<b>Older leaf tissues in younger plants are more susceptible to soybean rust

Abstract

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.

Predicting Asian Soybean Rust Epidemics Based on Off-Season Occurrence and El Niño Southern Oscillation Phenomenon in Paraná and Mato Grosso States, Brazil

“…Most forecasting models use only environmental factors; these usually include hours of wetness and temperature, considering that the pathogen and host (the other vertices of the triangular disease), are always present (M. C. Alves, Pozza, Costa, Carvalho, & L. S. Alves, 2011). In addition, the models usually not consider factors such as leaf age and susceptibility (Xavier, Martins, Fantin, & Canteri, 2017) and plant nutrition (Gaspar et al, 2015). The present study demonstrated that the presence of inoculum (the pathogen vertex) exerts a great influence on epidemics during the season.…”

Soybean Rust Epidemics as Affected by Weather Conditions in Brazil

“…Reductions in the disease severity in soybean might be related to depletion of the availability of free water for the pathogen infection process. A significant reduction in LWD was evidenced as soybean plants entered into the reproductive phenological stage, considered the most susceptible phase for disease occurrence (Young et al, 2011;Xavier et al, 2017). During this phenological stage mean LWD decreased from 13.4 to 9.0 h, reaching an average of 6.5 h during the third 10-day period of January (Figure 2), making the environment less unfavorable to SBR occurrence.…”

“…Thus, throughout the crop growing season in Pedra Preta neither the weather conditions nor the host characteristics favored the SBR occurrence in soybean production fields. However, by the time wet environmental conditions prevailed again and remained in a favorable range for SBR occurrence, soybean plants were at the beginning of flowering stage, coinciding therefore with a period of a high susceptibility to the SBR infection, and as a result disease progress turned out to be favored in the field (Table 3) (Young et al, 2011;Xavier et al, 2017).…”

Efeito da limitação hídrica no patossistema >i<Phakopsora pachyrhizi>/i<-soja

“…Na América do Sul o primeiro registro data de 2001 em algumas regiões do Brasil e do Paraguai, de onde o patógeno se alastrou rapidamente para outras áreas circundantes. Em 2004 o patógeno foi reportado no sul do domínio continental dos Estados Unidos da América, e algum tempo mais tarde infectando campos de soja no norte desse mesmo país (GOELLNER et al, 2010;YORINORI et al, 2005). O fungo P. pachyrhizi é um parasita obrigatório capaz de produzir urediniósporos e teliósporos.…”

“…A ferrugem-asiática da soja causada por Phakopsora pachyrhizi Syd & P. Syd é a principal e mais severa doença da soja no Brasil (LANGENBACH et al, 2016;XAVIER et al, 2017). A ocorrência da doença é atrelada às condições favoráveis ao desenvolvimento do patógeno.…”