Alexandre S. Ferreira

Reaction of sorghum genotypes to the anthracnose fungus Colletotrichum graminicola

The reactions of 22 sorghum (Sorghum bicolor) genotypes to six previously identified races of the sorghum anthracnose fungus Colletotrichum graminicola, were evaluated under greenhouse and field conditions. Races were inoculated in separate tests in the greenhouse. In the field, spreader rows of a susceptible genotype were artificially inoculated with a mixture of the six races of the pathogen. In the greenhouse tests, nine genotypes showed resistance to all six races. In the field high levels of dilatory resistance was observed in the sorghum genotypes CMSXS169 and CMSXS373.

Differences in competitive ability among races of Colletotrichum graminicola in mixtures

Mixtures of races of Colletotrichum graminicola, causing sorghum (Sorghum bicolor) anthracnose and differing in their virulence range, were inoculated for five and six generations on the susceptible sorghum cultivar BR009 (Tx623), in two experiments in a greenhouse. In each generation a sample of 50 single spore isolates was obtained and inoculated on a standard differential set to determine the proportion of each race in the mixture. Isolates of the race 30A, with the narrowest virulence range, predominated over isolates of the more complex races 31B, 31C and 31E indicating the existence of differences in the survival ability among races of this pathogen.

Incidência de Colletotrichum graminicola em colmos de genótipos de milho

Stalk rot (Colletotrichum graminicola) is one of the most serious disease affecting maize crop in Brazil, especially after the flowering phase, when yield losses can reach significant levels. The use of genetically resistant cultivars is the most efficient strategy to control the disease. The aim of this work was to evaluate the incidence of stalk rot in maize commercial hybrids sinde there is scarce information to allow the use of genetic resistance as a strategy to control this disease. Eighteen maize commercial hybrids were evaluated in 2005, 2006, and 2007 in the experimental area of EMBRAPA Maize and Sorghum Research Center Sete Lagoas, Minas Gerais State, Brazil, under conditions of natural infection. From each plot, three stalk segments of three plants were sampled: the second internode above the soil line, the internode of ear insertion, and the internode right bellow the tassel. Four tissue fragments of each stalk segment were surface sterilized and transferred to oatmeal agar plates, which were incubated under continuous fluorescent light at 25oC. Pathogen identification and quantification were performed after three to four days of incubation. The hybrids BR201 and BR206 showed the lowest infection level (below 30%) whereas the highest incidence (above 60%) was observed for the hybrid BRS1010. The pathogen was observed in all analyzed stalk segments, but was most frequently isolated from the internode right below the tassel. No evaluated hybrid could be considered to have high resistance to the pathogen.

Diversidade populacional de Colletotrichum sublineolum em seis localidades no Brasil

Foi objetivo deste trabalho caracterizar a populacao de Colletotrichum sublineolum Henn. por meio da avaliacao da virulencia de 289 isolados monosporicos do patogeno. Foram utilizadas como diferenciadoras 10 linhagens elites do programa de melhoramento genetico de sorgo da Embrapa Milho e Sorgo. Os isolados de C. sublineolum foram obtidos de folhas de sorgo provenientes de Palmeira de Goias e Goiânia, GO, Sete Lagoas, Ipiacu e Uberlândia, MG, e Jardinopolis, SP e designados de acordo com um sistema binario de classificacao de racas. As populacoes foram tambem caracterizadas quanto a diversidade fenotipica, por meio de indices de Shannon, de Gleason e de Simpson, e de um indice de complexidade, e quanto a sua distribuicao e frequencia nas seis localidades. Somente a raca 31.04 foi encontrada nos seis locais avaliados e foi a raca mais frequente em Uberlândia, Ipiacu e Palmeira de Goias. A raca mais complexa, 31.31, foi a mais frequente em Sete Lagoas e Goiânia e nao foi observada somente em Palmeira de Goias. Verificou-se que as racas mais frequentes em cada localidade apresentaram-se, em sua maioria, bem distribuidas nas seis regioes avaliadas. O local com maior diversidade fenotipica foi Jardinopolis, de acordo com os indices de Shannon, Simpson e Gleason. O maior indice de complexidade de racas foi encontrado em Sete Lagoas e Goiânia, seguidas por Jardinopolis, Ipiacu, Uberlândia e Palmeira de Goias, respectivamente. Houve correlacao entre os indices de Shannon e Gleason, mas nao entre os indices de diversidade e de complexidade.

Variability in isolates of Puccinia polysora in Brazil

O presente trabalho teve por objetivo avaliar a variabilidade de Puccinia polysora, agente causal da ferrugem polissora, com base na sua virulencia em cultivares de milho (Zea mays). Foram avaliados 60 isolados monopustulares, coletados em diferentes areas de ocorrencia da doenca, os quais foram inoculados em 50 hibridos experimentais de milho. Seis cultivares permitiram uma clara distincao entre a reacao de resistencia e a de suscetibilidade e foram utilizados para a caracterizacao dos isolados. Dezessete padroes de virulencia foram observados com base nas reacoes diferenciais e os padroes de virulencia mais frequentes foram observados em todos os locais amostrados, indicando uma possivel ausencia de diferenciacao geografica entre as populacoes de P. polysora prevalescentes no Brasil.

Resistance of Commercial Hybrids and Lines of Sorghum, Sorghum bicolor (L.) Moench., to Diatraea saccharalis (Fabr.) (Lepidoptera: Pyralidae)

Commercial sorghum hybrids and Embrapas lines were evaluated regarding resistance to sugarcane borer. Two field experiments were conducted in Sete Lagoas, MG, under natural infestation, during the second cropping season in a randomized complete block with 3 replicates. The plants were evaluated regarding to stand, maturity cycle, plant height, and number of broken or lodging stalks and yield. The borer incidence, number of galleries, number of healthy and damaged nodes were also evaluated to calculate the Infestation Index and Intensity Infestation Index. Among the commercial sorghum hybrids, Br 304 and CMSXS 9701 were the least infested wile Z 732 and Esmeralda were the least damaged. Among the lines, the least infested was 9815017 and least damaged 9816003. The two indexes were correlated positively with plant height in both experiments and negatively correlated with the lines cycle. However, there was no correlation between the Indexes for the hybrids. Among the lines the data fit to a logarithmic curve. Although there is a significant variability among commercial sorghum hybrids regarding sugarcane borer susceptibility, under high borer density, a significant yield loss can be computed to all hybrids. On the other hand, the genetic variability regarding sugarcane borer resistance among the sorghum lines indicated a significant potential for use in a breeding program of resistant cultivars.