Wheat stem rust (Puccinia graminis f. sp. tritici) is historically the most damaging disease of wheat. Under suitable conditions, yield losses of 70% or more are possible. In 1999, a new virulent race of stem rust was identified from wheat fields in Uganda – popularly known as Ug99 after the year and country of discovery. The unique virulence associated with Ug99, or variants, has rendered a large proportion of global wheat cultivars susceptible.
Regularly updated situation reports on cereal rusts are given based on information provided by a global network of rust workers. It is important to note that not all reports of stem rust relate to Ug99 or variants – other local races are also included.
Results of extensive lab tests of samples of stem rust have shown that the 2016 stem rust epidemics in Sicily were caused by a new, highly virulent variant of race TTTTF. The samples were collected during serious and unusual outbreaks of wheat stem rust on both durum wheat and bread wheat in Sicily during April – June 2016.
The epidemics were estimated to cover several thousands of hectares resulting in high inoculum load that could pose a threat to surrounding wheat areas in the forthcoming 2017 crop season, if environmental conditions prove suitable. Growers in at risk areas should be aware of the possible risk (on both durum and bread wheat), monitor crops for the early appearance of stem rust and undertake timely control if necessary.
Sicily 2016 Outbreak
Unusual, early stem rust infections (at heading stage) were first detected by Dr Biagio Randazzo on April 19th 2016 in experimental plots at Ciminna, Palermo province. High and unusual levels of yellow rust were observed as well. It is unknown if rust was extensively present in surrounding areas prior to this first detection. Low levels of stem rust were observed at Ciminna at the end of the previous season (June 2015) and a mild winter may have contributed to the early infections in 2016. The majority of wheat breeding lines being tested at Ciminna showed high susceptibility to both rust diseases.
Samples of rust infected leaves and stems were sent to the Global Rust Reference Centre (GRRC), Aarhus University, Denmark for race analysis. In the GRRC report: Samples of stem rust infected wheat from Italy, a single race of stem rust was reported among the 16 samples investigated – race TTTTF using the North American nomenclature – but additional virulence on resistance gene Sr13 and others were also identified. This is one of the few races known to be virulent to the combination of resistance genes Sr9e and Sr13, both being common sources of resistance in many durum wheat varieties. Although it should be noted that Sr13 virulence has been previously reported from Turkey. Race TTTTF has complex virulence, but is not related to the Ug99 race group and it is avirulent on genes Sr31, Sr24 and Sr25. Similar races have been detected in nearby regions and appear to be spreading rapidly, isolates from these regions are currently being investigated and compared with isolates from the Sicily epidemic. Yellow rust samples from Sicily revealed three races including a new one with a wide virulence spectrum (see – New races caused epidemics of yellow rust in Europe, East Africa and Central Asia in 2016). Rust samples were also sent to the John Innes Centre, UK for genotyping, and analysis is still on-going. Results on maps and charts are available from the GRRC, Aarhus University and RustTracker, CIMMYT. Continue reading
In 2016 surveys were carried out in all the four key wheat growing regions: South Rift (June, July), Mount Kenya region (July), and North Rift (September) and Central Rift (part of August and September). A total of 304 farms were sampled. Stem rust was detected in 235(78.3%), yellow rust in twenty-eight (9.3%) and leaf rust in fourteen (4.7%) of the farms. The disease severity was ranging from trace to 90S; trace to 60S and trace to 50S for stem rust, yellow rust and leaf rust respectively. Stem and yellow rusts were detected in all the wheat growing regions while leaf was detected in South, North and Central Rift. Stem rust infection ranged from TR to 90S with maximum infection in Central Rift( 88.3%), Mt. Kenya region (80.3%); South Rift(76.5%) and North Rift (72.4%). Yellow rust infection ranged TR to 60S with maximum infection in Central Rift (16.7%); North Rift(13.3 %) and minimum infection in South Rift( 4.9%),) and Mt. Kenya region ( 1.7%). Leaf rust infection ranged from trace to 50S with maximum infection in North Rift (10.2%) minimum infection in Central Rift (3.3 %) and South Rift (1.2%). Continue reading
The latest version of the Mehtaensis newsletter (a 6 monthly newsletter named after Prof. K.C. Mehta) has just been published by the IIWBR, Regional Station, Flowerdale, Shimla. Compiled and edited by Dr. S.C. Bhardwaj, O.P. Gangwar, Pramod Prasad and Hanif Khan with technical assistance from S.B. Singh and Subodh Kumar. Mehtaensis contains a detailed summary of all the rust activities and race analysis results from India and neighbouring South Asian countries during the period July – Dec 2015. The executive summary is reproduced here:
“During offseason 135 samples of three rusts of wheat were received from Himachal Pradesh, Punjab, Tamil Nadu and Uttarakhand. 49 samples of all three rusts of wheat were analyzed, some of which were spill over of main season. Ug99 type of virulence was not identified anywhere in India. In brown rust pathotype77-5 (121R63-1) of brown rust, 46S119 and 110S119 of yellow rust and 40A (62G29) of black rust were found most frequently among the analyzed samples. Nucleus inocula of three rusts were also supplied to 38 Scientists/Research centres to facilitate research work elsewhere in India. More than 2850 lines of wheat and barley were evaluated against the pathotypes of different rusts. The tested material included the breeding lines provided by breeders from various parts of India, exotic wheat lines from CIMMYT and ICARDA. In addition 150 lines of AVT I and II are being evaluated against different pathotypes to identify rust resistant wheat lines. For monitoring the occurrence/spread of different diseases of wheat, Wheat Disease Monitoring Nursery (WDMN) and SAARC-WDMN were planted at 50 and 28 locations, respectively. Early occurrence of yellow rust was reported from Ropar, Anandpur Sahib and Hoshiarpur district of Punjab. Three Ug99 resistant genetic stocks namely FLW31, FLW32 and FLW33 have been developed. FLW31 and FLW33 are completely resistant against black and brown rusts while FLW32 has resistance to black rust and adult plant resistance against yellow and brown rusts. Work on standardization of doubled haploid production in wheat using maize pollination induced chromosome elimination was initiated. Offseason nursery was used for selection and generation advancement of 325 wheat lines. Under frontline demonstration of wheat variety, HS542 was planted in five adopted villages (12 farmers) of Tehsil Arki (Solan district of Himachal Pradesh).”
Analysis of dead, single pustule stem rust pathogen samples (D-samples) using molecular diagnostic SNP assay at the USDA-ARS Cereals Disease Lab, Minnesota has detected a race TKTTF (“Digalu” race) genotype in Kenya for the first time. Five samples from 2014 and 7 samples from 2015, all collected by Ruth Wanyera and the pathology team from KALRO, Njoro, tested positive for race TKTTF genotype. These results indicated that a single genotype (clade IV-B) was present in Kenya. Clade IV-B is the predominant genotype in Ethiopia.
The D-sample results indicate that race TKTTF is distributed (probably at low frequency) throughout the major wheat growing regions of Kenya. The 2014 positive samples were collected from North Rift (n=2), Central Rift (n=1) and Mount Kenya (n=2). In 2015, positive samples were collected from South Rift (n=1), Central Rift (n=1) and Mount Kenya (n=5). Most of the TKTTF genotype positive samples were collected from the cultivar ‘Robin’ (n=9), but ‘KS Mwamba’ (n=1), ‘Kwale’ (n=1) and an unknown barley variety (n=1) also produced positive results for TKTTF genotype.
At present no race analysis studies on live samples collected in Kenya have detected the presence of race TKTTF. Testing of the SNP assay against known isolates of TKTTF and negative controls has proven 100% reliable, but until there is confirmation by race analysis the D-sample results are considered indicative.
Race TKTTF now totally dominates the stem rust pathogen population in all the wheat growing regions of neighbouring Ethiopia, so its presence in Kenya is not unexpected.
Combined results from race analysis undertaken at several different labs indicate that five new variants in the Ug99 lineage were detected from samples collected in East Africa during 2013 or 2014. Probably most significant were the two new SrTmp variants (Races TTKTK and TTKTT) previously reported (see “16 April 2015: BGRI report two new Ug99 variants with virulence to SrTmp detected in Kenya” and Patpour et al. 2015). Further analysis has revealed the presence of three additional new Ug99 lineage races. Details of these additional new races are:
- Race TTHST: Similar to race TTKST (Ug99 + Sr24 vir) but avirulent on Detected in Kenya in 2013 (1 isolate). Source lab: USDA-ARS Cereals Disease Lab, USA (Newcomb et al in prep)
- Race TTHSK: Similar to race TTKSK (Ug99) but avirulent on Detected in Kenya in 2014 (2 isolates). Source lab: AAFC Morden Research Centre, Canada (Fetch et al 2016)
- Race PTKTK: Similar to race PTKSK (Ug99 but avir on Sr21) but virulent on SrTmp. Detected in Kenya in 2014 (5 isolates). Source lab: AAFC Morden Research Centre, Canada (Fetch et al 2016)
These latest results bring the total of known variants within the Ug99 lineage to 13, with yet another SrTmp variant being detected in Kenya. All five of the new variants were detected in Kenya, but as previously reported race TTKTK was also detected in Uganda, Rwanda, Eritrea and Egypt in 2014. The results indicate that the Ug99 race group continues to evolve at a rapid rate.