How to plan soybean disease management

Disease management plans often do not meet expectations, whether due to climate reality or the resistance scenario. When planning a management program for soybean diseases, we seek to evaluate the main target, according to their occurrence in the region and economic impact. However, it is also necessary to assess which phenological phase of crop development is most important, from an epidemiological point of view, for the target. Monitoring is essential to prevent the environment from affecting the assertiveness of expectations and, of course, to avoid the use of isolated or repeated molecules, at the risk of not controlling due to resistance.

For the 2020/21 soybean harvest, it is necessary to pay attention to research results, showing that the process (how the fungicide is used) has more impact on productivity results than the fungicide itself. In other words, there is often concern about choosing the best product, when, in fact, the “care” in the use process is much more important. Application programs started more preventively, showed productivity of up to 8 sc/ha more when compared to programs started more curatively, “later” in the soybean cycle. The intervals between applications of a program, also important, with more than 15 days between applications, can steal up to 4 sc/ha in a plan with four applications.

The physiological component must be considered to structure a good disease management plan. In the development of soybeans, the leaves are the main actors, in which both diseases and pesticide applications seek to overcome. And during the vegetative phase, where the plant does not yet have photoassimilated sink structures, such as flowers and pods, the plant's energetic state offers a high supply of energy for defenses against diseases, causing them to develop slowly in this phase. Thus, the beginning of preventive control at this stage offers the possibility of building disease management as the plant is building its leaf area. Because the vegetative phase is the time when the plant can best respond to the stimuli of the technologies used, such as resistance inducers, nutrients and even fungicides. Thus producing a much more promising reality for the reproductive phase, and certainly a much more realistic expectation of results in relation to the desired plans.

On the other hand, the phytopathological component also presents a low possibility of diseases being epidemically strong in the initial phase of the crop. The inoculum, even in monoculture conditions, or in systems such as soybean/cotton, tends to be low in the vegetative phase. The diseases present in the area will present a slow initial epidemic phase, directly proportional to the amount of inoculum. Whether this initial inoculum is internal to the crop, present in the seeds or straw of the predecessor crop, or external to the crop in volunteer plants, especially preserving biotrophs, such as rust. Expected to sow according to recommendations, diseases will take time to produce enough inoculum, as well as the plant to produce enough leaf area, for a high-pressure epidemic to become established.

It is precisely in this tactical window that action must be taken to build disease control, managing resistance. After all, the best management of fungal resistance to fungicides is still preventive application, in the absence of symptoms, thus achieving the best economic results (productivity). However, planning and waiting is easy, it is difficult to get around the technical, operational and, even worse, climatic factors, in order to achieve the projected result. Where expectations generally hardly reach reality. But one of the main factors that have frustrated good results, even in situations of robust investments in fungicides for soybeans, is resistance, with its diverse and difficult dynamics in each harvest, and as if that were not enough, resistance to different diseases of soybean crops.

Continue reading in the Constructed Control Technical Notebook via the link.