In soybean crops, brown spot (Septoria glycones) and leaf blight of Cercospora (cercospora spp.) are also known in Brazil as the end-of-cycle disease complex (DFC). This name comes from the symptoms at the end of the cycle with early defoliation of the crop, but both pathogens can be present in the area throughout the cycle, since the fungi survive in crop residues. The fungi can be found in the tissues of soybean plants in a latent form since the vegetative stage, without causing symptoms.
Symptoms of brown spot or septoria may appear approximately two weeks after emergence as small, reddish-brown, angular spots or spots on unifoliate leaves. In favorable situations, the disease may reach the first trifoliate leaves and cause defoliation. The presence of straw reduces the incidence of brown spot by reducing the impact of raindrops on the soil and reducing the dispersion of the inoculum to the primary leaves of the soybean.
Cercospora kikuchii was the species known to cause blight, but other species have been associated with the disease, such as C. cf. flagellaris and C. cf. sigesbeckiae. Symptoms may occur on leaves, petioles, stems, pods, and seeds. On leaves, symptoms are characterized by dark, reddish-brown punctuations with irregular edges that coalesce to form large dark spots that result in blight and premature defoliation.
Necrosis can also be observed in the veins of the leaves. On the stems and petioles, the fungus causes reddish spots, usually superficial. On the pods, red dots appear that evolve into reddish-brown spots). The fungus also infects the seed and causes purple spots on the tegument. The color of the spots caused by Cercospora blight is caused by the cercosporin toxin produced by the fungus, which is activated by light, producing reactive oxygen species, causing the leakage of cellular contents, which causes cell death. The symptoms of Cercospora blight are more common at the end of the cycle and one of the reasons is the relationship between the production of cercosporin by the fungus and the production of simple sugars in the plant.
The two diseases can occur separately or simultaneously. The main damage is early defoliation, which is less severe than that caused by Asian rust. When rust occurs, competition for leaf tissue makes it difficult for CSD to occur, since rust defoliates the plant before these diseases occur.
Even with the intense use of fungicides in the crop, there has been a failure to control DFC, often perceived by the occurrence of purple spots on grains/seeds and reddish-brown coloration of leaves in the initial sowings. This may be associated with the reduced sensitivity of the fungus to fungicides.
These studies to compare the efficiency of fungicides in controlling DFC have been conducted in the cooperative experiment network since the 2020/2021 harvest. These trials involve sequential applications with single fungicides starting 35 days after sowing (DAS). However, this does not constitute a control recommendation. The information should be used within a management system, always prioritizing the rotation of fungicides with different modes of action, adapting management to the sowing season, cultivar, property size and application logistics, climate conditions, and disease incidence in the region and on the property.
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