Such as the use of fungicides in a preventive manner, the rotation of action mechanisms, respect for doses and intervals between applications, combined with an adequate control program can help in the conscious management of diseases that affect soybean crops.
In the 2019/2020 harvest, the estimated soybean production, according to the National Supply Company (Conab), is 121 million tons for a planted area of approximately 35 million hectares in Brazil. With a sowing window that begins during the month of September (due to limited sanitary voids) and extends until the beginning of December, a large part of the soybean area planted in Brazil is vulnerable to spots, powdery mildew and Asian rust. Among the leaf spots, in recent harvests, those commonly known in the field as end-of-cycle diseases (DFCs) can stand out as emerging, mainly Cercosporiosis (or Cercospora leaf blight) and Septoriosis (or Brown spot), which are caused by Cercospora kikuchii e Septoria glyconesrespectively.
Cercosporiosis, which attacks soybean cultivation, is caused by the fungus cercospora spp. whose main observed species are cercospora sojina, known as Frogeye Spot (Figure 1a) which normally occurs during the vegetative period and also due to the fungus Cercospora kikuchii (which causes Cercospora leaf blight), when it manifests itself on the leaf or Purple Spot, when it attacks the reproductive structures and consequently, the grains (Figure 1b). As it is a summer crop in Brazil, soybeans are grown under ideal temperature and rainfall conditions for the development of Cercosporiosis (from 22ºC to 30ºC), present throughout the national territory. The negative impact caused by Cercospora kikuchii on soybean grain yield, if not effectively controlled, can reach 20%, in addition to reducing the quality of grains and seeds.
In relation to Brown Spot (or Septoriosis), which is caused by the fungus Septoria glycones (Figure 1c), the occurrence is also relevant throughout agricultural Brazil. Normally, at the beginning of the vegetative stage, it is possible to identify the first symptoms of the disease on the leaves closest to the ground (mainly due to the fact that the spores are brought from the soil to the plant by rain splashes), which continue to infect the plant until the end of the soybean cycle, which can reduce grain yield by up to 15%.
When they occur simultaneously in a crop, Cercosporiosis and Septoriosis have a negative impact on productivity and cause severe defoliation in the lower part of the plant. From R5 onwards (in the reproductive period), the occurrence of both diseases in leaves and vegetables ends up being more visible and is then commonly referred to in the field by producers as end-of-cycle diseases (DFCs).
The target spot (Corynespora cassiicola) is a foliar disease that occurs predominantly in soybeans planted in the Brazilian cerrado, especially in those regions where the soybean-cotton cropping succession system is practiced. The use of cultivars tolerant to this disease has been characterized as a fundamental element in control, since the power of early defoliation and grain yield detractions can reach, on average, 25% in case of poor chemical control.
Another important disease, considering the context of soybean diseases, is Anthracnose (Colletotrichum dematium there. truncata) which can occur in the leaves, mainly attacking the cotyledons (Figure 2a), the leaf veins (Figure 2b), the petioles and, when in the reproductive stage, causing pod abortion (Figure 2c) in a very significant way, reducing yields between 15 % and 20%. The occurrence of Anthracnose is not restricted only to the soybean regions of the Cerrado, currently, as the favorable climatic conditions (high rainfall and high temperatures) are characteristic of the season in which soybeans are cultivated in Brazil.
Before any control method, soybean disease management must be conscious (see box below). The adoption of all possible tools is essential to guarantee effective control of the spots that attack soybean cultivation. Among the options that the producer can make use of are the use of fungicides in a preventive manner, the rotation of mechanisms of action (mainly carboxamides, triazoles and multisites), respecting the doses and intervals between applications in accordance with the recommendations.
Considering that strubilurins (QoIs) have already shown loss of sensitivity conferred by the G143A mutation in cercospora spp. and that the genetic resistance of current varieties are not effective options for control and, added to the fact that soybean leaf spots are capable of surviving in straw for long periods, chemical control associated with good technology and timing of application is characterized as the best alternatives for controlling spots (Cercosporiosis and Brown Spot) and Anthracnose in soybeans.
Table 1 highlights the main modes of action that provide efficient control over the main leaf spots (Cercosporiosis and Brown Spot) and Anthracnose that occur in a widespread manner in all soybean-producing regions in Brazil.
Considering that strobilurins are not highly effective against DFCs, management programs using chemical alternatives must consider carboxamides, triazoles and multisites. The best levels of control are achieved when the soybean plant receives fungicide applications from the vegetative stage (zero application, figure 3), where the plant can still be completely affected and subsequently, from the pre-closing of the lines (40 days to 45 days after emergence), with broad spectrum applications with a maximum interval of 14 days.
As spots and Anthracnose are necrotrophic pathogens and can be present in straw remaining from the previous crop, they begin attacking soybean plants very early and manifest themselves more strongly at the end of the cycle. Building plant health is essential, with emphasis on products based on Solatenol, difenoconazole and chlorothalonil, which provide an excellent level of control over Anthracnose, Brown Spot and Cercosporiosis.
In the 2018/2019 harvest (Figure 4), trials were conducted in representative regions for the spot complex in soybean cultivation, evaluating the effectiveness of the main multisites available on the market (Copper Oxychloride, Mancozeb and Chlorothalonil) and a combination with triazoles, due to to the potential complementary effect between multisites and triazoles in the control of Cercospora kikuchii e Septoria glycones, mainly due to the effect of difenoconazole as the main triazole with an effect on these pathogens. Based on the data, it is possible to identify chlorothalonil as the multisite with the best level of control and that the addition of difenoconazole further increases control over DFCs and return on grain yield.
In addition to end-of-cycle diseases, Anthracnose control is also essential to build the health of the plant as a whole. For the control of Colletotrichum dematium there. truncata in soybeans, the results demonstrated the effectiveness of Solatenol and difenoconazole (Figure 5), as well as the contribution of chlorothalonil as a partner to complement the control. Based on the data, the combination of Solatenol, cyproconazole and difenoconazole delivers a lower incidence (reduction in pod rot) of Anthracnose in pods and consequently, greater control, followed by chlorothalonil, with a positive impact on grain yield.
Lucas Navarini, Professor and Researcher/Doctor in Phytopathology. Plant (Knowledge/ha); Lucio Zabot, R&D Manager - Fungicides. Doctor in Plant Physiology. Syngenta Crop Protection; Bruno Zuntini, Marketing Manager - Fungicides. Master in Agronomy. Syngenta Crop Protection
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