ABSTRACT. The aim of this study was to clarify the relationship between soybean leaf/plant age and the susceptibility to infection by Phakopsora pachyrhizi. Two studies were conducted in the greenhouse during the 2009/2010 season using the BRS 232 cultivar. The experimental design for Study 1 was fully randomized in a 2 x 3 factorial arrangement (phenological stage x age of the 4th, 5th and 6th trifoliate at the time of inoculation), and in Study 2, the experimental design was fully randomized with five treatments (T) and four replicates. The variables assessed were disease severity, number of lesions, uredinia per lesion, and viable spores. In Study 1, it was observed that disease severity was lower when the plant was in the growth stage V6 (11.4%), compared to the reproductive stage R4 (16.9%). The regression for disease severity and leaf age at the time of inoculation showed that older trifoliates on the plants in the reproductive stage exhibited higher severity. However, in Study 2, for trifoliates of the same age on plants of different ages, the trifoliates of younger plants were more susceptible to the disease. It was concluded that soybean plant susceptibility to soybean rust was directly proportional to the age of the trifoliate on a given plant and the phenological stage of the plant at the time of inoculation.Keywords: Phakopsora pachyrhizi, phenological stage, age of trifoliates.Tecidos de folha velha em plantas mais jovens são mais suscetíveis à ferrugem da soja RESUMO. O objetivo do estudo foi esclarecer a relação entre a idade da folha/planta e susceptibilidade à infecção de Phakopsora pachyrhizi. Foram conduzidos dois estudos em cada de vegetação durante a safra 2009/2010 utilizando a cultivar BRS 232. O delineamento experimental do Estudo 1 foi inteiramente casualizado em arranjo fatorial 2 x 3 (Estágio fenológico x idade do 4th, 5th e 6th trifólio no momento da inoculação), e Estudo 2, delineamento inteiramente casualizado com cinco tratamentos (T) e quatro repetições. As variáveis avaliadas foram severidade da doença, numero de lesões, urédias por lesão e esporos viáveis. No Estudo 1 foi observado que severidade da doença foi menor quando as plantas estavam no estágio de crescimento V6 (11,4 %), comparado com estágio reprodutivo R4 (16,9%). A regressão para severidade da doença e idade da folha no momento de inoculação mostrou que trifólios mais velhos em plantas no estágio reprodutivo exibiram maior severidade. Contudo, no estudo 2, para trifólios com a mesma idade em plantas de diferentes idades, os trifólios de plantas jovens foram mais susceptíveis à doença. Conclui-se que a suscetibilidade de plantas a ferrugem asiática foi diretamente proporcional à idade do trifólio em uma dada planta e o estágio fenológico no momento de inoculação.Palavras-chave: Phakopsora pachyrhizi, estágio fenológico, idade do trifólio.
The objectives of this study were to evaluate the influence of soybean cultivar of two maturity groups, early/semi-early (E) or intermediate (I), on the management of Asian soybean rust (ASR) with fungicides and yield. Field trials were conducted during the 2006/07 and 2007/08 growing seasons. Seven cultivars of the two groups were tested in the first season and eight in the second season. All cultivars had plots that were treated (T) or non-treated (NT) with a commercial mixture of pyraclostrobin + epoxiconazole. ASR severity (%) was visually assessed several times during the crop cycle and yield (kg ha -1 ) was determined at harvest. Values of the standardized area under the disease progress curve (AUDPC) calculated from the severity assessments was higher in 2007/08 than in 2006/07, but no differences were found between cultivars of the E and I maturity groups. Differences in yield between between T and NT plots were lower in cultivars of the E group than those of the I group in both the 2006/07 (37.6% and 52.8% respectively) and the 2007/08 season (56.9% and 85.0%, respectively). A higher stability in yield was found for cultivars of the E maturity group compared to those of the I group.
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