Amarilis Labes Barcellos

The situation of common wheat rusts in the Southern Cone of America and perspectives for control

Approximately 9 million ha of wheat (Triticum aestivum and T. durum) is sown in the Southern Cone of America (Argentina, Brazil, Chile, Paraguay, and Uruguay). Two rust epidemiological zones separated by the Andean mountain range have been described in the region. Presently, leaf rust (caused by Puccinia triticina) is the most important rust disease of wheat. The utilisation of susceptible or moderately susceptible cultivars in a high proportion of the wheat area allows the pathogen to oversummer across large areas, resulting in early onset of the epidemics. Severe epidemics cause important economic losses if chemical control is not used. The pathogen population is extremely dynamic, leading to transitory resistance in commercial cultivars. Lr34 is commonly present in the regional germplasm, but there is limited knowledge about the presence of other genes conferring resistance in cultivars. Genes Lr28, Lr36, Lr38, Lr41, and Lr43 provide effective resistance in the region. The best strategy for the stabilisation of the pathogen population and resistance is considered to be the use of adult plant resistance conferred by minor additive genes including Lr34 and Lr46. Sources of this type of resistance from CIMMYT and the region have been made available to breeding programs in the Southern Cone. Stripe rust (P. striiformis f. sp. tritici) is endemic in Chile where chemical control is required to prevent severe losses in stripe rust susceptible cultivars. Although new virulent races emerge frequently, resistance genes Yr5, Yr8, YrlO, Yrl5, and YrSp are currently effective in Chile. Some important stripe rust epidemics have occurred in Argentina, Brazil, and Uruguay. Avoiding the use of highly susceptible cultivars appears to be an effective strategy to prevent stripe rust epidemic development in this area. There have been no serious stem rust (P. graminis f. sp. tritici) epidemics for over 2 decades; the disease was controlled by resistant cultivars. The most important genes conferring resistance in Southern Cone germplasm at the present time are probably Sr24 and Sr31. Other effective genes are Sr22, Sr25, Sr26, Sr32, Sr33, Sr35, Sr39, and Sr40. Several stem rust susceptible wheat cultivars have recently been released. The increased cultivation of susceptible cultivars may lead to higher stem rust incidence, increasing the probability of appearance of new virulent races. Since the 1BL.1RS translocation possessing Sr31 is present in a high proportion of the regional germplasm, the possible introduction of stem rust with Sr31 virulence from Africa is of great concern.

Especialização fisiológica de Puccinia triticina no Brasil em 2002

A ferrugem da folha e uma das principais doencas que afetam a cultura do trigo (Triticum aestivum). Nas regioes triticolas da Argentina, Brasil, Paraguai, Uruguai e Bolivia tem ocorrido todos os anos epidemias da molestia, as quais podem ocasionar perdas de ate 50% no rendimento de graos de cultivares suscetiveis, se nao for efetuado controle com fungicidas. No Chile, a importância da molestia tem aumentado, especialmente em cultivares de trigo de inverno. Na maior parte dessa regiao o patogeno esta presente durante todos os meses do ano, o que favorece o surgimento precoce da molestia, o desenvolvimento de epidemias, e a selecao e fixacao de novas racas, sendo frequente a superacao da resistencia em cultivares comerciais. O objetivo deste trabalho foi identificar as racas do fungo causador da ferrugem da folha do trigo que ocorreram no Brasil durante a safra de 2002 e determinar sua frequencia, distribuicao geografica e alteracoes de virulencia. Foram identificadas 38 combinacoes de virulencia, agrupadas em 12 racas distintas, de acordo com o sistema brasileiro. Esse conjunto representa um grande espectro de virulencia, abrangendo todos os genes da serie diferencial, embora a Lr16 e Lr21 a virulencia tenha se expressado em nivel intermediario. Duas novas combinacoes de virulencia foram identificadas, as quais correspondem, respectivamente, aos codigos SPJ-RS e MFT-CT/MFT-HT, de acordo com o sistema norte-americano de nomenclatura. O gene Lr19 ainda continua efetivo para resistencia a todas as racas, assim como as combinacoes de genes (Lr3+Lr9), (Lr9+Lr16), (Lr9+Lr3ka), (Lr9+Lr21), (Lr16 + Lr24).

Genetic analysis of adult-plant resistance to leaf rust in a double haploid wheat (Triticum aestivum L. em Thell) population

A genetic analysis of adult plant resistance to leaf rust (Puccinia triticina) was performed in in vitro obtained double haploid progenies from a cross between the Brazilian wheat cultivar Trigo BR 35, which, under the high inoculum pressure of the southern region, has been resistant to leaf rust for more than 12 years, and the susceptible cultivar IAC 13-Lorena. Haplodiploidization via in vitro gimnogenesis was done by somatic elimination of the pollen donor genome after maize pollination of the F1 plants. The advantages and usefulness of double haploids (DH) for genetic analysis of complex inherited traits like durable adult-plant resistance to wheat leaf rust were evident: it was possible to analyze inheritance patterns in this cross by using only the 35 DH homozygous segregant lines obtained by in vitro embryo culture of F1 flowers pollinated by maize, this number being equivalent to 1,225 conventional F2 lines because of lack of heterozygosity. After being infected with MCG and LPG races, the results indicated that Trigo BR 35 has two resistance genes. One of the genes expressed resistance only after the intermediate stage of plant development (5-6 leaves).

Postulação de genes (Lr) de resistência à ferrugem da folha em cultivares brasileiras de trigo

In 1996 and 1997 in the southern and central-southern regions of Brazil, a field trial evaluation was peformed on 55 wheat (Triticum aestivum) genotypes representing the recommended cultivars or the advanced inbred lines, as well as differentials for Lr known genes which confer resistance to leaf rust. The wheat genotypes were inoculated at seedling stage with 22 isolates having different avirulent/virulent genes of Puccinia triticina. The types of infection resulting from these inoculations were examined and by comparison, the occurrence of resistance genes in the tested Brazilian genotypes was determined. The seedling resistance gene more frequently identified in the Brazilian genotypes were Lr26, Lr23, Lr10 and Lr24. The Lr16 gene, that confers moderate resistance to most of the fungus races, was only found in two genotypes. Many of the analyzed wheat genotypes have one or more Lr genes probably not yet described.

Variabilidade isoesterásica associada a genes de resistência à ferrugem da folha do trigo

Yield redutions associated with leaf rust, caused by the fungus Puccinia triticina, are recorded for Brazilian wheat (Triticum aestivum) crops. Incorporation of genetic resistance and knowing the number of genes involved are important tools for plant genetic breeding programs, which are consistently introducing qualitative resistance in cultivars aiming to cope with new races of the pathogen. Biochemical techniques, based upon the analysis of the polymorphism of enzymes, provide a rapid and accurate detection of molecular markers and, also, play a valuable support role in breeding programs for predicting the identification of resistant and susceptible genotypes. The objective of this investigation was to carry out a phytopathological and molecular assessment of the haplodiploid population Trigo BR 35 (resistant)/IAC13-Lorena (susceptible) as to its adult plant resistance to leaf rust, as well as to the genetic similarity available in the progeny haplodiploidized in F1 generation. From a total of 96 double haploid lines evaluated for leaf rust, at adult-plant stage, 29 were resistant, 15 were susceptible and 52 showed less resistance than Trigo BR 35 than that presented by IAC 13-Lorena. The genetic analysis showed that the resistance was due to two partially dominant genes. In the biochemical analysis, made through isoenzymatic esterase system, six bands were detected, all of them anodic and with alpha-esterasic specificity the relative migration varied from 0,09 to 0,69. A high genetic similarity was detected, which confirmed isoesterasic analyses, for the 96 double haploid lines.