Reduced Asian Soybean Rust Control by Commercial Fungicides Co-formulations in the 2018-2019 Growing Season in Southern Brazil

It has been a growers concen the reduction of Asian soybean rust (ASR) control by commercial fungicide co-formulations in the last growing seasons in southern Brazil. The objective of this work was to assess the ASR control efficacy by the most used co-formulations in the 2018/19 season. In a field experiment, 19 fungicides in commercial formulations to control soybean rust caused by Phakopsora pachyrhizi, were evaluated. Chemicals at their recommended doses were sprayed at four soybean growth stages. The first application was performed with 1.82% leaflet incidence and coinciding with R1 phenological stage. The others were performed at 14-18 days intervals. At stage R6, end of the epidemic and coinciding with half of the defoliation in the control plots, the leaf severity was appraised. The experiment was conducted with Ativa soybean cultivar, in 3 × 6 m plots, four replications and randomized block design. The harvest was made with a plot combine and the yield expressed in grains kg/ha. The means were compared by the Scott-Knott test. The disease control efficacy by 17 fungicide co-formulation showed control less than 57%, one with 78% and none with ≥ 80%. The unsprayed treatment severity was 81% and the greatest control of 78% resulted in 3,876 kg/ha yield. Therefore, the hypothesis raised in this work was accepted showing that the site-specific fungicides co-formulations are showing efficacy reduction season after season.

The ASR control failure in Brazil was first reported six seasons after the begginning of rust control with the DMI tebuconazol (Fundação MT em Campo, 2008;Silva et al., 2008;Reis et al., 2017). In the first seasons of its use solo, control was > 80% and in the 2027/18 season, but in the national fungicide trials, its efficacy was reduced to 22% (Reis et al., 2017). Since than on, the evolution of the control reduction season-after-season tor the three MOA site-specific DMIs, QoIs, and SDHIs has been reported and did not stop yet. Co-formualations containing DMI, QoI and SDHI, in double or triple mixes, have been sprayed in a large soybean grown area without the additon of multi-sites, the main reason for the reduction of fungicides efficacy.
The hypothesis raised in this work was that the site-specific MOA fungicides co-formulations are showing efficacy reduction each season reaching in the last < 50% control.
Objective of this work was to assess the ASR control efficacy by commertial co-formulations in the 2018/19 season in Southern Brazil.

Materials and Methods
The experiment was carried out at Fazenda Carvalho ERS-324, Km  The experimental design was a randomized block with 19 treatments, four replications, with 3.5 m (6 rows) wide and 6.0 m long corresponding to 21.0 m 2 plots.
For rust assessment two pathometric methods were used: (a) For rust detection in the experimental area, weekly from V5 GS, five plants from the experiment borders were collected. Only central leaflets with leaves petioles inserted in the main plant stem were removed and assessed according to Ogle et al. (1979) and rust incidence was assessed under stereo microscope (30-50x); (b) The treatments effects on rust control were assessed based on leaflet severity. Severity notes, considering the percent leaflet area with symptoms/signs, were assigned in the central row according to Godoy et al. (2005).  Figure 4. Negative relationship between soybean grain yield and Asian soybean rust leaflet severity ASR damage, sensu Nutter et al. (1993), was calculated by the difference between the highest yield (3,876 kg/ha) and the yield in each treatment. The negative relationship between damage (kg/ha) and ASR control (%) was represented by the function y = -13.45x + 931.12 (R 2 = 0.819). For each percent point control reduction there was a 13.45 kg grain reduction for a maximum 931.12 kg/ha damage ( Figure 5). The highest yield 3,826 kg/ha was achieved with 78% control and considered without damage (Table 2). Thus, taking for example the lowest and the highest fungicides control in Figure 3, damage can be appraised with the generated function: for 9% control, 810.07 kg damage, and for 57% control, 164.47 kg reduction.

Figure 5. Negative relationship between Asian soybean rust damage and control
Related to the damage caused by plant diseases most of the published papers quantified the yield reduction only by adjectives instead of numbers and the used methodology for damage quantification is not described. The damage can be appraised knowing the relationship between different disease intensities and the resulting damage (Sah & MacKenzie, 1987). In our study, considering the maximum ASR severity of 83%, the maximum yield 3,876 kg/ha and the unsprayed treatment yield of 1,355 kg/ha, therefore the reduction was 1,355 kg/ha or 34.95%, lower than those mentioned in the literature. Each severity point reduced 17.0 kg in a 3,876 kg/ha actual gross yield.
In the first spraying cost was considered soybean kneading by sprayer wheels in commercial farms (32 m long boom, 35 cm width tires), fuel (75 HP engine), labor, fungicide price (dose/ha), yield potential kg/ha), soybean prince (one US$ = R$ 5.20; US$ 26.47/60 kg, Dec., 2020-Cotrijal, Não Me Toque county, Rio Grande do Sul state, Brazil), totaling US$ 46.00/ha, or 104 kg/ha soybean. In the calculation the costs were transformed into soybean grains weight (kg/ha) ( Table 2).  Note. v Total cost: kneading by sprayer wheels, fuel, labor, fungicide price (four spraying/ha), yield potential kg/ha, soybean price (US$ 26.47/60 kg); x Damage: the highest gross yield 3,876 kg/ha minus the yield in each treatment; y Gross yield: actual yield; z Net yield: Gross yield minus the spraying cost (kg/ha). Means follow by the same letter in the columns do not differ by Scott-Knott's test.
Our work shows that the profit is relate to the control efficacy and cost of application (Table 2).
In the last growing seasons experiments have shown a reduction in the efficacy of Asian soybean rust control by commercial fungicides (Reis et al., 2017). This statement can be confirmed by the mixtures performance with control less than 50% (Figure 3). Most of the commercial chemicals used by growers do not contain in their formulation multisite such as chlorothalonil, mancozeb or copper oxychloride (Figure 2), and there is a need for tank mixing which is little used. This has delayed the use of multisite across the soybean grown area and in all sprayings. Therefore, there is a need to shorten the time for all fungicides sold for soybean rust control to contain multisite in their formulation, as happened with those used to potato, tomato and grapevine downy mildew control (Duvauchelle & Ruccia, 2015).