Soybean end-of-cycle diseases: inheritance from crop to crop

In the 2023/24 harvest, more than 45 million hectares were sown with soybeans in Brazil. Due to the intensive cultivation system practiced in most areas, there is great selection pressure on pathogen populations, which can lead to the emergence of new diseases, an increase in the importance of existing diseases and the resistance of fungi to fungicides.

Diseases caused by necrotrophic fungi, which include target spot, anthracnose and end-of-cycle diseases, survive in crop residues, which favors the maintenance of inoculum from one harvest to the next.

End-of-cycle diseases (CLDs) are transmitted by two fungi: Septoria glycones e cercospora spp., which cause diseases known as brown spot (or septoria) and Cercospora leaf blight or purple seed spot. The name “end of cycle” came about because symptoms are more easily observed at the end of the soybean cycle, but the name can lead to management errors.

Although they are known as DFCs, in favorable years, the symptoms of these diseases can occur very early in the crop, still in the vegetative phase, through inoculum from seeds or crop residues. Another important factor is that, even though symptoms may appear later in the cycle, these fungi can already be found in the tissues of soybean plants in a latent form since the vegetative stage, without causing symptoms, precisely due to the inoculum sources. cited.

The brown spot (S. glycines) is generally one of the first diseases to affect soybeans, causing the yellowing of single-leaf leaves and the first trefoils. Symptoms are characterized by small, angular, irregular spots with yellowish halos and a brown color, which can coalesce and cause defoliation of the lower third of the plant. Under favorable environmental conditions and inadequate management, at the end of grain filling, the symptoms manifest themselves intensely, reducing the active photosynthetic area and crop yield. Depending on the region and time of incidence of the disease, losses between 7% and 40% are reported, but an average loss of 30% is considered, increased by the association with cercospora spp.

Purple seed spot and leaf blight of Cercospora (cercospora spp.) have a widespread distribution in the country, especially in regions with more abundant rainfall during the crop's maturation phase, although symptoms, like those of brown spot, can occur at any stage of the plant. Until recently, Cercospora kikuchii it was the species best known for causing blight. However, recently other species have been associated with the manifestation of the disease, such as C. cf. flagellaris e C. cf. sigesbeckiae.

Blight symptoms are characterized by reddish-brown spots, which can coalesce to form large dark spots with a purplish color and a leathery appearance on the leaves. The fungus produces a toxin called cercosporin, which naturally has a reddish color. When activated by light, cercosporin produces reactive oxygen species (ROS), which lead to the rupture of plasma membranes and death of plant cells, which explains the appearance and color of symptoms in the plant, caused by the extravasation of this cellular content. internal. Early defoliation due to blight is common under high severity conditions.

Red dots can be seen on the pods that evolve into purplish-brown spots. It is through the pod that the fungus reaches the seeds, causing the purple spot on the seed coat. Although there is considerable disagreement about the effects of purple spot on seeds, several studies demonstrate that grains with a high rate of infection have a lower concentration of oil and protein, in addition to generating seedlings with less vigor. Regardless of whether it affects germination or not, the fact is that the presence of the fungus in the seed could serve as an inoculum for the subsequent harvest. Losses due to blight also vary depending on cultivars, regions and climate, but can be greater than 30%.

Brown spot and blight can occur isolated or simultaneously in crops. The main joint damage of these DFCs is early defoliation, which makes integrated management important. In this sense, crop rotation has a direct impact on reducing the initial inoculum of patches, as soybean crop residues can remain in the field for between 20 and 30 months. It is important to maintain a good straw cover to reduce the impact of rain drops and the spread of inoculum to the first soybean leaves. Another measure is the use of less sensitive soybean cultivars, the use of seeds of good physiological and sanitary quality and the carrying out of specific seed treatment, guided by laboratory analysis of seed pathology.

The application of fungicides is essential in the management of DFCs. Planning which fungicides to use in management is necessary, as there are reports of reduced sensitivity of Cercospora spp. to fungicides from the strobilurins and benzimidazoles group. However, the decision to choose products must consider other diseases that occur concomitantly in the crop. Furthermore, the combination of site-specific and multisite fungicides is necessary.

Just as important as the choice of fungicide to be used is the timing of this application. The decision is based on knowing the history of disease incidence in the area, whether or not there is crop rotation, the sanitary quality of the seeds used and the climate. In many cases, applications started during the vegetative period have a significant impact on the occurrence of DFCs and, consequently, on grain yield. In the 2022/23 harvest, among the different treatments tested in early applications (up to 30 DAE), average increases in the control of DFCs of 18% were observed in relation to the treatment with later application.

The use of biological tools also contributes significantly to the management of DFCs. In field studies, we observed synergy in the combination of chemical + biological management in controlling S. glycines e cercospora spp., with very significant control increments, through the combined use of fungicides + products based on a mixture of Trichoderma harzianum, Trichoderma asperellum e Bacillus amyloliquefaciens or just Bacillus pumilus.

The integration of several control measures makes disease management more effective, maintaining the producer's profitability and preserving the effectiveness of fungicides.

By Caroline Wesp Guterres (UFRGS); Camila Cristina Lage de Andrade, Marisa Dalbosco e Rita de Cássia Madail Santin (Agronomics - Phytosanitary Diagnosis Lab)

Article published in issue 297 of Cultivar Grandes Culturas Magazine