Soy Technical Notebook Part 3: Challenge scenario

The occurrence of diseases in soybeans has always directly impacted the productive potential of crops, especially after the emergence of Asian rust (Phakopsora pachyrhizi) in the 2001/02 harvest in Brazil. More recently, disease control costs have increased almost linearly, highlighting the concern of soybean producers with the greater severity of diseases and, consequently, the impact they have on crop productivity.

Almost 20 years after the arrival of soybean rust in Brazil, the disease has been efficiently controlled when it is possible to combine sanitary conditions, sowing at the opening of the window and concentrated in a short period, the adoption of early cycle cultivars and protection of plants with fungicides. However, in cases where sowing is carried out later or with cultivars with a longer cycle, the disease has caused serious damage and exposes the fragility of the system, when it is based only on protection with fungicides.

In addition to rust, several other aerial diseases have grown in importance year after year in the Brazilian cerrado, notably brown spot (Septoria glycines), cercosporiosis (Cercospora kikuchii), target spot (Corynespora cassiicola) and anthracnose (Colletotrichum sp.). We also have outbreaks of white mold (Sclerotinia sclerotiorum) and powdery mildew (Microsphaera diffusa), in addition to an increase in the occurrence of downy mildew (Peronospora manshurica) in some cultivars. Regardless of the crop or region where soy is grown, there is invariably the presence of one or more of these diseases, causing damage to productivity.

Another aspect of great relevance has been the increasingly earlier occurrence of these diseases, especially when it comes to leaf spots. Despite variations between soybean cultivars, for the vast majority of commercially available germplasm, the presence of anthracnose symptoms from the cotyledonary phase of the soybean, brown spot and cercosporiosis already in the first trefoils, and target spot from closing onwards is very common. between the lines (Figure 1). This scenario tends to be more severe in areas with soybean monoculture. 

Experiments conducted at the Phytus Group, in Planaltina, Distrito Federal, show that starting applications in the vegetative phase of soybeans has a significant contribution to controlling the disease complex, with consequences for increased productivity (Figure 2). An assertive start not only has a preventive effect on rust, but also contributes to reducing the progression of other diseases mentioned above, such as leaf spots and anthracnose. The absence of this protection in the vegetative stage makes it difficult for these diseases to be effectively controlled later, due to a series of factors, including the difficulty in penetrating droplets and the lower performance of fungicides after a long established disease.

Figure 1. Symptoms and reproductive structures of Cercospora kikuchii and Septoria glycines (top left), cotyledons attacked by anthracnose (bottom left) and appearance of a healthy leaf and another severely attacked by the target spot (right). These diseases have manifested themselves at an increasingly earlier age in crops.

 

Figure 2. Soybean productivity (BMX Desafio RR) depending on the application of fungicide in the vegetative and reproductive phase, compared to the control treatment, in an experiment conducted during the 2017/18 harvest, in Planaltina/DF.

In recent harvests, there is also growing concern about the reduction in the effectiveness of fungicides and the risks of control failures. The producer knows that control failure is synonymous with economic losses. There are cases of reduced effectiveness of triazoles, strobilurins and carboxamides in the control of Phakopsora pachyrhizi, of benzimidazoles and carboxamides in the control of Corynespora cassiicola and of strobilurins on Cercospora sp. In this scenario, the importance of good practices in the use of fungicides is reiterated, aiming to extract maximum control from these tools. In addition to the positioning issue already mentioned, another important practice to increase control and, consequently, soybean productivity, lies in reinforcing site-specific fungicides with multi-site fungicides. This practice has been increasingly used due to clear and consistent field results, especially in the control of rust, anthracnose and target spot.

Another aspect of great importance that will certainly need to be better explored is related to soybean cultivars, both due to their morphophysiological and genetic characteristics. Despite the large supply of cultivars on the market, little is known about the reaction to diseases and response to the application of fungicides on a regional and microregional scale. Knowledge of how each cultivar interacts with pathogen populations and fungicides applied in each region could be a powerful tool to assist in harvest planning. Additionally, expanding the supply of germplasm, which combines resistance and a high production ceiling, continues to be a major challenge for genetic improvement professionals.

Finally, it is essential to keep in mind that most of the disease control failures that we encounter on a daily basis are still related to basic aspects, both operational and planning. In this sense, factors such as seed quality, nutritional balance, physical and biological quality of the soil, pesticide application technology, interval between fungicide applications, tank mixes and operational capacity are responsible for most of the problems.

Check out part 1 here.

Check out part 2 here.