How to maximize results from Asian rust management

The main disease in soybean crops, Asian rust always draws attention due to the potential for damage, the aggressiveness of the fungus and the demands regarding control. The addition of multisite fungicides to treatments and the identification of the appropriate interval between applications tend to maximize management results.

The intensification of the use of fungicides in Brazil occurred following the entry of Asian soybean rust, caused by Phakopsora pachyrhizi Syd. & P. Syd, in 2001. Since then, the disease has been the main disease of the crop in the country, causing 90% damage. Initially, the fungicides used for control had molecules with site-specific action, that is, the chemical compound acts only on one or a few processes, as is the case with triazoles and strobilurins.

However, with the emergence of less sensitive populations of the fungus, a decrease in the control efficiency of these products has been observed, as indicated by results obtained by the Antirust Consortium and the Chapadão Foundation. The use of products with multisite action, such as mancozeb, can provide additional benefits for disease control, and also for resistance management.

In order to verify the effect of adding multisite fungicides to control rust and determine the best management program for the disease, Fundação Chapadão carried out trials to evaluate the behavior of the fungicide mancozeb 750g/l, associated with azoxystrobin + cyproconazole, 200g /l + 80g/l, in different periods and intervals between applications.

Experiments

The trial was conducted in the 2014/15 harvest, at the experimental unit of Fundação Chapadão, located in Chapadão do Sul, Mato Grosso do Sul. The objective was to evaluate the best interval between fungicide applications (azoxystrobin + cyproconazole, 200g/l + 80g /l) added or not to the fungicide (Mancozeb, 750g/l), for the control of Asian soybean rust, caused by P. pachyrhizi. For this, applications were carried out starting at different stages of development, combined with application intervals, varying between 14 days and 21 days. The trial included 12 treatments, plus the control without application (Table 1). The design adopted was randomized blocks, with four replications. The Valiosa cultivar was used, sown on 21/11/2014. The fungicide applications of the treatments used a spray volume equivalent to 150L/ha, and in all, except the control, Nimbus mineral oil was added, at a dose of 0,6L/ha.

To identify the moment when the disease began in the area, soybean leaves were collected from control plots three times a week and sent to a disease analysis and diagnosis laboratory. Severity assessments began upon detection of the disease. The estimate of the percentage of injured leaf area was given from the evaluation of ten leaves collected at four different points in each plot. The evaluators assigned the scores following the severity assessment scale proposed by Godoy, Koga and Canteri (2006). The results were subjected to analysis of variance and Scott-Knott test, at 5% significance, using the Sasm-Agri software (Canteri et al.

Results Application interval trials have been conducted by the Chapadão foundation since the 2010/11 harvest, but without the addition of mancozeb treatments. The summarized results for these works are found in Figure 1. 

It was observed during this period that treatments with shorter application intervals presented better productivity results.

The onset of the disease in the 2014/15 harvest, diagnosed via laboratory, occurred on February 2nd with the culture at stage R4. Assessments began in R5.1, occurring at weekly intervals (Table 2). The results of the severity assessments demonstrated the rapid progression of the disease after its identification, reaching an average severity of 95% in the control. Table 1 shows the evolution of the disease and defoliation in the control.

In the evaluation of severity at R5.5, treatments containing applications at R1, R1+14, R1+28, R1+42 and R1+56 showed the lowest percentages of severity, however the addition of mancozeb did not imply a significant difference in severity, in relation to treatment with only azoxystrobin + cyproconazole. For treatments with application intervals of 21 days (V8, R1, R1+21, R1+42 and R1+56 days and R1, R1+21, R1+42 and R1+56 days) and treatments with a lower number of applications (R1, R1+21 and R1+36), the use of mancozeb associated with azoxystrobin + cyproconazole contributed significantly to the lower rust severity, compared to treatments with only azoxystrobin + cyproconazole (Table 2). The evolution of the disease was shorter in treatments with mancozeb, indicating a possible additional effect on the residual disease control.

The area under the AACPD disease progress curve and defoliation were significantly lower compared to the control for all treatments (Table 3). The lowest values ​​were presented by treatments with the shortest application interval R1, R1+14, R1+28, R1+42 and R1+56 days (Table 3, Figure 1).

The highest productivity averages were found in treatments with applications starting at R1, and at intervals of R1, R1+14, R1+28, R1+42 and R1+56 (treatments 8 and 9) and R1, R1+21, R1 +42 and R1+56 (treatments 12 and 13). These results may be linked to the emergence of the disease only in R4. In general, applications that persisted until 56 days after the R1 stage allowed greater crop protection and less disease progression, resulting in greater productivity.

In a trial testing only the application of a multisite fungicide against a mixture of triazole + strobilurins (cyproconazole + azoxystrobin), Silva et al (2015) found a significant increase in productivity for treatments with isolated application of mancozeb. The authors also mention the potential of adding mancozeb to other fungicides. 

Conclusion

Before the adoption of multisite fungicides, treatments with shorter intervals between applications showed better productivity results. In the multi-site trial in the 2014/14 harvest, under conditions of a late onset of the epidemic, applications in the final stages of the crop cycle promoted better control of the disease and increased productivity. The addition of mancozeb to azoxystrobin + cyproconazole applications reduced the speed of the epidemic, evidenced by the severity of the treatments, indicating a possible additional effect of the multisite on the residual disease control.

Lucas Henrique Fantin, João Vitor Andrade, Marcelo Giovanetti Canteri, Edson Pereira Borges, Alfredo Riciere Dias, State University of Londrina, Londrina, PR., Fundação Chapadão - Chapadão do Sul, MS

Article published in issue 199 of Cultivar Grandes Culturas.