Breeding for resistance to gray leaf spot, caused by Cercospora zeae-maydis (Cz) is paramount for many maize environments, in particular under warm and humid growing conditions. In this study, we mapped and characterized quantitative trait loci (QTL) involved in the resistance of maize against Cz. We confirmed the impact of the QTL on disease severity using near-isogenic lines (NILs), and estimated their effects on three major agronomic traits using their respective near isogenic hybrids (NIHs), which we obtained by crossing the NILs with an inbred from a complementary heterotic pool. We further validated three of the four QTL that were mapped using the Multiple Interval Mapping approach and showed LOD values>2.5. NILs genotype included all combinations between favorable alleles of the two QTL located in chromosome 1 (Q1 in bin 1.05 and Q2 in bin 1.07), and the allele in chromosome 3 (Q3 in bin 3.07). Each of the three QTL separately significantly reduced the severity of Cz. However, we found an unfavorable epistatic interaction between Q1 and Q2: presence of the favorable allele at one of the QTL allele effectively nullified the effect of the favorable allele at the other. In contrast, the interaction between Q2 and Q3 was additive, promoting the reduction of the severity to a greater extent than the sum of their individual effects. When evaluating the NIH we found significant individual effects for Q1 and Q3 on gray leaf spot severity, for Q2 on stalk lodging and grain yield, and for Q3 on grain moisture and stalk lodging. We detected significant epitasis between Q1 and Q2 for grain moisture and between Q1 and Q3 for stalk lodging. These results suggest that the combination of QTL impacts the effectiveness of marker-assisted selection procedures in commercial product development programs.

Crosses involving eight commercial upland cotton cultivars were performed in all possible combinations to generate 28 hybrids. The eight parental genotypes were also evaluated. A field experiment was conducted using a triple Latice design (6 x 6) in Itumbiara, Brazil, during the 2000/01 growing season. Data of the following agronomic and fiber traits were collected: seedcotton yield, lint yield, seed index, picked lint percent, index of production and earliness, micronaire index, fiber strength, fiber length, uniformity index, short-fiber index, fiber elongation, CSP index, reflectance and yellowness and used in the analysis of variance as proposed by Griffing (1956), method 4. Significant differences were detected among treatments and the estimates of combining abilities. Additive gene action prevailed for most traits studied. Both positive and negative heterotic values were detected, demonstrating the potential of hybrid combinations for trait improvement in breeding programs.

The objectives of this study were to adapt a screening method previously used to assess seedling drought tolerance in cereals for use in cotton (Gossypium hirsutum L.) and to identify tolerant accessions among a wide range of genotypes. Ninety genotypes were screened in seven growth chamber experiments. Fifteen-day-old seedlings were subjected to four 4-day drought cycles, and plant survival was evaluated after each cycle. Three cycles are probably the minimum required in cotton work. Significant differences (at the 0.05 level or lower) among entries were obtained in four of the seven experiments. A "confirmation test" with entries previously evaluated as "tolerant" (high survival) and "susceptible" (low survival) was run. A number of entries duplicated their earlier performance, but others did not, which indicates the need to reevaluate selections. Germplasms considered tolerant included: `IAC-13-1', `IAC-RM4-SM5', `Minas Sertaneja', `Acala 1517E-1' and `4521'. In general, the technique is simple, though time-consuming, with practical value for screening a large number of genotypes. Results from the screening tests generally agreed with field information. The screening procedure is suitable to select tolerant accessions from among a large number of entries in germplasm collections as a preliminary step in breeding for drought tolerance. This research also demonstrated the need to characterize the internal lack of uniformity in growth chambers to allow for adequate designs of experiments.

Foram instalados nove ensaios no período de 1973 a 1976 em solos LVA, LE e LR de três localidades do Triângulo e duas no Norte do Estado de Minas Gerais, Brasil, a fim de se estudar as épocas críticas de competição de plantas daninhas com o algodoeiro (Gossypium hirsutum L.). Os tratamentos foram: capinas até 2, 4, 6, 8 primeiras semanas e durante todo o ciclo; e capinas após 2, 4, 6, 8 primeiras semanas e todo o ciclo sem capinas. Os resultados mostraram que a competição das plantas daninhas, quando não controladas, com a cultura, provocou 90,22% de perda na produção no Triângulo Mineiro e 70,73% no Norte de Minas. Em relação à testemunha, mantida livre de competição durante todo o ciclo, o melhor rendimento foi obtido quando se manteve a cultura livre de competição durante seis: emanas após a emergência, no Triâgulo Mineiro, e durante oito semanas, no Norte de Minas. Não houve, entretanto, diferença significativa entre os tratamentos com 4, 6, 8 semanas e também com todo o ciclo sem competição, tanto no Triângulo quanto no Norte do Estado.

Heterodera glycines Ichinohe, the soybean cyst nematode (SCN) was detected in Brazil in 1992 and since then it has been causing losses to Brazilian soybean crop yields. SCN populations have great genetic diversity which makes it difficult to manage this disease. The objectives of this research were to investigate the genetic variation of 16 SCN populations sampled in middle western and southeastern Brazil, utilizing random amplified polymorphic DNA (RAPD) techniques, and to establish useful and specific RAPD markers for SCN race 3. RAPD markers demonstrated genetic variability among and within SCN populations, and they could be used for monitoring nematode population dynamics. The OPA-07 primer was a reliable molecular marker for race 3, while electrophoretic profile analysis of DNA fragments amplified with OPA-10 primer detected slight variation within those populations identified as race 3. The SCN population from Chapadão do Céu, GO - sample 2 - was the most genetically distant from the other populations.
Heterodera glycines Ichinohe, o nematóide de cisto da soja (SCN), foi detectado no Brasil em 1.992 e desde então tem provocado perdas consideráveis à produtividade da soja brasileira. Populações de SCN apresentam grande diversidade genética, o que dificulta o manejo dessa cultura em áreas infestadas. Os objetivos desta pesquisa foram investigar a possível variação genética de dezesseis populações de SCN amostradas nas regiões Centro Oeste e Sudeste do Brasil, utilizando a técnica do RAPD (amplificação casualizada de DNA polimórfico), bem como estabelecer marcadores RAPD específicos para a raça 3 do SCN. Nossos resultados mostraram que os marcadores RAPD podem ser utilizados em estudos de variabilidade genética entre populações de SCN e dentro de raças deste nematóide, bem como podem ser usados no monitoramento de dinâmica populacional deste patógeno. O "primer" OPA-07 foi considerado satisfatório como marcador molecular para a raça 3, enquanto a análise eletroforética dos fragmentos de DNA amplificados com a utilização do "primer" OPA-10 detectou ligeira variação nas populações de SCN identificadas como raça 3. A população do SCN oriunda de Chapadão do Céu, GO - amostra 2 - foi a geneticamente mais distante, quando comparada às demais usadas neste estudo