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A avaliação de manchas em amostras de sementes de arroz (Oryza sativa L.) tem sido feita mediante o cálculo de porcentagem de sementes manchadas, o que não dá informação quantitativa sobre sua incidência. Com o objetivo de obter dados quantitativos de manchas de sementes de arroz, procurou-se adaptar o índice de intensidade de infecção proposto por Amaral e exemplificado por Silva, comparando-se os dois métodos. Foram utilizadas 128 amostras de 5g de sementes provenientes de ensaios conduzidos nos municípios paulistas de Jaboticabal e Pindorama, no ano agrícola de 1982/83, e compostos de 16 cultivares de arroz-de-sequeiro, com quatro repetições. Para calcular o índice de intensidade de infecção, cada amostra foi separada visualmente em quatro categorias: n0 = sem manchas; n1 = poucas manchas (até 5%); n2 = muitas manchas (6-25%), e n3 = extremamente manchadas (acima de 25% da superfície com manchas). A mesma amostra foi separada em sementes com e sem manchas, independente da quantidade de manchas que cada semente apresentasse, calculando-se a porcentagem das manchadas. A comparação da análise da variância e da correlação entre ambas as avaliações revelou que o índice de intensidade de infecção, denominado pelos autores de índice de intensidade de manchas, proporcionou maior discriminação entre os tratamentos que a porcentagem de sementes manchadas, sugerindo-o para a avaliação quantitativa de manchas de sementes de arroz. The evaluation of spots in rice seeds has been performed through the calculation of the percentage of spotted seeds. However this measurement is not quantitative. The objective of this experiment was to compare the percentage of spotted seeds with an index of infection intensity, which gives a more precise quantitative information about the extension of the seed surface area infected. For this comparison, 5g-rice-seed samples from two field trials, carried out in Jaboticabal and Pindorama, in the State of São Paulo, Brazil, in 1982/83, were used. Each trial had the same 16 cultivars with four replications, totalizing 64 samples per site. Seeds of each sample were visually separated in one of four categories: n0 = spotless seeds; n1 = up to 5% of the seed surface spotted; n2 = from 6 to 25% of seed surface spotted; and n3 = over 25% of the seed surface spotted. The number of seeds was counted in each category in order to calculate the index of infection intensity. The same seed samples were separated in two groups: spotted and spotless seeds, independently of the surface area attacked. The number of seeds in each group was counted in order to calculate the percentage of spotted seeds. After the statistical analysis (F test and correlation) performed, the results showed highly significant correlation between both methods, and the so called "spot intensity index" presented better discrimination among cultivars than the percentage of spotted seeds. The authors suggest the use of the new index for selection of rice cultivars aiming to less spotted seeds

Índice de intensidade de infecção adaptado ao estudo de manchas de sementes de arroz