Response of an International Triticale Collection to Puccinia triticina and Puccinia recondita sensu stricto and Assessment of Temperature Sensitivity in Leaf Rust Isolates

Triticale is derived from a cross between wheat and rye and the leaf rust pathogen of wheat, Puccinia triticina (Pt), and that of rye, P. recondita sensu stricto (Pr), can potentially cause disease in this crop. Recent studies showed that wheat rust fungi could adapt to warmer temperatures. In this paper, we report on the comparative virulence of three Pt races and one Pr isolate (all were collected in South Africa) on triticale as well as their in vitro response to temperature. Seedling infection types (SITs) of 169 triticale entries to Pt races 3SA144 (North American code SDDN), 3SA145 (CCPS) and 3SA248 (CFPS) and Pr isolate UVPr2 revealed that 3SA144 is the most virulent with 106 triticale entries found susceptible to this race. The three Pt races were avirulent to the four rye cultivars included as controls. UVPr2 was avirulent on all the triticale entries and 49 entries were considered resistant to the Pt races tested. Freshly harvested urediniospores of the above isolates were tested at constant temperature regimes of 10 °C, 22.5 °C and 35 °C to study germination characteristics. Mean urediniospore germination percentages as determined for 3SA144 (61.3%) and UVPr2 (62.6%) were significantly lower when compared to 3SA145 (83.7%) and 3SA248 (84.9%). Race 3SA144 was most sensitive to the higher temperature regime of 35 °C (5.2% germination). Among the investigated races, 3SA144 showed significantly lower mean germ tube elongation rates at all three incubation temperatures. This is the first report of differences in temperature adaptation between Pt races from SA

Isozyme characterisation of Fusarium graminearum isolates associated with head blight of irrigated wheat in South Africa

Head blight, caused by certain species in the genus Fusarium, has become an economically important disease of wheat under centre pivot-irrigation along the Orange River region in South Africa. Isozyme analysis, by means of starch gel electrophoresis, was employed to compare 73 Fusarium isolates, obtained from three farms in this region, with isolates of Fusanum acuminatum, F. chlamydosporum, F. compactum, F. crookwellense, F. culmorum, F. equiseti, F. graminearum Group I and Group II. F. moniliforme, F. oxysporum, F. solani, and F. subglutinans. For all 11 Fusarium species, nine loci were identified in eight enzyme systems and 19 electrophoretic phenotypes (EPs) were detected, each composed of isolates from one species only. The extent of isozyme polymorphisms found among these Fusarium spp. provides a potential method for delineating isolates at species level. Six EPs, of which the most common one comprised 36 isolates, were identified within isolates obtained from the three farms. These isolates clustered together with the F. graminearum Group II reference isolates. Based on perithecial production, their identity was confirmed as F. graminearum Group II

Modification of the Aegilops neglecta - Common wheat Lr62/Yr42 translocation through allosyndetic pairing induction

The Lr62/Yr42 translocation comprises mostly allen chromatin and retains only the very distal end of wheat chromosome arm 6AL, including the telomere. The large amount of foreign chromatin prohibits commercial use of the resistance and an attempt was therefore made to exchange some of the introgressed chromatin for wheat chromatin. Plants heterozygous for the translocation, but lacking the Ph1 locus were testcrossed with Chinese Spring nullisomic 6A tetrasomic 6B or nullisomic 6A tetrasomic 6D plants. Resistant (Lr62) testcross F1 progeny were evaluated for the presence of three marker loci and the data used to do a three-point genetic mapping analysis. Forty one recombinants were identified and characterised with further markers. The data revealed that Lr62/Yr42 occurs towards the distal end of 6AS and that the 6AS telomeres of the wheat and translocation chromosomes were homoeologous. While the wheat (Chinese Spring ph1b mutant) and translocated chromosomes 6A shared sufficient homoeology to allow for regular allosyndetic recombination, there were also major structural differences between them, including a duplication (marked by Xgwm334) and translocation (marked by XsopwT). The duplicated region occurred on chromosome 6A of the CS ph1b mutant and probably resulted from its inherent genomic instability. The structural differences caused irregular meiotic pairing and complex segregation data that were difficult to interpret. It was, however, possible to explain the majority of recombination products in keeping with the expected low frequency of allosyndetic recombination, and to identify the most promising recombinants. These retained both Lr62 and Yr42 within a comparatively small region of foreign DNA at the 6AS telomere.Articl

Transfer of rust resistance genes from Triticum species to common wheat

A programme aiming to transfer leaf rust resistance genes identified in a collection of wild Triticum species was initiated in 1993. In 2000, 25 promising backcross populations were available, 19 of which bred true for resistance. Seedlings of the above lines were tested with nine leaf rust, four stem rust and two stripe rust pathotypes endemic to South Africa. A subset of five lines in which resistance (derived from T. dicoccoides, T. sharonense, T. speltoides and T. peregrinum) appeared to be integrated on wheat chromosomes and six addition lines with added chromosomes from T. kotschyi, T. peregrinum, T. umbellulatum, T. macrochaetum and T. neglectum appeared to have wide spectrum resistances, and were retained. In several instances promising stem rust and/or stripe rust resistance genes were co-transferred with leaf rust resistance. The stripe rust resistance was also effective to four Australian pathotypes and appeared to be novel. Temporary gene designations were assigned to the resistance genes in four euploid derivatives.Articl

Transfer of rust resistance genes from Triticum species to common wheat

A programme aiming to transfer leaf rust resistance genes identified in a collection of wild Triticum species was initiated in 1993. In 2000, 25 promising backcross populations were available, 19 of which bred true for resistance. Seedlings of the above lines were tested with nine leaf rust, four stem rust and two stripe rust pathotypes endemic to South Africa. A subset of five lines in which resistance (derived from T. dicoccoides, T. sharonense, T. speltoides and T. peregrinum) appeared to be integrated on wheat chromosomes and six addition lines with added chromosomes from T. kotschyi, T. peregrinum, T. umbellulatum, T. macrochaetum and T. neglectum appeared to have wide spectrum resistances, and were retained. In several instances promising stem rust and/or stripe rust resistance genes were co-transferred with leaf rust resistance. The stripe rust resistance was also effective to four Australian pathotypes and appeared to be novel. Temporary gene designations were assigned to the resistance genes in four euploid derivatives.Articl