Latest Survey Data
- Rust Issues: Stem and leaf rust are serious constraints to wheat production.
- Ug99 Status: Three variants of Ug99 have been detected in Zimbabwe. Race TTKSF (avirulent on Sr31) was first detected in 2009, race PTKST (virulent on both Sr31 and Sr24) was first detected in 2010 and race TTKSF+ (avirulent on Sr31) detected on samples collected in 2010.
Situation Updates:
Credit: Bruce Mutari
Following the widespread distribution of rusts diseases observed in 2011 main season, a survey team co-led by Mr Bruce Mutari (Crop Breeding Institute) and Mrs Mutisi (Plant Protection Research Institute) carried out surveys throughout the key wheat growing areas of Zimbabwe during the period 10th – 16th September 2012. The primary focus of the surveys was to assess the status of wheat stem rust in Zimbabwe compared to the situations in 2009, 2010, 2011 and to monitor the potential spread of virulent races of wheat stem rust (Ug99 race lineage) in Zimbabwe and Southern Africa as a whole. The majority of the crops observed were at dough to maturity, with some crops already harvested. In the survey area, wheat was grown at elevations ranging from 422m to 1605m. A total of 22 wheat fields, distributed throughout the main agro-ecological zones, were surveyed using standard BGRI survey methodology. The highveld region contained the highest density of wheat fields. Surprisingly, wheat stem rust was not widespread and only positively recorded at 1 (4.5%) of the 22 survey sites; Rattray Arnold Research Station, in the middleveld region. Whilst not at the same level of disease pressure as 2010 and 2011, leaf rust was the most widespread disease as it was recorded at 9 (40.9%) of the 22 survey sites (Nyanga 1, Nyanga 2, Sisal farm, Save Valley, Birchnough Bridge, Panmure Research Station, Gwebi Variety Testing Center, Rattray Arnold Research Station and ARTfarm. For the first time in four years, stem rust was not recorded in the Lowveld. At Rattray Arnold Research Station, high levels of stem rust infections were observed in a single experimental line which exhibited highly susceptible reactions. At Birchnough Bridge, high incidence and severity of leaf rust was observed in an irrigation scheme with SC Nduna, Kana and SC Sekuru (improved varieties) showing highly susceptible reactions. Low incidence and very high severity of leaf rust was recorded at GVTC in a Kana breeder’s seed field. Trace amounts of leaf rust were recorded at Agricultural Research Trust farm in contrast to last year where very high incidence and severity was recorded at the same station. In addition to one stem rust sample, nine leaf rust samples were collected and sent to Prof.Z.Pretorius, University of Free State, RSA for race analysis. One stem rust sample was sent to Dr.L. Szabo, Cereal Disease Laboratory (CDL), U.S.A for DNA – molecular diagnostic analysis. As compared to the previous seasons, powdery mildew was much more widespread as it was regularly recorded at survey sites. No stripe rust was observed at any of the survey sites visited.
Results from the DNA analysis indicated that the stem rust race present was TTKSF, although it must be noted that the CDL molecular diagnostic assay used is currently unable to differentiate between races TTKSF and TTKSF+. Conventional race analysis at the University of the Free State confirmed that the stem rust race was TTKSF.
The difference in the incidence and severity of stem and leaf rust between 2010, 2011 and 2012 may be the result of the unfavorable environmental conditions that prevailed during the 2012 season. The 2012 wheat cropping season was characterized by long periods of dry conditions and cool temperatures which are not favorable for rust development. The decline in wheat hecterage in Zimbabwe (from 10 000ha to 5 000ha) might also have resulted in this decrease in rust incidence because of the reduction in suitable hosts. In addition, for the past two seasons, most farmers in Zimbabwe have been adhering to the recommended planting dates which is very important for rust escape. By planting early, the crop escapes stem rust infection since it is common/develops late in the season when temperatures begin to rise.
The breakdown of leaf rust resistance on improved cultivars, SC Sekuru, SC Nduna, Kana and Insiza (highly susceptible reactions) may indicate the presence of new biotypes of leaf rust. However further laboratory analysis needs to be undertaken to confirm if this is the case.
Photo Credit: ZA Pretorius
Stem rust samples collected from Afrikaskop, Eastern Free State, South Africa in Dec. 2010 and Birchenough, Eastern Zimbabwe in Sept. 2010 have been confirmed to be a new variant in the Ug99 lineage. The confirmed findings have just been published in Plant Disease by Pretorius et al. (2012). The new variant has been designated as race TTKSF+, this is the eighth race detected in the Ug99 group. The new race was collected from the winter wheat cv. Matlabas in South Africa. This was the first time this cultivar had been observed to be infected with stem rust. Microsatellite analysis showed that all single pustule isolates established from the original Matlabas isolate formed part of the Ug99 group. Race analysis using the 20 North American (NA) stem rust differential lines and letter code system classified the new race as TTKSF. However, cv. Matlabas is resistant to race TTKSF and characterization with selected single nucleotide polymorphisms (SNPs), revealed that all single pustule isolates of the Matlabas race shared an identical genotype that differed from race TTKSF. The Zimbabwe isolate was identical and was also classified as race TTKSF+. Studies are under way to determine the identity of the defeated Sr gene in Matlabas. Since no other cultivars or advanced lines were found to carry the Matlabas gene, it is unlikely that race TTKSF+ will threaten wheat production in South Africa. However, the occurrence of a new Ug99-related race emphasizes the variability within this internationally important group.
[Photo Legend: Left to right: race TTKSF+ on Matlabas; race TTKSF on Matlabas; race TTKSF (original isolate) on Matlabas, Susceptible Control]
Photo Credit: B. Mutari
Wheat rust surveys were undertaken by the Crop Breeding Institute and Plant Protection Research Institute throughout the key wheat growing areas of Zimbabwe during the period 19th – 24th September 2011. The primary focus of the surveys was to assess the status of wheat stem rust in Zimbabwe compared to the situations in 2009 and 2010 and to monitor the potential spread of new virulent races of wheat stem rust (Ug99 race lineage) in Zimbabwe and Southern Africa as a whole. In the survey area, wheat was grown at elevations ranging from 413m to 1535m. A total of 20 wheat fields, distributed throughout the main agro-ecological zones, were surveyed using standard BGRI survey methodology. Wheat stem rust was recorded at 6 of the 20 survey sites – Gwebi Variety Testing Center (GVTC), Chiredzi Research Station, Chisumbanje Research Station, Birchenough*2, and Sisal Farm Mutare. For the third year running, stem rust was widespread in the Lowveld, with 4 out of 6 Lowveld sites below 800m recording the disease. High or moderate stem rust severity (>20%) was recorded at 3 of these Lowveld sites. At Chiredzi, high levels of stem rust infections were observed in Preliminary Variety Trials with SC Sekuru (improved variety) showing highly susceptible reactions together with three other experimental lines. At GVTC, low incidence of stem rust was observed in Intermediate Variety Trials with Dande (improved variety) and four other experimental lines showing highly susceptible reactions. Low incidence and very high severity of stem rust was recorded at Sisal farm in a CIMMYT rust resistance screening nursery. Trace amounts of stem rust were recorded at Chisumbanje Research Station in contrast to last year where very high incidence and severity was recorded at the same station. Stem rust samples were collected and sent for molecular diagnostic analysis and race analysis.. In addition to stem rust, leaf rust was recorded at 13 of the 20 sites surveyed. Severe leaf rust infections were observed at 4 of the survey sites. Powdery mildew was also regularly recorded at survey sites. No stripe rust was observed at any survey location.
Survey updates – Southern Africa
Mozambique: Wheat rust surveys were undertaken by IIAM, Mozambique and FAO in the key wheat growing areas of Manica and Tete provinces, Central Mozambique during the period 26th August – 1st September 2010. Wheat in Mozambique is grown in dispersed, small plots typically in remote highland areas. The Tsangano region bordering Malawi contained the highest density of fields. A total of 24 wheat fields were surveyed using standardized methodology. Wheat stem rust was not widespread and only positively recorded at one site; Rotanda, in Sussendenga district. Here a tall local wheat variety (Sitsonko) showed a highly susceptible reaction. It was noteworthy that virtually all farmers in Mozambique were growing the same variety; SC Nduna.
Zimbabwe: Wheat rust surveys were undertaken by the Crop Breeding Inst, SeedCo and FAO in the key wheat growing areas of Zimbabwe during the period 4th-9th September 2010. A total of 27 wheat fields, distributed throughout the main agro-ecological zones, were surveyed using standardized methodology. Wheat stem rust was recorded at 12 of the 27 survey sites, being much more prevalent and widespread than in the previous 2009 season. Stem rust was widespread in the Lowveld, with 8 out of 9 Lowveld sites (elevation below 800m) recording the disease. High or moderate stem rust severity (>20%) was recorded at 6 of these Lowveld sites. Highly susceptible reactions were observed on improved varieties such as SC Stallion. Very high incidence and severity of stem rust was recorded at Chisumbanje Research Station (Photo). Trace amounts of stem rust were also recorded at 4 highveld sites (Gwebi VTC, Rattray Arnold, Kadoma, Nyanga). Stem rust samples were collected at all sites where the disease was present. Until completion of the laboratory analysis the actual identity of stem rust pathotypes present in Zimbabwe is unknown. Given the recent (2009) confirmation of Ug99 lineage race PTKST (combined virulence to both Sr31 and Sr24) in South Africa, identification of the Zimbabwe pathotypes is a high priority.
Stem rust samples collected near Harare in Sept 2009 have been confirmed as race TTKSF, the presumed progenitor of Ug99. Analysis undertaken by the University of the Free State, South Africa and the USDA-ARS, Cereals Disease Lab, Minnesota, USA confirmed the presence of this race. TTKSF is similar to Ug99 except for avirulence on Sr31. TTKSF had only previously been known from South Africa, where it has become the most prevalent stem rust race since its detection in 2000.