stem rust

Short season, high latitude spring wheat is grown on 7 million ha in Western Siberia and 10 million ha in Northern Kazakhstan. Despite relatively low wheat yields (1.5 t/ha), the region is extremely important for regional and global food security. Leaf rust dominates, occurring three years out of five, especially in favorable years with higher rainfall. Since 2010, stem rust has been observed at an increasing number of sites. The first large-scale stem rust outbreak occurred in 2015 and affected about 0.5-1 million ha in Omsk, Western Siberia. In 2016, 2 million ha were affected in the Omsk and Altay regions, while 1 million ha in the Kostanay and Northern Kazakhstan regions were affected in 2017. Estimated yield losses reached 25-35% each year. Factors associated with the outbreaks included: higher rainfall in late June and July; cultivation of susceptible varieties; and an increased area planted to winter wheat, which serves as a source of inoculum. Sampling and race analysis revealed a diverse pathogen population, indicative of a sexual recombination. A total of 51 races were identified from 31 samples taken in 2015 and 2016. All races were avirulent on Sr31. The majority of varieties released and cultivated in the region are susceptible to stem rust and require replacing. A recent study of 150 local resistant varieties and breeding lines indicated that the genetic basis of resistance was limited to Sr25, Sr31, Sr36, Sr6Ai, Sr6Ai#2, and additional unknown major genes. Adult-plant resistance to stem rust was observed in less than 20% of the germplasm. The potential impact of these large stem rust outbreaks on other wheat growing regions is being investigated by analyzing spore wind dispersal patterns. Further research is required to understand and mitigate the sudden appearance of stem rust as a disease of economic importance.
Study at Omsk State Agrarian University was supported by the Russian Science Foundation (project No. 16-16-10005).

Emergence of Pgt race Ug99 and rapid proliferation of lineal highly virulent races imminently threaten Kenyan wheat. Devastating epidemics have led to huge losses among smallholder farmers who invariably are unable to spray appropriately and in situations where susceptible varieties are grown. To combat stem rust, the Kenya wheat improvement program seeks to release high yielding stable genotypes with suitable levels of disease resistance. Moreover, detection of genotypes that are adapted to rain-fed environments is an overarching objective. Six hundred and seventeen genotypes from various CIMMYT nurseries (PCBW, EPCBW, PCHPLUS, and 9th SRRSN) were selected based on plant type and reaction to stem rust at Njoro. The reconstituted nursery-KSRON, was sown in the main season of 2016 at Njoro and Timau for further evaluation. Forty red grained lines depicting R-MR infection types, severity of 30% or less, and average Thousand Kernel Weight of >40g were then selected to constitute a yield trial. At each of eight diverse environments, trials also comprising four commercial varieties as checks, were designed in RCBD, three replicates laid out in contiguous array of 8 rows x 10 m plots. Genotype (G), Environment (E) and GE interactions effects were estimated by fitting the AMMI model to yield data, supported by a biplot visualization of the results. Analysis revealed significant (P ?0.01) genotype (G), environment (E), and GE interactions. The first three principal components (PC) explained ~78% of the observed variation. Environment was the predominant source contributing over 85% to total sum of squares. The biplot pointed to at least four environments that were highly correlated. By classifying genotypes based on Shukla's stability variance and Kang's stability rating, six genotypes (R1402, R1411, R1424, R1481, R1484, and R1486) were deemed high yielding and stable, and thus suitable candidates for further testing through the release pipeline.

Stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is a threat to wheat production worldwide. To manage this important disease, new sources of genetic resistance are needed and common wheat landraces are a potential source of such resistance. Landrace accessions from the USDA-ARS National Small Grains Collection were evaluated for seedling resistance to the Ug99 race group. To identify accessions most likely to carry novel resistance genes, a bulked segregant analysis (BSA) approach was used. Seven resistant accessions were crossed to a susceptible parent line and F3 families were tested against Pgt race TTKSK. The resistant plants were identified and grouped into two bulks per population. The bulks, along with the parents and F1 progeny, were genotyped with the 90K wheat iSelect SNP genotyping platform. Four of the populations appeared to segregate in a 1:1 phenotypic resistant/susceptible ratio, one in a 1:2 ratio, and two in 1:3 ratios. However, chi squared tests indicated the ratios were statistically the best fit for only two of the 1:1 segregating populations and one of the 1:3 segregating populations. Initial BSA results indicate the markers associated with reduced stem rust infection are located on wheat chromosomes 1DL and 2B. These mapping populations are being advanced for further evaluation to ascertain if novel resistance to the Ug99 stem rust race group is present.

In recent years, wheat stem rust, caused by Puccinia graminis f.sp. tritici, has been reconsidered in Iran due to its prevalence and the emergence of the dangerous Ug99 race. This study was conducted to understand pathogenic variation in the population of P. graminis f.sp. tritici, detection of effective genes, and identification of resistance in Iranian commercial wheat cultivars or advanced lines, by planting stem rust trap nurseries under natural disease infection in several regions of Iran during the 2016-2017 cropping season. The trap nursery in each location included 48 wheat lines each carrying a single gene of stem rust (Sr) resistance, seven lines each carrying Sr multigenes, eight additional lines to confirm four Sr genes, 149 commercial wheat cultivars or advanced lines from Iran, plus several susceptible checks. The percentage leaf area affected (disease severity) and infection type were recorded at adult plant stage when disease was well developed on flag leaves of susceptible checks. Results showed presence of virulence for several Sr genes in one or more locations. However, the single genes of Sr13, Sr23, Sr24, and two complex genes of Sr7a+Sr6+Sr12 and Sr6+Sr24+Sr36+Sr1RS-Am were still effective against stem rust in all locations. The results of evaluations of commercial wheat cultivars or advanced lines showed that approximately 16% the genotypes tested including wheat cultivars Gonbad, Shiroudi, Chamran-2, Baharan, Dena, Karkheh, and Arya were resistant in all locations.