Alternative products alone and associated with fungicide to control Phakopsora pachyrhizi ABSTRACT
The aim of the present study was to evaluate the effectiveness of Asian soybean rust (Phakopsora pachyrhizi) management, considering the influence of sowing time and the use of alternative products, either alone or in combination with fungicides. Field experiments were conducted over three growing seasons (2015/2016, 2016/2017, and 2017/2018) with the soybean cultivar ‘NA 5909’. The treatments were: (1) control (water); (2) Bacillus subtilis; (3) B. subtilis combined with fungicide (azoxystrobin + benzovindiflupyr); (4) chitosan 1%; (5) chitosan 1% combined with fungicide; (6) elemental sulfur; (7) elemental sulfur combined with fungicide; (8) sodium hypochlorite; (9) sodium hypochlorite combined with fungicide; (10) fungicide applied at two stages, and (11) fungicide applied at three stages. Evaluations included disease severity, defoliation, yield components, and grain yield. In the three growing seasons, sowing in December was consistently associated with greater disease severity. In all seasons and sowing times, treatments with sulfur and chitosan applied alone, as well as alternative products combined with fungicide, and fungicide applied at two or three stages, reduced the disease severity by 75.85%. Defoliation was reduced by sulfur alone, alternative products combined with fungicide, and fungicide applied at two and three stages, in all seasons and sowing times. The most affected yield components were number of pods per plant and thousand-grain weight. In all three seasons, the yield was reduced when sowing occurred in December. Application of fungicide, sulfur and alternative products combined with fungicide prevented yield losses in all sowing times and seasons. Notably, sulfur combined with fungicide was equivalent to the fungicide applied at three stages and could therefore be considered a viable strategy for disease management.
Keywords
Glycine max
;
Bacillus subtilis
; chitosan;
Phakopsora pachyrhizi
; sulfur; productivity
RESUMO
O objetivo deste estudo foi avaliar a eficácia do manejo da ferrugem-asiática da soja (Phakopsora pachyrhizi), em função da época de semeadura e do uso de produtos alternativos, aplicados isoladamente ou em combinação com fungicida. Os experimentos de campo foram conduzidos ao longo de três safras (2015/2016, 2016/2017 e 2017/2018), utilizando a cultivar de soja NA 5909. Os tratamentos foram: (1) controle (água), (2) Bacillus subtilis, (3) B. subtilis combinado com fungicida (azoxistrobina + benzovindiflupir), (4) quitosana 1%, (5) quitosana 1% combinada com fungicida, (6) enxofre elementar, (7) enxofre elementar combinado com fungicida, (8) hipoclorito de sódio, (9) hipoclorito de sódio combinado com fungicida, (10) fungicida aplicado em duas fases e (11) fungicida aplicado em três fases. As avaliações incluíram severidade da doença, desfolha, componentes de rendimento e produtividade de grãos. Nas três safras avaliadas, a semeadura em dezembro esteve consistentemente associada a maior severidade da doença. Em todas as safras e épocas de semeadura, os tratamentos com enxofre e quitosana aplicados isoladamente, bem como os produtos alternativos associados ao fungicida e o fungicida aplicado em duas ou três fases, reduziram a severidade da doença em 75,85%. A desfolha foi reduzida pelos tratamentos com enxofre isolado, por produtos alternativos combinados com fungicida e pelo fungicida aplicado em duas e três fases, em todas as safras e épocas de semeadura. Os componentes de rendimento mais afetados foram o número de vagens por planta e a massa de mil grãos. Nas três safras, houve redução de produtividade na semeadura em dezembro. A aplicação de fungicida, enxofre e produtos alternativos associados ao fungicida preveniu perdas de produtividade em todas as épocas de semeadura e safras. Notadamente, o tratamento com enxofre associado ao fungicida foi equivalente ao fungicida aplicado três vezes, podendo, portanto, ser considerado uma estratégia viável para o manejo da doença.
Palavras-chave
Glycine max
;
Bacillus subtilis
;
Phakopsora pachyrhizi
; quitosana; enxofre; produtividade
Soybean (Glycine max (L.) Merrill) is one of the major crops considering global agriculture (7, 26). In Brazil, the suitable period for soybean sowing is between October and December, particularly in regions south of the Equator. (22). The yield of soybean cultivars is influenced by sowing time, which directly affects productivity, as well as disease occurrence and severity. Late sowings tend to face greater inoculum pressure and consequently greater disease severity (13).
Early planting is the recommended management strategy in Brazil to reduce the impact of Asian soybean rust (Phakopsora pachyrhizi Syd. & P. Syd.) (18, 29). This is considered the most detrimental disease to soybean; reported yield losses are between 10% and 90% (14, 29). Chemical control remains the predominant approach; however, extensive and repeated application can cause the emergence of fungicide-resistant fungal populations, thereby increasing the production costs and complicating the disease management (6).
Agricultural production technologies are under social pressure for sustainable food production without toxic residues. Alternative products offer advantages over synthetic ones for generating novel compounds that cannot be easily overcome by pathogens, besides showing lower toxicity to animals, rapid environmental degradation, and a broad mode of action (20).
However, the isolate use of alternative products for disease control may be ineffective (9). Integrating alternative products with chemical control can enhance overall efficacy and extend the effective lifespan of commercially available products (9).
Sowing time is hypothesized to affect Asian soybean rust severity since later sowings tend to result in higher disease rates. In addition, alternative products, such as elemental sulfur and chitosan, show limited effectiveness on disease control when applied alone but improve the disease management when combined with fungicides. Furthermore, this combination is expected to reduce disease severity and defoliation, besides helping preserve productivity over different sowing periods.
The aim of the present study was to evaluate the effects of foliar applications of alternative products, alone or in combination with fungicides, in two sowing dates (October and December) and multiple seasons, on Asian soybean rust severity, yield components, and overall crop productivity under monoculture and no-tillage systems.
MATERIALS AND METHODS
The experiments were conducted at “Fazenda Escola Capão da Onça”, which belongs to the State University of Ponta Grossa, Paraná State, Brazil. Its geographic reference coordinates are 25º50’58” S and 50º09’30” W; average altitude of 975 m above sea level, and the climate is classified as Cfb, according to the Köppen system.
Two sowing times (October and December) were adopted in three seasons (2015/2016, 2016/2017 and 2017/2018), using the cultivar NA 5909, row spacing of 0.45 m and 15 seeds per meter to obtain a density of 12 plants m-1 and a final population of 250,000 plants ha-1. The field plots were 4.5 x 4.0 m, totaling 18 m². Experimental design was a randomized complete block design with 11 treatments and four replicates.
Treatments consisted of: 1- control (water; stages V4, V6, R1 and R5.1); 2- Bacillus subtilis lineage QST (stages V4, V6, R1 and R5.1; 3 L a.i. ha-1); 3- Bacillus subtilis lineage QST (stages V4 and V6; 3 L a.i. ha-1) associated with azoxystrobin + benzovindiflupyr (stages R1 and R5.1; 200 g a.i. ha-1); 4- chitosan 1% (stages V4, V6, R1 and R5.1;2 L a.i. ha-1); 5- chitosan 1% (stages V4 and V6; 2 L a.i. ha-1) associated with azoxystrobin + benzovindiflupyr (stages R1 and R5.1; 200 g a.i. ha-1); 6- elemental sulfur S 26% (stages V4, V6, R1 and R5.1; 2 L a.i. ha-1); 7- elemental sulfur (stages V4 and V6; 2 L a.i. ha-1) associated with azoxystrobin + benzovindiflupyr (stages R1 and R5.1; 200 g a.i. ha-1); 8- sodium hypochlorite 2.5% (stages V4, V6, R1 and R5.1); 9- sodium hypochlorite 2.5% (stages V4 and V6) associated with azoxystrobin + benzovindiflupyr (stages R1 and R5.1; 200 g a.i. ha-1); 10- azoxystrobin + benzovindiflupyr (stages R1 and R5.1; 200 g a.i. ha-1), and 11- azoxystrobin + benzovindiflupyr (stages V6, R1 and R5.1; 200 g a.i. ha-1). In all fungicide treatments, Nimbus® (mineral oil, 0.5 v/v) was added as adjuvant.
The timing of the first spraying was determined based on specific phenological stages of the crop, rather than on the observed incidence of Asian soybean rust. The stages V4, V6, R1 and R5.1 correspond to the development of the third trifoliate leaf, the fifth trifoliate leaf, the beginning of flowering (up to 50% plants flowering), and grain formation detectable by touch (approximately 10% pod filling), respectively (23).
Treatments were applied using a CO2 pressurized backpack sprayer equipped with a bar containing simultaneous arrangement of four swing jet nozzles (XR 11002) spaced 0.50 m apart, which had constant pressure of 3 kg cm-2. Application was carried out under suitable climate conditions: relative humidity higher than 60% and temperatures below 30 °C. Pest and weed control were performed as required by the crop.
Asian rust severity was assessed at seven-day intervals throughout the crop cycle, beginning prior to the first spraying at stage V4. Seven plants at the two central rows of each plot were randomly selected for severity assessment on the leaves of the lower, middle and upper thirds of the canopy. Each third of the plant received three severity estimates based on Godoy’s diagrammatic scale (12). Disease severity assessments were employed to calculate the area under the disease progress curve (AUDPC) (27).
Defoliation was evaluated when plants of the control plot reached 80-85% defoliation, which was estimated based on the diagrammatic scale developed by Hirano et al. (15). The yield components evaluated for plants harvested from a one-meter row in each plot were: plants per meter, number of pods per plant, number of grains per pod, and thousand grain weight (TGW).
At the end of the crop cycle, when grain moisture was around 15%, the plants in the useful area of each plot were manually harvested. The grains were weighed, and their moisture content was determined using a universal moisture meter. The obtained values were converted to 13% moisture to estimate grain yield as kg ha- 1.
Data underwent analysis of variance using the F test; significant means were compared according to Scott-Knott test at 5% probability. Analyzes were carried out using the SASM-Agri software (3).
RESULTS
The AUDPC values from sowing in October were lower than those obtained from sowing in December in the three evaluated seasons; reductions were 0.17%, 30.94% and 34.71% for the lower, middle and upper thirds of the canopy, respectively, and 18.82% for the whole plant. Comparing plant thirds, the lowest AUDPC values were obtained for the upper third, whereas the highest values were observed for the lower third, which had an increase of 63.73% in all three seasons and both sowing times (Tables 1, 2 and 3).
Area under the disease progress curve (AUDPC) for Asian rust (Phakopsora pachyrhizi) in the upper, middle and lower thirds, and average for the whole plant after application of the products to soybeans (Glycine max), cultivar NA 5909, sowing in October and December, 2015/2016 and 2017/2018 seasons. Ponta Grossa, Paraná State, Brazil.
Area under the disease progress curve (AUDPC) for Asian rust (Phakopsora pachyrhizi) in the upper, middle and lower thirds, and average for the whole plant after application of the products to soybeans (Glycine max), cultivar NA 5909, sowing in October and December, 2016/2017 season. Ponta Grossa, Paraná State, Brazil.
Area under the disease progress curve (AUDPC) for Asian rust (Phakopsora pachyrhizi) in the upper, middle and lower thirds, and average for the whole plant after application of the products to soybeans (Glycine max), cultivar NA 5909, sowing in October and December, 2017/2018 season. Ponta Grossa, Paraná State, Brazil.
Regarding foliar application of alternative products alone, in the first sowing date and in the first season, sulfur treatment showed significant differences, compared to the control (Table 1). In the second and third seasons, chitosan and sulfur treatments differed significantly from the control (Tables 2 and 3).
The reduction in the AUDPC for chitosan treatment at sowing in October was 21.75%, 28.02% and 35.41% in the first (2015/2016), second (2016/2017) and third (2017/2018) seasons, respectively, compared to the control. Regarding sowing in December, such reductions were 16.65%, 14.68% and 11.11% in the first, second and third seasons, respectively in relation to the control (Tables 1, 2 and 3).
As regards sulfur treatment, in the first sowing date, there was an AUDPC reduction of 39.46%, 46.52% and 46.79% in the first (2015/2016), second (2016/2017) and third (2017/2018) seasons, respectively. In the second sowing date, 23.88%, 24.35% and 24.20% AUDPC reduction was obtained in the first, second and third seasons, respectively (Tables 1, 2 and 3). For both chitosan treatment and sulfur treatment alone, the AUDPC reduction percentage was lower in the second sowing date, probably due to the greater inoculum pressure.
In the three seasons and in the two sowing dates, treatments with B. subtilis and sodium hypochlorite alone only differed from the control when they were associated with the fungicide. Bacillus subtilis treatment associated with the fungicide reduced the AUDPC in the first sowing date by 47.28%, 56.85% and 71.04% in the first (2015/2016), second (2016/2017) and third (2017/2018) seasons, respectively. Regarding the second sowing date, 57.80%, 71.61% and 65.20% AUDPC reduction was obtained in the first, second and third seasons, respectively (Tables 1, 2 and 3). Sodium hypochlorite associated with the fungicide, in the first sowing date, reduced the AUDPC by 41.03%, 48.91% and 70.08% in the first (2015/2016), second (2016/2017) and third (2017/2018) seasons, respectively. In the second sowing date, 57.46%, 67.29% and 61.63% AUDPC reduction was obtained in the first, second and third seasons, respectively (Tables 1, 2 and 3).
The fungicide was effective in reducing the disease severity in both sowing dates and over the three seasons. Alternative products showed superior results when associated with the fungicide than when applied alone (Tables 1, 2 and 3) and were equivalent to two fungicide applications. However, only chitosan and sulfur treatments associated with the fungicide were equivalent to three fungicide applications in all three seasons.
In the first sowing date (Table 1) during the 2015/2016 season, three fungicide applications were more effective than two applications. This is probably due to the greater disease intensity observed in that period (1,061.51 for the control). In the second and third seasons (Tables 2 and 3), there was no significant difference between the number of applications; however, disease intensity was lower in the first sowing date (446.62 and 128.60, respectively), making the additional application unnecessary to reduce severity.
In the second sowing date, there was a difference in severity considering the fungicide treatment. Three applications were more effective than two applications in the first season (276.84) (Table 1). In the second and third seasons (Tables 2 and 3), there was no difference between the number of applications, probably due to the lower disease severity (for the control: 820.10 and 878.28, respectively); in the first season, the disease was more severe (1,307.63 for the control).
In the second sowing time (Tables 1, 2 and 3), only alternative treatments associated with the fungicide differed from the control, equating to two fungicide applications; however, once again sulfur treatment associated with the fungicide was superior and equivalent to three fungicide applications (except in the first season).
There was no effect of isolated alternative treatments on crop defoliation in the first season (Table 4). In the second season (2016/2017), only sulfur treatment differed from the control for sowing in October. In the 2017/2018 season, sulfur and chitosan treatments differed from the control for both sowing times (disease severity was lower in this season; Table 3). Fungicide treatments applied two or three times, and their association with alternative products, were equivalent among seasons and between sowing times for defoliation (Table 4).
Defoliation (%) and yield components based on the products applied to soybeans (Glycine max) and the sowing time. Cultivar NA 5909, in the 2015/16, 2016/17 and 2017/18 seasons. Ponta Grossa, Paraná State, Brazil.
Yield components were affected by the studied treatments (Table 4). The number of pods per plant did not differ between sowing dates, while TGW differed among the three seasons. For sowing in December 2015/2016, there was a difference in both TGW and number of pods. Considering the yields obtained (Table 5), values reduced when the crop was sown in December, comparing the influence of sowing time on seasons (control). The reduction in productivity was 1,241.48 kg ha-1, 972.90 kg ha-1 and 262.81 kg ha-1 in the 2015/2016, 2016/2017 and 2017/2018 seasons, respectively.
Productivity (kg ha- 1) based on the products applied to soybeans (Glycine max) and the sowing time. Cultivar NA 5909, in the 2015/2016, 2016/2017 and 2017/2018 seasons. Ponta Grossa, Paraná State, Brazil.
In the 2015/2016 season, in the first and second sowing dates, average productivity was 2,080.07 and 838.59 kg ha-1 for the control, while maximum productivity was 2,771.49 kg ha-1 and 1,269.96 kg ha-1 for isolated alternative products and 3,988.19 kg ha- 1 and 2,744.41 kg ha-1 for the fungicide, respectively (Table 5). In this season, fungicide application led to a reduction of 47.84% in damage and 69.44% in productivity, compared to control, in the first and second sowing times, respectively (Table 5).
During the 2016/2017 season, in the first and second sowing times, average productivity was 3,523.01 and 2,550.11 kg ha-1 for the control, while maximum productivity was 4,029.46 and 2,816.28 kg ha-1 for isolated alternative products and 4,622.29 and 3,504.21 kg ha-1 for the fungicide, respectively. Compared to the control, fungicide application resulted in a reduction of 23.78% in damage and 27.23% in productivity in the first and second sowing times, respectively (Table 5).
Considering the 2017/2018 season, in the first and second sowing times, average productivity was 3,206.94 and 2,944.13 kg ha-1 for the control, while maximum productivity was 3,638.89 and 3,392.15 kg ha-1 for isolated alternative products and 4,796.48 and 3,916.20 kg ha-1 for the fungicide, respectively. Compared to the control, fungicide application led to a reduction of 24.82% in damage and 33.13% in productivity in the first and second sowing times, respectively (Table 5).
DISCUSSION
The disease was more severe in the second sowing date (Tables 1, 2 and 3). Reis (22) stated that there is greater inoculum pressure during this period, resulting in increased disease severity. Alternative treatments with chitosan and sulfur have reduced the disease severity and have the potential to be included in crop management to control Asian rust.
According to Zanão Junior & Zambolim (30), sulfur is a lipophilic element that acts through the fungal cell wall, destabilizing the redox reaction of the pathogen’s metabolism. Those authors stated that such a product is considered a contact fungicide, which eliminates and/or eradicates fungal structures on the plant surface, also participating in amino acid and protein formation, photosynthesis process, and in the plant’s defense mechanisms. This may justify the results obtained in the present experiment (Tables 1, 2 and 3).
Chitosan is derived from chitin, which is found in fungi, arthropods and other invertebrates. Commercially, the exoskeleton of crustaceans is the most commonly used source for chitin production (21). Foliar application of chitosan improves the plant growth and yield, induces the synthesis of secondary metabolites such as polyphenols, flavonoids, lignin and phytoalexins, and increases the activities of peroxidase, phenylalanine ammonia-lyase, tyrosine ammonia-lyase and catalase (4).
The antimicrobial potential of chitosan applied to fruits or leaves was demonstrated by Hong (16) in postharvest guava fruits, using concentrations of 0.5%, 1% and 2%. The authors concluded that 2% chitosan formed a protective barrier in the fruits, increasing their antioxidant capacity and delaying the ripening process during refrigerated storage. In the present study, the adopted concentration of 2% reduced the severity of P. pachyrhizi (Tables 1, 2 and 3).
Camili (2) applied chitosan at 0.00%, 0.25%, 0.50%, 1.00%, 1.50% and 2.00% to postharvest grapes against Botrytis cinerea; however, pathogen control was only achieved at 1.5% and 2.0%. Maia (24) also reported positive effects of chitosan at 160 mg L-1 in controlling the fungi Plasmopara viticola and Elsinoe ampelina, causal agents of downy mildew and grape anthracnose, respectively.
Chitosan and sulfur treatments associated with the fungicide were equivalent to three applications of the fungicide in the three seasons analyzed, which indicates that such a management strategy can be adopted, especially for the first sowing time, saving one pesticide application. This is also an excellent strategy to prevent the selection of resistant genotypes and extend the useful life of the fungicide on the market.
Comparatively, alternative products and treatments associated with the fungicide were more likely to succeed in the first sowing time (October) due to the lower inoculum pressure, while alternative products alone failed to control the disease under higher inoculum pressure (Tables 1, 2 and 3). When the disease intensity was low, there was no need of three fungicide applications. The greater the number of fungicide applications on crops, the higher the production costs, worsening the cases of selection of resistant isolates (19).
In the first sowing time, the reduced inoculum pressure contributed to increasing the chemical and alternative control efficacy, in addition to preventing damage to productivity (25). Late sowing expose the host to a greater quantity of pathogen inoculum, resulting in lower product efficacy, higher disease levels and reduced productivity.
Gabardo (9), studying the soybean rust pathosystem using isolated alternative products, concluded that alternative treatments only slow the disease progress under conditions of low inoculum pressure and low disease severity. Those authors highlighted the need for future experiments using combinations of alternative fungicide products with the aim of both improving their efficiency in reducing the disease progress and introducing other control methods into soybean cultivation. The present study confirmed that the association between alternative products and fungicide has a better performance than the products used alone.
Yorinori (29) stated that Asian rust control involves several management strategies, especially the sowing of early-maturing cultivars at the most recommended times for each region, along with the implementation of a soybean-free period (60 to 90-day interval during the off-season − without soybean plants in the field) to reduce fungal survival and prevent the early onset of the disease in the crop.
The AUDPC values were higher for the middle and lower thirds of the plants (Tables 1, 2 and 3). According to Igarashi (17), this occurs because the crop density makes it difficult for products to penetrate the plant canopy, especially in the middle and lower thirds, and provides a microclimate favorable to the development of diseases, with mild temperatures and higher humidity. Godoy (10) reported that, in addition to microclimate formation, shading protects the spores from UV and direct solar radiation, which has a deleterious effect on urediospore survival.
Phakopsora pachyrhizi infection causes rapid yellowing and premature defoliation of crop leaves (14). The defoliation observed in the present study was more severe in the second sowing time due to the greater disease severity (Table 4). Excessive defoliation can affect soybean productivity, which depends on photosynthesis generated by the leaves; therefore, early defoliation reduces the crop productivity by interfering with physiological processes and decreasing the number of normal pods, the number of seeds per pod and the TGW (1).
Considering the yield components evaluated, only the number of normal pods per plant and TGW were affected by the treatments (Table 4). Comparison of sowing times evidenced that sowing in October increased both the number of pods per plant and TGW averages, while the results of fungicide-associated treatments were superior to those of alternative products applied alone.
The crop cycle in the first season lasted an average of 137 days; for sowing in December the average cycle was 122 days, corroborating the results of Zanon (31), while there was a 15-day reduction when sowing was late (December).
Soybean plants severely infected with P. pachyrhizi present early defoliation, which compromises formation, pod filling and final grain weight (14). The sooner the defoliation, the smaller the grain size and, consequently, the greater the damage to the yield and quality. This justifies the obtained data, since sowing in December led to the lowest averages of number of pods per plant and TGW due to the higher disease severity at this sowing time (Tables 1, 2 and 3), resulting in lower productivity (Table 5).
Late sowing progressively and drastically reduced crop productivity (Table 5). Similar results were observed by Navarini (25), who demonstrated that Asian soybean rust significantly affected the grain yield of the studied soybean cultivars (RS 10, BRS 154, CD 201, BRS 153, CD 206 and CD 209), which had variable reduction, between 10.0% and 50.4%, in the first and second sowing dates, corroborating the present results.
Thus, the higher yields observed in early sowing times are assumed to be the result of the less exposure of the crop to the pathogen that causes Asian rust and other diseases. The present results confirm that the estimated productivity may have been influenced not only by Asian soybean rust, but also by the different sowing times (Table 5).
The yields obtained in the presence of the fungicide are close to those of Paraná State, Brazil: 3,090.00 kg ha-1 in the 2015/2016 season, and 3,731.00 kg ha-1 in the 2016/2017 season (Table 5). In the 2017/2018 season, the yields found in the presence of the fungicide are above the average yield of 3,508.00 kg ha-1 obtained in Paraná State (5).
Comparing the productivity obtained with isolated alternative treatments, chitosan and sulfur stood out, but their association with the fungicide provided superior results, which in most cases were equivalent to those of the treatment with three fungicide applications. Associating alternative products with the fungicide improved their performance, compared to the application of these products separately (Table 5). In the three seasons, the alternative treatment that presented the highest averages was sulfur in both sowing times and in all seasons, which evidences the possibility of inserting this product into the crop management.
According to Vitti (28), soybean crops respond in productivity to the application of soluble sulfur sources, but this result only occurs for soils poor in this nutrient. In the present experiment, there was no lack of this nutrient in the soil, and the highest averages obtained with sulfur treatments were attributed to the reduction in the AUDPC (Tables 1, 2 and 3). The treatments sulfur alone and sulfur associated with the fungicide showed an average gain of 401 and 651 kg ha- 1, respectively.
Chemical control is the most recommended method for controlling Asian rust in soybeans. Intensive and inappropriate use of fungicides, including repeated application of molecules with the same mechanism of action, use of isolated site-specific products, changes in the recommended levels, insufficient coverage due to application of fewer drops than necessary to deliver the lethal fungicide dose, and eradicative use of systemic products, contributes to the selection of less sensitive fungal isolates, consequently reducing the fungicide efficiency in controlling the disease over time (11).
Newly launched fungicides, containing molecules of the carboxamide chemical group, were widely used in the 2014/2015 season (8) in an attempt to achieve better disease control. Lower sensitivity of P. pachyrhizi to demethylation inhibitor, quinone outside inhibitor and succinate dehydrogenase inhibitor fungicides has already been identified and confirmed in Brazil (8, 19). These data indicate that the chemical control of this disease has become increasingly difficult, evidencing the need for including alternative products to prevent resistance cases and extend the lifespan of commercially available products.
In the present experiment, the carboxamide fungicide provided 63.74% control in the first season (2015/2016) when sowing was in October and 78.83% control when sowing was in December. In the 2016/2017 season, such a reduction was 75.85% when sowing was in October and 80.85% when sowing was in December. In the 2017/2018 season, control was 71.20% for sowing in October and 73.20% for sowing in December, indicating lower sensitivity of the pathogen to the fungicide (Tables 1, 2 and 3).
Considering the search for management strategies against pathogen resistance to fungicides, incorporating alternative protective products with multi-site mechanism of action and broad-spectrum control appears as a possibility on the market.
The superior result of the sulfur treatment with fungicide indicates that the mixture of two or more active ingredients with different mechanisms of action can more efficiently control Asian soybean rust. Furthermore, these combinations can increase the product’s spectrum of action in the field, ensuring a greater residual effect, in addition to reducing the risk of emergence of resistant pathogen populations.
According to the recommendations, applications were carried out at phenological stages V4 and V6 for sulfur and R1 and R5.1 for the fungicide, in both sowing times, as a preventive strategy in the absence of disease symptoms. Sequential applications based on plant phenology, as found by Godoy (11), have presented better results in reducing Asian rust severity and increasing plant productivity. Navarini (25) also observed that greater yield gains tended to be associated with applications timed according to the plant phenology, even in the absence of visible symptoms of soybean rust (Phakopsora pachyrhizi Sydow).
CONCLUSION
The lowest AUDPC values were consistently observed when the fungicide mixture was applied.
In all seasons and sowing dates, sulfur and chitosan treatments applied alone, as well as alternative products combined with fungicide and the fungicide alone, reduced the disease severity by 75.85%.
Defoliation was reduced, compared to the control (52%), with sulfur treatments applied alone, alternative products combined with fungicide, and the fungicide applied two or three times in all seasons and sowing dates.
Productivity decreased by 825.73 kg ha⁻¹ when sowing occurred in December. In all sowing dates and growing seasons, application of fungicide, sulfur, and alternative products combined with fungicide resulted in an average productivity loss mitigation of 40.32%.
Sulfur treatment associated with fungicide was equivalent to the fungicide treatment applied in three stages and can be recommended for the crop.
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Gabardo, G.; Pria, M.D.; Silva, H.L. Alternative products alone and associated with fungicide to control Phakopsora pachyrhizi. Summa Phytopathologica, v.52, p.1-9, 2026.
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Edited by
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EDITOR CIENTÍFICO:
Edson Luiz Furtado https://orcid.org/0000-0002-6924-835X
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EDITOR ASSOCIADO:
José Otávio Machado Menten https://orcid.org/0000-0002-9644-5770
