Inheritance of resistance to anthracnose stalk rot (Colletotrichum graminicola) in tropical maize inbred lines

Generation means was used to study the mode of inheritance of resistance to anthracnose stalk rot in tropical maize. Each population was comprised of six generations in two trials under a randomized block design. Inoculations were performed using a suspension of 105 conidia mL(-1) applied into the stalk. Internal lesion length was directly measured by opening the stalk thirty days after inoculation. Results indicated contrasting modes of inheritance. In one population, dominant gene effects predominated. Besides, additive x dominant and additive x additive interactions were also found. Intermediate values of heritability indicated a complex resistance inheritance probably conditioned by several genes of small effects. An additive-dominant genetic model sufficed to explain the variation in the second population, where additive gene effects predominated. Few genes of major effects control disease resistance in this cross. Heterosis widely differed between populations, which can be attributed to the genetic background of the parental resistant lines.Comissao de Aperfeicoamento de Pessoal de Ensino Superior (CAPESPICDT)Comissao de Aperfeicoamento de Pessoal de Ensino Superior (CAPES-PICDT)FAPESP [01/02793-0]FAPES

Progeny evaluation for resistance to Phaeosphaeria leaf spot in tropical maize

Maize breeding programmes in Brazil and elsewhere seek reliable methods to identify genotypes resistant to Phaeosphaeria leaf spot. The area under the disease progress curve (AUDPC) is an accurate method to evaluate the severity of foliar diseases. However, at least three data points are required to calculate the AUDPC, which is unfeasible when there are thousands of genotypes to be assessed. The aim of this work was to estimate the heritability of disease resistance, evaluate disease severity at different times using a nine-point scale in comparison to the AUDPC, and establish the most suitable phenological period for disease assessment. A repeated experiment was conducted in a 11 x 11 lattice experimental design with three replications. Disease assessments were carried out at flowering, 15 and 30 days post-anthesis for the parental lines DS95, DAS21, the F1 generation and 118 F2:3 progenies. Then, the AUDPC was obtained and results compared with the single-point evaluations used to calculate it. Individual and joint analyses of variance were conducted to obtain heritabiliy estimates. The assessments performed after the flowering stage gave higher estimates of heritability and correlation with AUDPC. We concluded that one assessment between the 15th and 30th day after flowering could provide enough information to distinguish maize genotypes for their resistance to Phaeosphaeria leaf spot under tropical conditions.Brazilian institution Fundacao de Amparo a Pesquisa do Estado de Sao Paulo FAPESP[99/09170-7]Brazilian institution Fundacao de Amparo a Pesquisa do Estado de Sao Paulo FAPESP[01/02793-0

Characterisation of turnip mosaic virus isolates reveals high genetic variability and occurrence of pathotype 1 in Brazil

Turnip mosaic virus (TuMV) infects many plant species, being the only potyvirus able to infect brassicas. TuMV isolates have been classified into 12 pathotypes according to symptoms induced in lines of Brassica napus, and molecularly clustered into six lineages (basal-B, basal-BR, Asian-BR, world-B, Iranian and OMs). Despite being considered one of the most important viruses infecting brassicas worldwide, there is little information on this virus in the Neotropical region. Aiming to fill in this gap and advance knowledge on occurrence, genetic variability, and biological aspects of TuMV in Brazil, 40 isolates were identified and characterised. Five of these isolates were selected to determine their host range, sequence their genomes, and for phylogenetic, recombination and diversity analyses. Mechanical inoculations performed on plant species from 10 families showed differences in symptom expression among isolates. Inoculations of 13 TuMV isolates in B. napus lines revealed occurrence only of the pathotype 1. According to phylogenetic analyses of the coat protein, TuMV Brazilian isolates clustered into the groups: world-B (subgroups world-B2 and world-B3) and basal-BR. In the latter, there was a formation of a subclade named Brazilian subgroup composed by 31 Brazilian TuMV isolates. Intralineage and interlineage recombination events of world-B, basal-B and basal-BR suggest that Brazilian TuMV isolates had a European origin. Our diversity analysis suggest that a strong negative selection is acting on polyprotein coding region. We confirmed that Brazilian TuMV isolates showed high variability, which together with their ability to infect wild brassicas and to circumvent resistance genes highlight their genetic and epidemiological potential in causing damages in cultivated species of brassicas and other crops in Brazil