Pod abortion in soybeans

Episodes of pod abortion and deficiency in soybean grain filling in crops with high productive potential has worried researchers and producers in several Brazilian growing regions. The causes of these disorders are complex and cannot be attributed to the climate alone. The coincidence of several factors stressors, with a possible synergistic effect, associated with excess humidity in the soil and air and low light during the reproductive period may be among the explanations for this phenomenon.

In Brazil, approximately 35 million of hectares of soybeans in the 2017/18 harvest, in the most diverse environmental and of production systems. In this broad universe, there are countless success stories and overcoming, but also many problems. Brazilian soybean farmers recurrently encounter phytosanitary, climatic, logistical, etc., which can be localized, regional or national, causing losses with different magnitudes.

Between the problems with soybeans in this harvest, including pod and/or pod abortion. poor grain filling. These cases attracted attention not because they been more costly and comprehensive than others, generated by mistakes in the crop installation, inadequate soil management, fungal diseases, insect pests, weeds, nematodes, rain during the harvest, etc., but because they occurred in crops that had been showing high productive potential, in areas with good agronomic records, led by experienced soybean growers and assisted by competent professionals.

Most of the reports about these disturbances in soybeans came from the western and northern regions of Paraná, medium Paranapanema in São Paulo and southwest of Mato Grosso do Sul. However, despite the considerable amount of affected farmers, it is noteworthy that the distribution of cases was random and with a relatively low area extension, in relation to the total number of crops trade in these regions.

As for symptoms, the crops came presenting normal development, visually with high potential productive, until the phenological stages of pod formation (technically called R3 and R4). When the grain filling period began (stage R5.1), problems began to become evident. There were situations where the pods were aborted from soybean plants, at different levels of damage, and, in others, the pods remained connected to the plants, without the grains being able to complete its development.

In general, pod abortion has been attributed to the climate. Indeed, in the last two weeks of December 2017 and in the first two weeks of January 2018, practically all soybean regions in central-southern Brazil remained for several days under rain, overcast sun and soil with high water content. However, it was not uncommon for situations in which a soybean field was found to have pod problems and, a few meters away, plots with soybeans at similar phenological stages to the damaged area, presenting pods with normal and high development productive potential. In view of this, it is clear that the isolated approach to climate is not sufficient to elucidate the physiological disturbances observed. To the evidence points to a complex agronomic problem, resulting from interaction of several factors.

Genetically, soybeans are programmed to discard a large number of flowers, which are normally produced in excess, just as pod abortion is common when plants have need to adjust grain filling depending on the supply of resources in the environment (water, light, nutrients, etc.). This complex control is controlled by genetic attributes inherent to each cultivar, which respond to signals from the environment (climate and soil) and management (sowing time, fertilization, agrochemicals, etc.). Therefore, although it was not possible to assess the degree of susceptibility of soybean cultivars, it is essential to consider the factor genetics in the study and elucidation of this problem.

Another cause wrongly cited for the pod abortion was the occurrence of anthracnose, a disease caused by the fungus Colletotrichum truncatum, that can attack leaves, stems, petioles and pods. The infection and development of this disease are favored by high relative air humidity associated with high temperatures and this environmental condition makes anthracnose frequent in the region of Closed. Although the fungus is a common occurrence, in most cases, outside the Cerrados region, behaves as an opportunist, that is, it grows in plant tissues died from other causes. And, even in the Cerrados, where the disease is common and serious, does not cause green pods to fall, as occurred in this harvest.

In crops with poor grain filling, the result of the adversity suffered is becoming more evident as the harvest. Even in crops where the producer had not noticed the problem, there are reports of high rates of damaged grains. Therefore, there are quantitative and qualitative losses in soybeans.

When it comes to soybean farming in Brazil, it is correct consider that the vast majority of crops remain constantly under some stress level, from installation to harvest. From the point of view agronomic, solutions are sought to minimize or avoid the causes of these stresses, so that cultivars can express their potential to the maximum genetic. Stress factors are didactically divided into biotic (pathogens, insect pests, weeds, etc.) and abiotic (temperature, photoperiod, water, nutrients, etc.). However, in practical terms, it is It is essential to consider that, in a given production environment, the factors stress act concomitantly and can interact with each other.

Therefore, it is reaffirmed that the causes that generated pod abortion and/or poor grain filling are complex and cannot be solely attributed to the climate. It is therefore suggested that there was a coincidence of several stressful factors, with a possible synergistic effect. The excess of humidity in the soil and air and low light during the reproductive period of soy increased the action of these factors. It is also necessary to consider the degree of sensitivity of soybean cultivars and possible erroneous management practices soil and culture.

Although there are no techniques that can solve or minimize the already established problem, the recommendation remains, for next harvests, the inclusion of soybeans in a diversified production system, with cultivars adapted to the growing region and adequate soil management, in addition to careful management of the crop with due attention to the management of insect pests, diseases and weeds, avoiding exaggerations in the application of agrochemicals and other products. 

Article published in issue 226 of Cultivar Grandes Culturas, March, 2018.