COMMON NAME:Stem rust, black rust
SCIENTIFIC NAME: Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. E. Henn.
SYMPTOMS: Uredinia generally appear as oval lesions on leaf sheaths, true stem, and spike. Uredinia can appear on the leaves if other diseases have not killed them. Uredinia are brick red in color and can be seen to rupture the host epidermis, on the leaves uredinia generally penetrate to sporulate on both surfaces. Infected areas are rough to the touch.
ENVIRONMENTAL CONDITIONS: Stem rust is favored by hot days 25-30 C, mild nights 15-20 C with adequate moisture for night time dews. Wind can effectively disperse urediniospores over great distances. Rain is necessary for effective deposition of uredinospore involved in regional spore transport.
INOCULUM SOURCE AND INFECTION:
Urediniospores and aeciospore germinate when in contact with free water. Infection by penetration through the stoma. Penetration requires at least a low light intensity. Germination optimum is 18 C, latent period varies from 10 to 15 days in the field with temperatures of 15-30 C.
SURVIVAL: Stem rust can survive as teliospores during winter when aeciospores are a major source of inoculum. It generally survives as mycelium or uredinia on volunteer wheat during the non-wheat growing season. Uredinospore can be spread by wind into disease-free areas. Sporulating uredinia are active in tropical and some subtropical areas throughout the winter. Occasional dormant mycelium may survive beneath the snow pack in more northern temperate regions.
METHOD OF DISSEMINATION:Urediniospores and aeciospores are wind borne. Teliospores remain with the straw.
A. Uredinia of Puccinia graminis f. sp. tritici B. Scanning electron micrograph (SEM) view of a single uredinium
Telia on wheat plants and Teliospores of Puccinia graminis f. sp. tritici. Note dark color and thick cell walls.
On barberry and other alternate hosts:
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 genes that 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. 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.