Control of foliar rusts of wheat in South Africa with special emphasis on Puccinia striiformis f. sp.Tritici

English: Stripe rust, caused by Puccinia striiformis Westend. f. sp. tritici Eriks., has become an endemic disease of wheat (Triticum aestivum L.) in South Africa after being observed for the first time near Moorreesburg, Western Cape, during August 1996. Pathotypes (pts.) detected in surveys were 6E16A- with virulence to Yr2,6,7,8,11,14,17 and Yr19, and 6E22A- with added virulence to Yr25. Stripe rust isolates found on Hordeum murinum L. in the Western Cape were identified as pt. 6E 16A- whereas both pts. 6E 16A- and 6E22A- were collected from Bromus catharticus Vahl in the eastern Free Sate. The possible role that grass species may play in the over-summering of the stripe rust pathogen has not yet been fully established. However, stripe rust infections have been found on summer-sown wheat in the south Western Cape during 1998, volunteer wheat growing in the summer and autumn months in the eastern Free State from 1998 to 2000, and on summer-sown wheat in Lesotho. The reaction of 55 South African and 18 foreign wheat cultivars was determined to pts. 6E16A- and 6E22A- in both the seedling and adult plant stage. The occurrence of stripe rust head infections was studied using 16 spring wheat cultivars and 17 supplemental lines. Six of the 55 local wheat cultivars expressed seedling resistance, 18 appeared heterogeneous and 31 were susceptible. The mean area under the disease progress curve (AUDPC) determined in the field for 42 cultivars over a three year period showed that 11 cultivars expressed high levels of complete or adult plant resistance (AUDPC <200). Twelve cultivars expressed intermediate levels of resistance (AUDPC 200 to 500) and 19 displayed AUDPC values of 500 to 1598. The percentage head infection was positively correlated to stripe rust severity on flag leaves. Of the 18 foreign cultivars evaluated 10 were resistant in both seedling and adult plant stages. The remaining eight cultivars were susceptible as seedlings but showed high levels of adult plant resistance in the field. Field trials were conducted from 1997 to 1999 to determine the effect of stripe and leaf rust (P. triticina Eriks.) epidemics on yield and quality of wheat. Five triazole fungicides, applied at two growth stages on three cultivars in the south Western Cape during 1997, resulted in a mean decrease of 31% in the AUDPC calculated for stripe rust infection. The application of fungicides closely to, or just after head emergence, resulted in a 65 to 74% decrease in the occurrence of stripe rust head infections. In contrast, head infection was reduced by only 8% when fungicides were applied at the seven leaf stage. Combined seven and flag leaf treatments with propiconazole, averaged over the three cultivars, resulted in a 56% yield increase, followed by increases of 49%, 44%, 39% and 25% with tebuconazole, flutriafol, bromuconazole, and flusllazole, respectively. In the absence of disease during 1998 no fungicide treatment resulted in a significant yield or hectolitre mass increase in any of the trials. The application of eight fungicides at both seven and flag leaf stages at Langgewens in the Western Cape during 1999, resulted in a mean decrease of 65% in the AUDPC, calculated for leaf rust severity. Combined seven and flag leaf treatments resulted in a mean yield increase of 56%, followed by 50 and 15% for the flag leaf and seven leaf treatments, respectively. Over treatments applied, yield increases varied from 24% for bromuconazole to 53% for epoxiconazole/carbendazim. Furthermore, the application of a flag leaf, and combined seven and flag leaf treatments, resulted in a significant increase in hectolitre mass. During 1999 the combination of triticonazole seed treatment with a propiconazole flag leaf spray on the cultivar Gariep in the eastern Free State resulted in a 91% decrease in the stripe rust AUDPC, and an associated 36% yield increase. Hectolitre mass increased by 3% and protein content decreased by 4% for the latter treatment. Triticonazole seed treatment had a 54% decrease in the AUDPC resulting in 16 and 2% yield and hectolitre mass increase, respectively. The best control of stripe rust was obtained with a combined seven and flag leaf treatment with propiconazole, as well as triticonazole seed treatment combined with a seven and flag leaf treatment of propiconazole. The latter two treatments resulted in a 49% yield increase. The results obtained in yield loss studies emphasise the importance of research aimed at the genetic control of rust diseases of wheat in South Africa. Effective and longlasting genetic control can only be obtained by coordinating future research, continuous monitoring of changes in the pathotype population, the regular collection of germ plasm carrying new or unused sources of resistance, characterising current sources of resistance, and by deploying available sources of resistance in a responsible manner. The result of successful genetic control is not only aimed at preventing the repeated application of fungicides, but also at reducing risk in wheat production, thereby ensuring more stable yields of high quality.