Asian soybean rust control efficacy calculated with AUDPC and with final severity data

Estimativas da eficácia de fungicidas para controle da ferrugem asiática da soja com base em dados da AACPD ou da severidade final da doença

Erlei Melo Reis Mateus Zanatta Luiz Henrique Carrega Pereira da Silva Andrea Camargo Reis About the authors

ABSTRACT

Data from experiments conducted in the field during four growing seasons aiming at the chemical control of soybean rust caused by Phakopsora pachyrhizi, the control values calculated for the area under the disease progress curve (AUDPC) were compared with those of the final severity. Data were analyzed according to a factorial experiment: (i) treatments with fungicides and (ii) rust control. Out of 70 treatments, the control was always lower when calculated with the AUDPC than with the final severity. In the general means for four growing seasons, the AUDPC control was of 48.5 (range 36 to 65%) and for calculation with the final severity 43.25% (range 29 to 66%). It can be concluded that, in the case of soybean rust, the control calculation shows advantage when performed with final severity data.

Keywords
Glycine max ; disease quantification; plant pathometric methods; Phakopsora pachyrhizi

RESUMO

Com dados de experimentos conduzidos no campo durante quatro safras visando ao controle químico da ferrugem da soja, causada por Phakopsora pachyrhizi, foram comparados os níveis de controle com base na área abaixo da curva de progresso da doença (AACPD) com os da severidade final. Os dados foram analisados segundo um experimento fatorial com dois fatores: (i) tratamentos com fungicidas e (ii) método para calcular eficácia. Dos 70 tratamentos, as estimativas de eficácia de controle foram maiores quando calculadas com dados de AUDPC do que com a severidade final da doença em três das quatro safras. No entanto, considerando a média das quatro safras de cultivo, a estimativa da eficácia do controle dos fungicidas com base no AUDPC foi de 48,5 ± 11,2% (variando de 36 a 65%), semelhante à estimativa baseada na severidade final da doença (43,3 ± 14,0%, variando de 29 a 66%). Portanto, para simplificar e reduzir custos experimentais, a eficácia do fungicida para o manejo da ferrugem da soja pode ser estimada com base na severidade final da doença.

Palavras-chave
Glycine max ; quantificação de doenças; métodos patométricos; Phakopsora pachyrhizi

Asian soybean rust (ASR), caused by the basidiomycete fungus Phakopsora pachyrhizi Sydow. & Sydow (66 Sydow, H.; Sydow, P. Beitrag zur Kenntnis der parasitischen Pilze der Insel Formosa. Annales Mycologici, Berlin, v. 12, p. 105-112. 1914.) in soybean plants [Glycine max (L.) Merr.], was first reported in South America in the 2001 growing season (55 Morel, W. Roya de la soja. Comunicado técnico – Reporte oficial. Serie Fitopatológica no.1 – Junho de 2001. Ministerio de Agricultura y Ganaderia. Subsecretaria de Agricultura. Dirección de Investigación Agrícola. Centro de Investigación Agrícola (CRIA) Capitán Miranda, Itapúa, Paraguay.).

Experiments conducted to identify the most efficacious fungicide treatments or treatment programs that result in economic ASR control have proposed several methods to estimate control levels and consequently determine the efficacy of fungicides. Each of those studies adopt multiple methods to quantify ASR severity and its effects on the host, which makes the collection of disease severity data time-consuming and laborious, poorly contributing to interpretation. As a common feature, most of those studies have included estimates of the area under the disease progress curve (77 Vale, F.X.R.; Jesus Jr, Liberato, J.R.; Souza, C.A. Quantificação de doenças e do crescimento do hospedeiro. IN: do Vale, F.X.R.; Jesus Jr, W. C.; Zambolin, L. (ED..). Epidemiologia aplicada ao manejo de doenças de plantas. Belo Horizonte: Editora. Perfil. P. 89-123, 2004.).

Asian soybean rust quantification demands considerable time and human resources. To determine the AUDPC, rust intensity (incidence or severity) on the leaves is measured in sequential evaluations at various soybean development stages. Some of the above-mentioned studies used only AUDPC values corresponding to each treatment, not reporting the control efficacy levels. Fungicide trials with the aim of identifying economic treatment programs require that control efficacy values are estimated for comparison.

In the current study, we hypothesize that control values estimated based on the AUDPC are similar to those obtained with the final disease severity level.

The analyzed data were obtained from a national cooperative fungicide trial conducted in Rio Verde, Goiás State, in the 2013/14, 2014/15, 2015/16 and 2016/17 growing seasons. Asian soybean rust severity was assessed at four phenological stages, and the AUDPC was calculated. On average, in each season, 16 different fungicides were tested for their effects on rust severity. Ten central trifoliate leaves were removed from the main stem of the plant and used for severity assessment. The AUDPC was calculated based on four periods, and final severity data underwent statistical analysis to compare the control efficacy estimated based on AUDPC with that obtained based on final severity. Fungicide efficacy, determined by ASR control level, was calculated as [1-(disease severity for the corresponding treatment/disease severity for the unsprayed control)*100].

For the 2013/14 growing season, final severity ranged from 10% to 59% (unsprayed) and control efficacy varied from 9% to 82%. The AUDPC ranged from 309 to 1398 (unsprayed) units, resulting in fungicide control efficacy of 19% to 78%. The general means of fungicide efficacy level were 35% based on final disease severity and 41% based on AUDPC, which were statistically different at p≤0.05 (Table 1).

Table 1
Asian soybean rust control levels calculated based on final disease severity and AUDPC data from 2013/14 season.

Regarding the 2014/15 growing season, final ASR severity ranged from 5% to 75% (unsprayed) and control levels varied from 33% to 94%. The AUDPC ranged from 43 to 532 (unsprayed) units and control efficacy varied from 33% to 92%. The general means of fungicide control level were 66% based on final disease severity and 65% based on AUDPC, which were not significantly different at p≤0.05 (Table 2).

Table 2
Asian soybean rust control efficacy calculated based on final disease severity and AUDPC data from 2014/15 season.

For the 2015/16 soybean growing season, final ASR severity ranged from 40% to 100% (unsprayed) and fungicide control levels varied from zero to 60%. The AUDPC ranged from 576 to 1591 (unsprayed) units and control varied from 15% to 64%. The mean fungicide efficacy level based on final severity was 29% and that based on AUDPC was 36%, which were statistically different at p≤0.05 (Table 3).

Table 3
Asian soybean rust control calculated for final severity and AUDPC data from 2015/16 season.

Considering the 2016/17 growing season, final severity ranged from 30% to 93% (unsprayed) and control efficacy varied from 6% to 68%, while the AUDPC ranged from 462 to 1425 (unsprayed) units and fungicide control levels varied from 33% to 68%. The mean fungicide efficacy level based on final severity was 43% and that based on AUDPC was 52%, which were statistically different at p≤0.05 (Table 4).

Table 4
Asian soybean rust control levels calculated based on final disease severity and AUDPC data from 2016/17 season.

According to the overall analysis of the four seasons, fungicide efficacy (control efficacy) calculated based on the final severity ranged from 29% to 66%, presenting an overall mean of 43.25(±14.04)%. Similarly, fungicide control levels obtained with the AUDPC ranged from 36% to 65%, showing an overall mean of 48.5(±11.15)%. Thus, considering the data from the four growing seasons, fungicide efficacy values estimated based on the final disease severity were similar to those calculated with the AUDPC.

The AUDPC has been recommended especially to: (i) determine the damage caused by plant diseases (55 Morel, W. Roya de la soja. Comunicado técnico – Reporte oficial. Serie Fitopatológica no.1 – Junho de 2001. Ministerio de Agricultura y Ganaderia. Subsecretaria de Agricultura. Dirección de Investigación Agrícola. Centro de Investigación Agrícola (CRIA) Capitán Miranda, Itapúa, Paraguay.); (ii) assess the reaction of cultivars (33 Jeger, M.J.; Viljanen-Rollinson, S.L.H. The use of the area under the disease-progress curve (AUDPC) to assess quantitative disease resistance in crop cultivars. Theoretical and Applied Genetics, Wien, v. 102, n.1, p. 32–40, 2001., 44 Jesus Jr, W. C.; do Vale, F.X.R.; Bergamin Filho, A. Quantificação de danos e perdas. IN: do Vale, F.X.R.; Jesus Jr, W. C.; Zambolin, L. (ED..). Epidemiologia aplicada ao manejo de doenças de plantas. Belo Horizonte: Editora. Perfil. P. 273-297, 2004.), and (iii) compare data obtained in several years and places or with different disease management strategies. Data on the AUDPC, if not expressed as control efficacy levels, are more difficult to interpret.

It must be considered that >80% control efficacy is required for matching the fungicide spraying costs in soybean crops. Another important outcome from our long-term study is that the majority of the tested fungicides and co-formulations did not control ASR (63 out of 70 tested fungicide treatments, approximately 90%, showed low ASR control efficacy). This is a very serious scenario for ASR management in Brazil, indicating that spraying of fungicides with these actives should not be recommended by extension plant pathologists.

Therefore, in the current case study on ASR, control efficacy calculated based on the final severity (one single evaluation) can be considered similar to that obtained with the AUDPC (four evaluations in a season). Thus, to simplify and reduce experimental costs, there is no need for estimating the AUDPC in studies involving fungicide efficacy trials for the management of soybean rust.

REFERENCES

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    Jesus Jr, W. C.; do Vale, F.X.R.; Bergamin Filho, A. Quantificação de danos e perdas. IN: do Vale, F.X.R.; Jesus Jr, W. C.; Zambolin, L. (ED..). Epidemiologia aplicada ao manejo de doenças de plantas. Belo Horizonte: Editora. Perfil. P. 273-297, 2004.
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    Morel, W. Roya de la soja. Comunicado técnico – Reporte oficial. Serie Fitopatológica no.1 – Junho de 2001. Ministerio de Agricultura y Ganaderia. Subsecretaria de Agricultura. Dirección de Investigación Agrícola. Centro de Investigación Agrícola (CRIA) Capitán Miranda, Itapúa, Paraguay.
  • 6
    Sydow, H.; Sydow, P. Beitrag zur Kenntnis der parasitischen Pilze der Insel Formosa. Annales Mycologici, Berlin, v. 12, p. 105-112. 1914.
  • 7
    Vale, F.X.R.; Jesus Jr, Liberato, J.R.; Souza, C.A. Quantificação de doenças e do crescimento do hospedeiro. IN: do Vale, F.X.R.; Jesus Jr, W. C.; Zambolin, L. (ED..). Epidemiologia aplicada ao manejo de doenças de plantas. Belo Horizonte: Editora. Perfil. P. 89-123, 2004.

Publication Dates

  • Publication in this collection
    06 June 2022
  • Date of issue
    Jan-Mar 2022

History

  • Received
    06 Nov 2017
  • Accepted
    28 Nov 2019
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