C. R. Casela

Virulence and molecular diversity in Colletotrichum graminicola from Brazil.

Genetic diversity among 37 isolates of the sorghum anthracnose pathogen Colletotrichum graminicola, from four geographically distinct regions of Brazil, was evaluated by RAPD and RFLP-PCR markers and virulence characters on a set of 10 differential sorghum genotypes. Twenty-two races were identified and race 13B was the most frequent, but present in only two regions. RAPD analysis revealed 143 polymorphic bands that grouped the isolates according to their geographic origin, but not by their virulence phenotypes. RFLP with HaeIII, MspI, HinfI, HhaI, HpaII, EcoRI, HindIII, PstI, RsaI, Taq αI, and AluI enzymes over ITS domains and 5.8 rDNA genes of C. graminicola did not show differences among the isolates, indicating high conservation of these restriction sites. Molecular polymorphism was observed among isolates belonging to the same race. No association between virulence phenotypes and molecular profiles was observed.

Localization of Pantoea ananatis inside lesions of maize white spot disease using transmission electron microscopy and molecular techniques

The etiological agent of maize white spot (MWS) disease has been a subject of controversy and discussion. Initially the disease was described as Phaeosphaeria leaf spot caused by Phaeosphaeria maydis. Other authors have suggested the existence of different fungal species causing similar symptoms. Recently, a bacterium, Pantoea ananatis, was described as the causal agent of this disease. The purpose of this study was to offer additional information on the correct etiology of this disease by providing visual evidence of the presence of the bacterium in the interior of the MWS lesions by using transmission electron microscopy (TEM) and molecular techniques. The TEM allowed visualization of a large amount of bacteria in the intercellular spaces of lesions collected from both artificially and naturally infected plants. Fungal structures were not visualized in young lesions. Bacterial primers for the 16S rRNA and rpoB genes were used in PCR reactions to amplify DNA extracted from water-soaked (young) and necrotic lesions. The universal fungal oligonucleotide ITS4 was also included to identify the possible presence of fungal structures inside lesions. Positive PCR products from water-soaked lesions, both from naturally and artificially inoculated plants, were produced with bacterial primers, whereas no amplification was observed when ITS4 oligonucleotide was used. On the other hand, DNA amplification with ITS4 primer was observed when DNA was isolated from necrotic (old) lesions. These results reinforced previous report of P. ananatis as the primary pathogen and the hypothesis that fungal species may colonize lesions pre-established by P. ananatis.

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.

First report of pathogenicity of Pantoea ananatis in sorghum (Sorghum bicolor) in Brazil

Bacterial isolates from sorghum plants showing leaf spot symptoms were identified through molecular and phenotypic traits, showing that the isolates belong to Pantoea ananatis. Sorghum plants inoculated with those isolates showed a pathogenic reaction. The causal agent was reisolated and Koch’s postulates were fulfilled. This is the first report of P. ananatis causing leaf spots on sorghum plants in Brazil.

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.

Adubação nitrogenada e potássica na severidade da antracnose em dois cultivares de milho

O equilibrio nutricional pode contribuir para a resistencia das plantas as doencas. Com o objetivo de avaliar o efeito da interacao entre as adubacoes nitrogenada e potassica na severidade das lesoes da antracnose foliar e na nutricao mineral da cultura do milho, foram instalados dois experimentos, em casa de vegetacao com dois cultivares, o DAS 2B710 (moderadamente resistente a doenca) e o BRS 1010 (susceptivel), cinco doses de N (75, 150, 300, 600 e 1200 mg dm-3) e cinco doses de K 63, 125, 250, 500 e 1000 mg dm-3). O delineamento experimental para cada cultivar foi em blocos inteiramente ao acaso, em esquema fatorial 5 x 5, com 25 tratamentos e quatro repeticoes. Cada vaso com quatro plantas constituiu uma parcela experimental. As doses de N e K foram divididas em quatro parcelas, com intervalos de dez dias a partir da semeadura, para os dois cultivares. Aos 21 dias apos a semeadura, as plantas foram inoculadas e levadas para câmara umida, com fotoperiodo de 16 horas, no escuro durante tres dias consecutivos. Avaliou-se a severidade das lesoes diariamente, ate 11 dias apos a inoculacao. Aos 43 dias apos a semeadura, a parte aerea das plantas foi colhida, secada e moida para determinar os teores de N e de K. A quantidade de area foliar lesionada dependeu da interacao entre os nutrientes. Em ambos os cultivares, os menores valores de severidade foram observados na menor dose de N, combinada com a maior dose de K. A severidade das lesoes observada no cultivar moderadamente resistente foi 41% menor que a observada no cultivar susceptivel. A adubacao nitrogenada influenciou de forma negativa o teor de K da parte aerea.

Identification of sources of resistance in sorghum to Peronosclerospora sorghi

The main objective of this work was to identify sources of resistance in sorghum (Sorghum bicolor) to Peronosclerospora sorghi, the causal agent of downy mildew, through the evaluation of 42 sorghum genotypes under natural infection in the field. Genotypes were planted in single row plots between two rows of the susceptible line SC283, planted 30 days before, to act as spreader rows, in two separate nurseries. The experimental design was a completely randomized block design with three replications. Sorghum genotypes CMSXS156, CMSXS157, CMSXS243, TxARG-1, 8902, 9902054, 9910032, 9910296, Tx430, QL-3, SC170-6-17, CMSXS762 and BR304 were classified as highly resistant in both nurseries. Among these, SC170-6-17 and 9910296 showed 0% systemic infection. Results indicated the possible occurrence of different pathotypes of P. sorghi in the two nurseries.