Márcia Soares Chaves

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.

Resistência quantitativa à ferrugem da folha em genótipos de aveia branca: I - Caracterização da reação em condições de campo

Quantitative resistance to crown rust in white oat genotypes: I – Characterization of reaction in field conditions Among diseases that attack oat (Avena sativa) crop, crown rust, caused by Puccinia coronata f. sp. avenae, has been the most destructive, being responsible for a decrease in the quality and yield of grains. Control through qualitative resistance has been restricted by the capacity of the pathogen to overcome this resistance type. Seeking an alternative strategy for disease control, 31 white oat genotypes were investigated for the occurrence of quantitative resistance. Trials were accomplished from 1996 to 2000, in the Agronomic Experimental Station of UFRGS, in Eldorado do Sul, RS, Brazil. Disease progress was evaluated in the field for three years, and some genotypes were tested for five years. Weekly evaluations of rust severity in each plot were performed, providing data from which to determine the disease progress curves, and calculate the area under the disease progress curve (AUDPC), and the rate of development of disease (r). There was great variability among genotypes for all the years in regard to rust reaction, allowing the genotypes to be classified in four groups: resistant, moderately resistant, moderately susceptible and susceptible. The first two groups have good levels of quantitative resistance and can be used in the future as parents in breeding programs.

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).

The importance for food security of maintaining rust resistance in wheat

Wheat is one of the main sources of calories and protein of the world’s population and therefore the pathogens that cause rust diseases of the crop are a real threat to food security. Besides the continuous evolution of rust pathogens which repeatedly results in overcoming the resistance of commercial varieties throughout the world, plant breeders are also now challenged by the impacts of global climatic changes. Agricultural practices will need to keep pace with the intensification of sustainable food production in order to face the challenge of feeding a world population estimated to reach about nine billion by 2050. Contemporary wheat breeding has increasingly focused on the future, culminating in the emergence of a global partnership for breeding new wheat varieties with resistance to rust pathogens. Plant breeding now employs a wide range of both long-established and frontier technologies aimed at achieving the United Nations Millennium Development Goals of ending hunger and extreme poverty (MDG1), while concurrently promoting environmental sustainability (MDG7) through global partnerships for development (MDG8).

Resistência Quantitativa à Ferrugem da Folha em Genótipos de Aveia Branca: II - Avaliação de Componentes de Resistência*

Quantitative resistance to crown rust in white oat genotypes: II - Evaluation of resistance components The use of quantitative resistance as a form to control oat (Avena sativa) crown rust can be a viable alternative, once there is differentiated reaction among oat genotypes in field, which shows great variation. The slow disease progress observed in the field is the result of combined effects of resistance components as low infection efficiency, longer latent period, low number of spores per pustule and little pustules size. This work was accomplished in 1999 and 2000, and its objective was to quantify the resistance components above mentioned in 31 white oat genotypes. The reaction of these genotypes was evaluated in the field, from 1996 to 2000, and they were classified in four groups. Evaluation of resistance components was accomplished in seedlings and adult plants maintained in controlled conditions, and the last two components were also quantified in leaves collected in the field trials. Genotypes showed variability for all measured characteristics, except for latent period in seedlings, and those genotypes classified as resistant in the field presented the best combination of desirable components.

Inheritance of resistance to oat crown rust in recombinant inbred lines

Crown rust is the main disease affecting oats (Avena sativa L.), and genetic resistance has been the chief method utilized to control this disease. A population composed of 135 recombinant inbred lines, F 5:6 , generated by crossing the oat cultivar UFRGS 8 with the genotype Pc68/5*Starter, was assessed on the inheritance of resistance to crown rust (Puccinia coronata f. sp. avenae P. Syd. & Syd.). The evaluation of resistance in F 5:6 seedlings was based on the type of infection resulting from inoculation with the race SQPT of P. coronata f. sp. avenae. The proportion of Resistant: Susceptible seedlings (R:S) was 62:64, which indicates that inheritance was governed by a single gene. The assessment of resistance inheritance in adult plants was performed in the field during the years 2004 and 2005. The distinction between resistant and susceptible classes was based on the final severity (FS) as well as the area under the disease progress curve, which was normalized and corrected (AUDPC*c). F 5:6 and F 5:7 were evaluated under field conditions in 2004 and 2005, demonstrating a ratio of approximately 1R:3S, which fits with a typical two genes inheritance model.

Virulence Variability of Puccinia coronata f. sp. avenae Isolates Collected in Three Counties from Rio Grande do Sul State, Brazil

Using isolates collected in three counties of Rio Grande do Sul State, Brazil, the goals of this work were to determine (i) the pattern of virulence or avirulence of the isolates to 25 Pc resistance genes, (ii) the similarity in virulence among Puccinia coronata f. sp. avenae isolates considering their pattern of virulence or avirulence, (iii) the race code for each isolate by the North American system of nomenclature, and (iv) the supplemental Pc genes potentially useful as local differentials for P. coronata f. sp. avenae races. The results indicate that the southern Brazilian rust isolates presented a high level of virulence, because 66% of inoculations manifested the high infection type. Only the Pc 68 gene was effective against all tested isolates. In general, each isolate presented a different pattern of virulence or avirulence, which indicates the high variability for virulence that the fungus presents at the sampled sites. However, the North American system of nomenclature was not completely sufficient in distinguishing southern Brazilian races. Thus, the genes Pc 36, Pc 53, Pc 55, and Pc 63 represent a possible gene combination to be incorporated into the North American system of nomenclature.

Resistência quantitativa à ferrugem da folha em genótipos de aveia branca: III - Correlação de componentes de resistência entre si e com a intensidade de doença no campo

Selection of genotypes for quantitative resistance to oat crown rust, based on its components, needs the knowledge of the association of these components with each other and with the amount of disease in the field, represented by AUDPC. This work aimed to determine these correlations, based on results of previous trials, in which the reaction of 31 white oat genotypes was tested and the resistance components were quantified. Correlations were usually low for the resistance components with each other and for these components with the amount of disease in the field. Higher correlations, consistent among years, with AUDPC were infection frequency in seedlings and latent period and pustule size in adult plants. All other possible correlations were very low. These data suggest that the resistance components mentioned can influence the expression of quantitative resistance in the field in a somewhat important manner. Nevertheless, the low correlation of resistance components with each other indicates that genotype selection, in this host-pathogen interaction, must not be based on a single resistance component assessment. The correlation analysis suggests that the expression of quantitative resistance to oat crown rust is not conditioned by a single resistance component but by the group effect of all of them. In addition, it is possible that different mechanisms are involved for each genotype, in such a way that the more important resistance component in one genotype may not be the same for another.

Histopathology of durable adult plant resistance to leaf rust in the Brazilian wheat variety Toropi

Leaf rust, caused by the fungus Puccinia triticina is a major disease of wheat (Triticum aestivum) worldwide. This disease is prevalent in southern South America where the environmental conditions and high genetic variability of P. triticina favour epidemics. The primary means of controlling pathogenic P. triticina races has been through using wheat varieties containing race-specific resistance genes. The defence mechanisms involved in durable race non-specific resistance to P. triticina are probably distinct from those involved in non-durable race-specific resistance. We investigated the histological components of resistance to P. triticina present in three wheat genotypes: the race non-specific resistant Brazilian variety Toropi; the race-specific resistant line RL6010 Lr9; and the susceptible Brazilian variety BRS 194. Plants of these three genotypes were inoculated with P. triticina race MFP and tissue samples excised from flag leaves at various times after inoculation to assess the number of infective structures, frequency of cell death and the accumulation of autofluorescent cells and hydrogen peroxide. The genotypes showed different resistance mechanisms active at different times during the infection process. Our results for Toropi indicate that there was a reduction in the extent of formation of stomatal appressoria and all subsequent structures. During attempted penetration we also observed the production of autofluorescent compounds and late cell death, but not peroxide formation. This non-specific resistance to P. triticina involves both pre-haustorial and post-haustorial mechanisms which may be responsible for maintaining the low disease severity observed in this variety even under high inoculum pressure.

Padrão de resistência de genótipos de aveia à ferrugem-da-folha na definição de hibridações

Abstract – The objectives of this work were: to determine the resistance/susceptibility pattern of 20 elite oatgenotypes to 40 isolates of Puccinia coronata f. sp. avenae ; to determine the pattern of virulence/avirulence ofisolates collected in three counties of Rio Grande do Sul to the oat genotypes studied; and to indicate potentialparents for the generation of populations with high crown rust resistance. The resistance pattern of oat genotypesand the virulence/avirulence of the fungi were determined by the analysis of the reaction incited by the inoculationof Puccinia coronata f. sp. avenae isolates into seedlings of oat genotypes. The selection of genitors wasbased on the cultivar complementation index proposed in this work. Genotypes expressing resistance to thelarger number of isolates were FAPA6, URS20, UPFA20, CFT1 and FAPA5, while UFRGS15, UPF15, UPF18,UPF19 and UPF16 were susceptible to a higher number of isolates. The following crosses are indicated:FAPA6 x Albasul, URS22 x FAPA6, CFT1 x URPEL15 and CFT1 x UFRGS19.Index terms: