EFFICIENCY OF FUNGICIDES FOR OIDIUM CONTROL IN SOYBEAN CROP

One of the most relevant factors in soybean productivity is fungal diseases such as oidium (Microsphaera diffusa), which can be controlled through resistant cultivars or preventive control through the application of fungicides. The aim of the present study was to evaluate the efficiency of chemical fungicides for oidium control and their effects on yield and weight of one thousand seeds (WTS) of soybean crop. Seven treatments were conducted with fungicides for oidium control, and control treatment without fungicide (Treatment 8). Fungicides were effective in controlling oidium incidence and severity and also increased yield. Fungicides tested were: Picoxystrobin 100 gL-1 + Benzovidiflupir 50 gL-1 (Treatment 1), Trifloxystrobin 150 gL-1 + Proticonazol 175 gL-1 (Treatment 2), Fenpropimorph 750 gL-1 (Treatment 3), Picoxystrobin 200 gL + Ciproconazol 80 gL-1 (Treatment 4), Azoxystrobin 300 gL-1 + Benzovindiflupir 150 gL-1 (Treatment 5), Mancozeb 750 g.kg-1 (Treatment 6), Metominostrobin 110 gL-1 + Tebuconazol 165 gL-1 (Treatment 7). A difference of 962 kg.ha-1 (16 sc.ha-1) was observed between the most efficient treatment (T3 Fenpropimorph 750 g.L-1) and the control treatment.


INTRODUCTION
Soybean (Glycine max) is the oilseed most cultivated in Brazil, being of great importance for the generation of income, heat up the domestic economy and generation of surplus in the trade balance. According to CONAB (2018), a planted area of 35.794 million hectares is estimated for the 2018/2019 crop season, reaching production of 118.019 million tonnes. Among Brazilian states, Rio Grande do Sul is the third largest soybean producer, reaching an estimated area of 5.777 million hectares and production about 18.690 million tonnes with an average yield of 3.235 kg. ha -1 (53.9 sc. ha-1) (CONAB, 2018).
Oidium, caused by fungus Microsphaera difusa, stands out as one of the most economically important diseases in soybean crop (Barbosa, 2017). Mild temperatures favor its occurrence. According to Alves et al. (2009), temperature from 23 to 24°C associated with leaf wetting period of 8 h provided the maximum progress of its severity. Other factors such as medium to high air humidity (50 to 90%) and low incidence and intensity of precipitation at the beginning of flowering and seed formation are pointed out by Blum et al. (2002). Since 1996, from the Southern region to the Southeastern and Midwestern regions of Brazil, several outbreaks of this disease have been reported (Michel et al., 1998). In particular, when environmental conditions are favorable to disease development, the occurrence of oidium in soybeans may affect the crop and cause significant losses in grain yield (Toigo et al., 2008).
In leaves, the white color of the fungus turns to a grayish-brown color over time, giving the appearance of dirt covering both sides of the leaf. Under conditions of severe infection, mycelium development and fungus fruiting prevent photosynthesis and leaves dry and fall prematurely, giving the crop the appearance of soybean desiccated by herbicide, and color turns from grayish-brown to bronze (Yorinori, 1997).
Genetic resistance is considered the preferred method to control diseases, since it is a strategy of greater durability and does not impact production costs (Maciel and Danelli, 2018). On the other hand, several cultivars that were resistant to oidium became susceptible, demonstrating the fungi variability. In this case, one the most efficient methods is chemical control, which until now is the main control method. Information on the efficiency of fungicides is increasingly essential and necessary to guide and select products with correct use and guidance in the field (Yorinori, 2002).
Due to the significant productivity losses that may be associated to the occurrence of oidium in soybean crops, as well as the costs related to the use of pesticides for disease control, investigating the efficiency of fungicides in the control of oidium is of great importance. The aim of the present study was to evaluate the efficiency of fungicides in the control of oidium and its effects on yield and weight of one thousand seeds (WTS) of soybean crop.

MATERIAL AND METHODS
The experiment was conducted in a notillage area on oat straw in commercial soybean monoculture in the municipality of Estação -RS, in the "Alto Uruguai" region, with coordinates 27º 53 '53''S and 52º 15' 07'W and altitude of 754 m a.s.l. Oat desiccation was performed 21 days before sowing with the use of Paraquat herbicide at dose of 2.5 g. ha¹ plus adjuvant recommended for the product at dose of 0.5 l.ha¹. The cultivar used was Active BMX RR ® of maturation group 5.6 with determined growth habit and medium size. Sowing was carried out on October 28, 2017, with Semeato® planter of 09 rows with mechanical seed distribution system and disk furrower and spacing of 0.45 meters pulled by a Massey Ferguson tractor model 292 ®, with sowing speed of 6 km.h -1 . Planter was regulated to deposit 16 seeds per linear meter, totaling approximately 300,000 plants/ha¹. Seed treatment was carried out at dose of 1mL.L seed -1 (pyraclostrobin-25 g.L -1 , methyl thiophanate 225 g.L -1 , fipronil 250 g.L -1 ). Base fertilization was carried out with 400 kg. ha¹ of NPK 02.23.23 fertilizer distributed in the sowing furrow according to soil analysis.
A completely randomized design (CRD) was used, consisting of 8 treatments and 4 replicates for each treatment. Each Experimental Unit (EU) presented total area of 12.5 m² (5.0 x 2.5m) and spacing between plots of 2.0m, totaling 32 experimental units ( Figure 1). Table 1 shows the treatments evaluated for the control of oidium in soybean crops, which were composed of different active ingredients and their respective doses recommended for the control of the disease. The first fungicide application was carried out at vegetative stage V6 (sixth node, sixth fully developed trifoliate leaf). At 21 days after the first application, the second application was performed at reproductive stage R1 (beginning of flowering); the third application was performed after 14 days at reproductive stage R4 (fully developed pod) and the fourth and last applications were performed 14 days after reproductive stage R5 (beginning of grain filling).
Applications were carried out with costal CO 2 sprayer, with application volume of 150L.ha -¹ and constant pressure of 3.5 bar. Application bar 2.5 meters long, with four spray tips ATR 80 full cone type with the purpose of applying 20 drops per cm² with spacing of 0.50 cm. Application height was 50 cm on the plant and displacement velocity used in fungicide application was 1 m.s -1 . Anti-drift and adjuvant were used in all applications at dose of 0.100 L. ha¹.
In the present study, diagrammatic scales based on Polizel and Juliatti (2010) were developed to aid in the identification of oidium incidence and severity (Figures 2 and 3). Evaluations of oidium incidence and severity were carried out on the days of fungicide application at reproductive stages R5 (beginning of grain filling) and R6 (full grain). Three random evaluations were performed in the useful area of each replicate, and in each evaluation, 10 leaflets in the lower third, 10 leaflets in the middle third and 10 leaflets in the upper third of the plant were collected, totaling 30 leaflets within the plot. In each evaluation, 960 leaflets were evaluated.
Harvest was carried out on April 21, 2018, in the two central rows of each plot, contemplating the length of five linear meters, totaling an area of 4.5 m² per plot. Harvest was manually performed, and plants were manually husked and cleaned with the aid of a soybeanspecific sieve, and after cleaning, grains were bagged in paper bags and labeled according to the respective treatment and plot and taken to the IDEAU laboratory, Campus of Passo Fundo for moisture correction to 13% and weighed with the aid of a precision scale to determine the weight of one thousand seeds (WTS) and yield in Kg.ha -1 . Data obtained such as productivity, weight of one thousand seeds (WTS), oidium incidence and severity were submitted to analysis of variance with the Tukey test at 5% error probability, with the aid of the SISVAR statistical software (Ferreira, 2000).

RESULTS AND DISCUSSION
The mean oidium incidence and severity values obtained at reproductive stages R5 and R6 stages are presented in Table 2.  In T3 treatment (Fenpropimorph 750 g.L -1 ), the lowest oidium incidence and severity values in soybean crops were observed, providing the best results, which combined the lowest incidence and severity values of the disease under study (Table 1). T5 treatment (Azoxystrobin 300 gL -1 + Benzovindiflupir 150 gL -1 ) provided results equivalent to those observed with T3 treatment for the incidence and severity indexes, but not differing from T4 treatment (Picoxystrobin 200 gL -1 + Ciproconazole 80 gL -1 ) for the incidence parameter.
Despite the importance of controlling the incidence and severity of phytopathies for crop health, the use of chemical control of diseases through the application of fungicides is aimed at increasing grain yield (Cunha et al., 2008). Results regarding weight of one thousand seeds (WTS) and yield are presented in table 3. Table 3. Average yield and weight of one thousand seeds (WTS) of soybean crop submitted to different treatments for oidium control. 1.94 7.67 Means followed by the same letter in the column did not differ significantly from each other by the Tukey's test (P <0.05).
T3 (Fenpropimorph 750 g.L -1 ), in which the lowest incidence and severity indexes were observed, was the treatment with the best weight of one thousand seeds (WTS) values, being superior to all other treatments for this parameter. In addition, with the exception of T6 treatment (Mancozebe 750 g kg -1 ), all treatments provided better WTS than control.
Regarding grain yield, T3 treatment (Fenpropimorph 750 g.L -1 ) was the only treatment superior to control, not differing from the other treatments. Grain yield is associated to greater photosynthetically active foliar area, due to the high sanity of leaves, which certainly favors the best use of solar radiation and consequently contributes in the best way to grain filling. On the other hand, in T8 treatment (control), in which the highest oidium incidence and severity were observed, the lowest grain yield and WTS values were also verified. In this sense, according to McGee (1992), the reduction of the photosynthetic layer of the leaf by the action of the fungus is associated with reduced grain yield.
It was observed that Microsphaera difusa fungus has a very great damage potential, as evaluated in the experiment, with difference of 962 kg ha -1 (16 sc.ha -1 ) between the most efficient treatment (T3 -Fenpropimorph 750 gL -1 ) and control treatment. It was concluded that even if the farmer has expenses with disease prevention, the simple fact of preventing the potential of disease damage will compensate in terms of greater profitability. In this sense, it is observed that treatments are efficient enough to cover prevention costs. Similar results were observed by Orso et al. (2018), regarding the increase in soybean crop productivity due to the chemical control of oidium incidence.
Regarding productive parameters, greater weight of one thousand seeds (WTS) was observed for T3 treatment (Fenpropimorph 750 g.L -1 ) compared to the other treatments, being also efficient to increase grain yield.