Soy technical notebook Part 1: Strategic management

Due to the great diversity of diseases in soybean crops, management depends on operational and technical strategies. Furthermore, it is increasingly necessary to bring together different tools to increase efficiency, making integrated management a mandatory strategy for effective and sustainable control. 

In previous soybean harvests, producers in Rio Grande do Sul were faced with a mild scenario of Asian rust pressure on soybeans, which occurred later, mainly influenced by weather conditions. However, in the 2018/19 harvest, due to a combination of some factors, disease pressure was high, higher than previous harvests, causing more severe damage to crops throughout the State. The greater severity of Asian rust may be related to the maintenance of volunteer soybean plants in the field, in the winter of 2018, which may have contributed to the survival and multiplication of Phakopsora pachyrhizi inoculum, already in the months of September and October. 

Furthermore, the soybean sowing window extended for 60 days, due to high accumulated volumes of rain that affected plant establishment and led several producers to have to reseed their areas. This long sowing period made field management difficult. As for the climate, it was favorable, with frequent rains, high humidity and minimum temperatures on average 1,0ºC higher than those of the 2017/18 harvest (Figure 1).

In this scenario favorable to diseases, in addition to Asian rust, the occurrence of leaf spots (Septoria glycines and Cercospora kikuchii) and anthracnose (Colletotrichum truncatum) were observed in the field in all regions of the State. These are commonly present diseases, which, depending on environmental conditions, may present greater severity, such as, for example, rainier harvests. In addition to these foliar diseases, root problems caused by soil fungi, predominantly Phytophthora soyae, were observed in many areas, which can occur since the establishment of the crop and also attack adult plants, causing stem dryness and consequently death of plants in reboleiras. 

Disease damage was greater in the 2018/19 harvest, compared to what occurred in previous harvests. This can be seen in Figure 2, which shows the productivity of control plots (without fungicide treatment) of the cultivar M5947 Ipro, sown in the same area and in the same period in the last three harvests. The productivity obtained in the 2018/19 harvest was 16 sc/ha lower than in 2016/17. It is likely that this reduction in soybean productivity was influenced, among several factors, by the greater severity of diseases, mainly rust. The 16/17 harvest had low pressure of Asian rust, while in the 18/19, the rust started earlier and reached greater levels of severity, partly explaining this result. Notably, this greater disease pressure in this last harvest, in commercial areas, challenged producers regarding management and had a direct influence on their profitability. Furthermore, it makes it clear that diseases are the result of plant x pathogen x environment interactions and that their behavior cannot be expected to be the same in all crops.

Regarding the use of fungicides to control rust in this harvest, it was noticed that, in many areas, success was determined by the moment in which the program was started. Figure 3 shows the increase in productivity related to disease management with three applications positioned from the reproductive stage, and the increase in the fungicide program with four applications, starting in the vegetative stage. The trial's sowing time was October 30, a period considered early for Rio Grande do Sul. The data indicate that the first application started in the vegetative period was responsible for 55,5% of the productive increase provided by the fungicide program. In general, in the 2018/19 harvest, in the vast majority of fungicide programs evaluated, the first application was decisive for the success of disease control and for protecting the productive potential of soybeans.

In crops in Rio Grande do Sul, producers faced some operational difficulties due to having crops at different management stages and somewhat short application windows, due to weather conditions, especially long periods of rain. In many cases, applications were delayed due to the impossibility of entering the areas, resulting in very long application intervals. Data from several experiments conducted at the Phytus Institute show that, starting from the reproductive stage, intervals of 15 days between applications are adequate for greater consistency in protection, and ideally this interval should not exceed 18 days, considering a margin due to application difficulties. . 

Another very relevant result of the 2018/19 harvest, which reiterates those obtained in previous harvests, is the need to reinforce applications with protective multisite fungicides or systemic fungicides (morpholine or triazoles). The addition of multisites resulted in 15% to 20% increases in Asian rust control effectiveness. Furthermore, in relation to reinforcement with multisites, increases ranging from 0,4sch/ha to 10,5sc/ha in productivity were noted. In addition to the benefits in increasing control and productivity, the adoption of multisites has been fundamental as a resistance management strategy, both in existing cases and in attempts to prevent new cases. This contribution of multisites in relation to resistance is important for everyone involved in the production chain, including producers. Reinforcements reduce the risk of control failures, which are an imminent risk to producers, as they can result in major economic losses. Historically, there are reports of cases of control failures in which cost increases were noted, not only due to the need for new applications, but essentially due to production lost to the disease. 

Faced with the evolution of phytosanitary problems, it is essential to adopt different strategies within an integrated management program. Specifically in the case of rust, it is important to sow seeds at the beginning of the season recommended for each region, using earlier cultivars, with a good plant arrangement and adequate nutritional levels. In relation to the use of fungicides, assertive positioning is essential, especially at the beginning of applications, which must be carried out preventively, preferably in the vegetative stage, allowing greater deposition of drops in the lower third of the plant, and providing not only protection against rust, but also to the entire complex of diseases, including powdery mildew, spot and anthracnose. 

In addition to previous practices, it is important to maintain adjusted intervals between applications, diversify and rotate fungicides with different mechanisms of action, use reinforcement products, especially multisite fungicides, and adopt good application technology. It becomes evident throughout the harvests that small adjustments are often responsible for important gains in production, which can not only pay the control costs, but also bring more profitability to the producer.

Figure 1 - Comparison of the average minimum temperature (ºC) and accumulated rainfall (mm) in the months of November to March, of the 2017/18 and 2018/19 harvests

Figure 2 - Average productivity of plots without fungicide treatment of the soybean cultivar Monsoy 5847 Ipro, over three harvests, always sown in the first week of December, in Itaara/RS

Figure 3 - Productivity of soybeans (BMX Tornado RR) sown on 30/10/2018, depending on the application of fungicide started in the vegetative and reproductive phase, compared to the control treatment, in an experiment conducted during the 2018/19 harvest in Itaara/RS

Check out part 2 here

Check out part 3 here.