problem in ancient Greece and Rome. Rust was observed and recognized as early as the time of Aristotle B. The ancient Romans sacrificed red animals such as dogs, foxes, and cows to the rust god, Robigo or Robigus, each spring during the festival called the Robigalia in hopes that the wheat crop would be spared from the ravages of the rust Figure This festival was incorporated into the early Christian calendar as St.
Mark's Day or Rogation on April Historical weather records suggest that a series of rainy years, in which rust would have been more severe and wheat harvests reduced, may have contributed to the fall of the Roman Empire. Although the parasitic nature of stem rust was not known until the s, farmers in Europe had recognized much earlier that barberry was somehow connected to stem rust epidemics in wheat.
Laws banning the planting of barberry near wheat fields were first passed in Rouen, France, in The Italian scientists Fontana and Tozzetti independently provided the first detailed descriptions of the stem rust fungus in wheat in Persoon named it Puccinia graminis in By , the Tulasne brothers recognized that some autoecious single host rust fungi could produce as many as five spore stages.
They were the first to link the red urediniospore and black teliospore stages as different spores of the same organism, but the remaining stages of P. Anton deBary was puzzled by the lack of infection when basidiospores of P.
Using the farmers' belief that barberries increased wheat rust, he successfully inoculated barberries with the basidiospores and observed the remaining spore stages develop on the alternate host. Once the heteroecious nature of the life cycle was established, many other known rust fungi were discovered to be heteroecious, and their hosts could be paired up.
Both wheat and barberry plants were brought to North America by the European colonists. Barberry has a number of practical uses including a yellow dye from the bark, jams and wines from the berries, tool handles from the wood, and fast-growing, thorny hedges to help retain animals. As in Europe, farmers began to recognize the connection between barberry and stem rust epidemics in wheat.
Barberry laws were enacted in several New England colonies in the mids. However, barberry continued to spread as pioneer farmers moved west. From farmyard plantings, barberry spread into fencerows and woodlots.
Barberry bushes can be 3 m 9 ft high and produce abundant berries that are attractive to birds and animals that feed on them and spread their seeds. After the devastating North American stem rust epidemic, a cooperative state and federal barberry eradication program was established in Figure This program was partially motivated by the concern about food supplies during war. A "war against barberries" was established that enlisted help from the general population through radio and newspaper ads, extension pamphlets, and booths at fairs urging them to aid in the destruction of barberries.
Even school children were encouraged to help find sites where barberry bushes existed Figure From through , the program was gradually returned to the jurisdictions of various states. A federal quarantine is still maintained against sale of stem rust-susceptible barberry in states that were part of the barberry eradication program.
A barberry testing program was established to ensure that only barberry species and varieties, such as the popular ornamental Japanese barberry, that are immune to stem rust will be grown in the quarantine area.
Cereal Disease Laboratory website, University of Minnesota. Bushnell, W. Roelfs, The Cereal Rusts. Origins, Specificity, Structure, and Physiology. Academic Press, Orlando. Cook, R. Veseth, Wheat Health Management. American Phytopathological Society Press, St.
Dubin, H. Rajaram, Breeding disease-resistant wheats for tropical highlands and lowlands. Annual Review of Phytopathology Leonard, K. Stem rust of small grains and grasses caused by Puccinia graminis. Molecular Plant Pathology Littlefield, L.
Biology of the Plant Rusts: An Introduction. Iowa State University Press, Ames. McIntosh, R. Brown, Anticipatory breeding for resistance to rust diseases in wheat.
Peterson, P. APS Press. Roelfs, A. Effects of barberry eradication on stem rust in the United States. Plant Disease Epidemiology of the cereal rusts in North America.
Canadian Journal of Plant Pathology Bushnell, Diseases, Distribution, Epidemiology, and Control. Singh, and E. Saari, Singh, R. Hodson, J. Huerta-Espino, Y. Jin, P. Njau, R. Wanyera, S. Herrera-Foessel, and R. Go here to the download software webpage 2.
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