Studies on chemical control of wheat stem rust

[摘要] English: Stem rust caused by Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. & E. Henn., is an economically important disease of bread wheat (Triticum aestivum L.) worldwide. The disease has often reached epidemic proportions in South Africa. The release of rust-susceptible cultivars in South Africa is of great concern and therefore research is needed to determine if chemical control is an economically viable method to control rust diseases. The recent increase of stem rust in the Western Cape production area of South Africa has led to a renewed interest in the use of fungicides.In addition to chemical control, effective genetic control of rust diseases requires a coordinated effort, including pathotype monitoring, collection and characterization of sources of resistance, and resistance breeding. Data generated by pathotype surveys thus form an essential component of breeding programmes and are conducted by most wheat producing countries, principally to recognize pathogenic changes. The timely detection of stem rust pathotypes with new virulence is considered important to the South African wheat industry. A stem rust survey is conducted annually to monitor pathogenic variability in Puccinia graminis f. sp. tritici in the major bread wheat and triticale producing areas of South Africa. More than 70% of the isolates obtained from 2002 to 2004 were characterized as pathotype 2SA88, which unusual for local pathotypes, was virulent for Sr8b and Sr38. During the six year survey period, stem rust pathotypes 2SA4, 2SA55, 2SA88, 2SA99 and 2SA100 were detected on bread wheat and 2SA102, 2SA102K and 2SA103 on triticale.To optimize in vitro testing conditions the effect of temperature on germination and germ tube growth of P. graminis f. sp. tritici as well as incubation periods were evaluated. Seventy eight percent of urediniospores had germinated on water agar in petri dishes after 6 h of incubation at 20 and 25°C. Low germination rates were observed at 30°C. Twenty-nine fungicides in nine chemical classes were evaluated in vitro for toxicity to stem rust urediniospores. Water agar petri dish plates were amended with fungicides at concentrations ranging from 10 to 0.0001 μl active ingredient / ml medium. Low EC50 (effective concentration that results in 50% inhibition) values were obtained with azoxystrobin, trifloxystrobin, kresoxim-methyl, mancozeb, azoxystrobin/ difenoconazole, iprodione, chlorothalonil and hexoconazole.A shorter residual effect of trifloxystrobin occurred in wheat infected with stem rust in a glasshouse assay, whereas longer efficacy periods were determined for azoxystrobin/difenoconazole and tebuconazole.Fungicide field trials were conducted at Greytown, South Africa to determine the effect of rust diseases on yield and test weight. Information on the efficacy of different foliar treatments in controlling these diseases was obtained. Although the emphasis was on stem rust, it was not possible to eliminate leaf rust from the field trials. In 2005, 92% of the yield reduction experienced was contributed by leaf rust infection whereas in 2006 32% of losses could be attributed to this disease. Stem rust on the other hand contributed 8% to losses in 2005 and 68% in 2006. Yield increase resulting from fungicide applications ranged from 36% to 45% over the two years. With a flag leaf treatment the mean yield increase per plant was 21% compared to 43% with a multiple-spray treatment in 2005. The highest yield increase was obtained with tebuconazole resulting in a 58% gain. Test weight increased by 29% with fungicide applications over the control plots. Further research is needed to optimize control by means of fungicides.

[发布机构] University of the Free State