Genetic differentiation within the Puccinia triticina population in South America and comparison with the North American population suggests common ancestry and intercontinental migration.

Leaf rust, caused by Puccinia triticina, is the most prevalent and widespread disease of wheat in South America. The objective of this study was to determine whether genetically differentiated groups of P. triticina are currently present in South America and to compare the South American population with the previously characterized population in North America. In total, 130 isolates of P. triticina from the wheat-growing regions of Argentina, Brazil, Chile, Peru, and Uruguay, mostly from the 1990s to 2008, were tested for virulence on 20 lines of wheat with single genes for leaf rust resistance and for molecular genotypes with 23 simple-sequence repeat (SSR) markers. After removal of isolates with identical virulence and SSR genotypes, 99 isolates were included for further analysis. Principal coordinate analysis plots indicated five different groups of isolates based on SSR genotypes that also differed for virulence to leaf rust resistance genes. All pairs of groups, except for one pair, were significantly differentiated for SSR genotypes according to R(ST) statistics. All but two pairs of groups were significantly differentiated for virulence phenotype according to Phi(PT) statistics. Isolates in all five groups had high values of fixation index for SSR alleles and linkage disequilibrium was high across all isolates that indicated the clonal reproduction of urediniospores. Only one of the five P. triticina groups from South America was differentiated for SSR genotypes from all of the six P. triticina groups from North America. The high degree of similarity for SSR genotype of isolates from both South America and North America suggested a common European origin of P. triticina that was introduced to both continents. The emergence of the same P. triticina virulence phenotypes with highly related SSR genotypes in the United States in 1996 and in Uruguay in 1999 indicated the likely intercontinental migration of these genotypes from Mexico to both South America and North America.

First Report of Spot Type of Barley Net Blotch Caused by Pyrenophora teres f. sp. maculata in Uruguay.

In September 2003, leaves exhibiting spot-type lesions similar to those produced by Cochliobolus sativus Drechs. ex Dastur were widely observed in six commercial barley crops of cvs. Norteña Daymán, Norteña Carumbé, and MUSA 936 in Soriano and Río Negro provinces, the main barley production region in western Uruguay. Spot lesions were tan to dark brown, circular to elliptical, and 3 to 10 mm in diameter. Larger lesions were surrounded by a chlorotic margin of varying width. Affected leaf pieces (10 to 15) from each field were placed in a moist chamber for 2 days to promote sporulation. A fungus identified morphologically as Pyrenophora teres (Died.) Drechs. (1) was consistently isolated from infected leaves. However, symptoms did not correspond to the net-type lesions of net blotch commonly produced by P. teres f. sp. teres in Uruguay. Three monoconidial cultures were obtained by transferring single conidia to potato dextrose agar and then to 10% V8 juice agar and incubated at 20 to 22°C with a 12-h photoperiod for 10 days. Adding sterile water to each plate and gently rubbing the surface with a microscope slide prepared inoculum for pathogenicity tests. Conidia concentration was adjusted to 1 × 104 conidia per ml. Sixty-eight barley genotypes from Uruguay, ICARDA/CIMMYT, and North Dakota were grown in the greenhouse for 2 weeks at 20 to 22°C with a 14-h photoperiod. For each monoconidial isolate, three seedlings of each genotype were inoculated at the three-leaf stage 15 to 16 days after seeding with 0.4 ml of the inoculum suspension with an airbrush inoculator. A drop of Tween 20 was added per 40 ml of inoculum suspension. One set of each genotype was inoculated with sterile water as a control. Seedlings were placed in a dew chamber at 20°C and 100% relative humidity in the dark for 24 h and then returned to prior conditions. The first lesions developed after 7 to 9 days. Leaves two and three of the plants were visually rated for disease (3) 13 days after inoculation. Control plants were disease free. The most susceptible reactions were observed on cvs. Norteña Daymán, MUSA 936, and line CLE 230 (Uruguay). Symptoms were similar in shape and size to those observed in the fields. The most resistant infection types were observed on several Uruguayan and North Dakota advanced lines. The fungus was consistently reisolated from inoculated plants. On the basis of morphology and symptoms produced, the pathogen was identified (2) as P. teres. f. sp. maculata Smedeg. To our knowledge, this is the first report of this fungus causing spot-like symptoms of net blotch in Uruguay. References: (1) M. B. Ellis. Dematiaceous hyphomycetes, CABI, Oxon, UK, 1971. (2) V. Smedergaard-Petersen. Pages 124-144 in: R. Vet. Agr. Univ. Yearbook, Copenhagen, 1971. (3) A. Tekauz. Can. J. Plant Pathol.7:181, 1985.

Effect of gene Lr34 in the enhancement of resistance to leaf rust of wheat.

Leaf rust resistance gene Lr34 is present in many wheat cultivars throughout the world that have shown durable resistance to leaf rust. Fourteen pair-wise combinations of Lr34 and seedling leaf rust resistance genes were developed by intercrossing near isogenic 'Thatcher' lines. In both seedling and adult plant tests homozygous paired combinations of specific resistance genes with Lr34 had enhanced resistance relative to either parent to different numbers of isolates that were avirulent to the additional resistance genes. The TcLr34, 18 line also expressed enhanced resistance to specific isolates virulent to Lr18 in seedling and adult plant stages. In rust nursery tests, homozygous lines were more resistant than either parent, if the additional leaf rust gene conditioned an effective of resistance when present singly. The ability of Lr34 to interact with other genes conditioning effective resistance may contribute to the durability of leaf rust resistance in cultivars with Lr34.