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Disease Management

Barberry eradication:
Once the life cycle of P. graminis was determined, the potential effects of the removal of the barberry alternate host became clear (Figures 20, 21). An expensive and extensive barberry survey (Figure 22) and eradication program was initiated in 1918 in the U.S. (Figure 23) and continues to a limited extent today (Figure 24).


Figure 20	Figure 21	Figure 22

Figure 23	Figure 24	Figure 25

It was originally hoped that the program would eliminate stem rust as a significant disease in North America, because the basidiospores would have no barberry hosts to infect, and urediniospores could not usually survive harsh winter conditions. The importance of continental spread of stem rust epidemics was not understood until later. Urediniospores overwinter in wheat fields in the southern U.S. and northern Mexico and are then airborne northward via what is now called the "Puccinia Pathway" (Figure 25). If the weather is favorable for stem rust development in the South, urediniospores will arrive in time and in sufficient numbers to cause epidemics in northern wheat-growing areas.

Despite this problem, barberry eradication has had significant positive effects on the control of stem rust epidemics. First, it removed a significant, early source of inoculum. A single barberry plant can produce as many as 64 billion aeciospores. Second, it reduced the genetic variation in the fungal population by eliminating the sexual cycle, leaving only asexual urediniospores to maintain the fungus. Mutation is now the primary source of genetic variation. Consequently, there are no longer so many different races of wheat stem rust against which wheat breeders must seek resistance. Finally, epidemics are delayed by several weeks in many of the major wheat producing areas of the U.S. and Canada because aeciospores were released before the first arrival of urediniospores from the south.

Cultural practices
It has long been known that moisture on leaves and excessive foliar nitrogen favor infections by rust fungi. Farmers consider these factors in spacing, row orientation, and fertilizer schedules. Recent changes in production practices may have effects on stem rust. In some areas, summer wheat crops are irrigated, which may increase the survival of infected volunteer plants. In addition, many farmers are practicing no-till or minimum tillage. This increases the probability that rust fungi may successfully overwinter in the protective layer of stubble from the previous crop.

Use of earlier-maturing wheat varieties in the central Great Plains of the U.S. has helped reduce the threat of stem rust epidemics. Modern wheat varieties in that region mature about 2 weeks earlier than older varieties. This limits the length of time for stem rust epidemics to develop in the central Great Plains as well as the numbers of urediniospores that can contribute to epidemics farther north.

Genetic resistance
Genetic resistance is the most commonly used and the most effective means to control stem rust. Its success is directly linked to the reduced number of races present in the fungal population following the barberry eradication program. Because funding for the program has been reduced in recent years, scientists fear that the remaining barberry bushes will continue to spread into wheat-growing areas to serve both as a source of inoculum and as a means by which the fungus can complete its sexual cycle. The currently used resistance genes should not be expected to remain effective as new races of the fungus begin to appear.

Even without the presence of alternate hosts, the fungus is capable of overcoming resistance genes, primarily through mutation. For this reason, plant pathologists monitor the race populations each year and advise wheat breeders about which resistance genes will best protect the wheat crop in various areas. Wheat breeders use a combination of vertical resistance genes against specific races of P. graminis and horizontal resistance genesthat slow the development of the epidemic by offering some resistance to all pathogen races.

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.e., sterol biosynthesis inhibitors (SBIs) or demethylation inhibitors (DMIs)] are particularly effective, but the cost of application is generally prohibitive for routine use in most wheat-growing areas in the U.S.

Potential approaches to management
Urediniospores infect wheat only through stomata. Scientists have studied how germinating urediniospores locate stomata on leaf surfaces (Figure 26). 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.

Figure 26

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