The high productive potential of the soybean crop is defined from a set of biotic and abiotic factors. The diseases stand out as biotic factors, and when they attack the soybean crop at the end of the development cycle, they are called End of Cycle Diseases (DFCs).
Therefore, knowing the environmental conditions in which these diseases develop and knowing how to identify them, makes the control and placement of fungicides as assertive as possible, aiming at maximum management efficiency. Among the DFCs in soybeans, we can highlight anthracnose, leaf blight, brown spot and frog's eye spot.
anthracnose
Anthracnose, caused by the fungus Colletotrichum truncatum, can occur throughout the crop's development cycle, but its highest incidence is found in the reproductive stage. The transmission of this pathogen occurs through infected seeds and reproductive structures located in crop residues, associated with wind and rain. High rainfall and high temperatures favor the incidence of the disease.
This fungus can cause seedling death, darkening of veins and petioles, elongated brown spots on the stems and dark spots on the pods, which later have a twisted appearance and open prematurely.
Leaf blight (purple spot)
Leaf blight is caused by fungi of the cercospora genus, the best known being Cercospora kikuchii. The initial dissemination takes place through contaminated seeds, this fungus being the most found in seed lots, although it does not affect germination. It can also be disseminated by reproductive structures in cultural remains, as it is necrotrophic. The conditions that favor the development of leaf blight are intense leaf wetness and temperatures between 23ºC and 30ºC.
The fungus attacks all parts of the plant. On leaves and stems, symptoms are characterized by dark red and reddish brown spots that expand and result in severe blight and premature defoliation.
Through the pods, the fungus reaches the seed and causes the purple spot on the integument, getting a purplish tone. It is worth noting that seeds without the presence of this color can also be infected.
Brown spot (septoriasis)
Brown spot, which is also known as septoria, is caused by the fungus Septoria glycine. It occurs at the beginning of soybean development, in the grain filling phase and at the end of the crop cycle. The appearance of the symptoms of this disease is accentuated in conditions with temperature between 15? and 30?. One of the main problems is that this fungus survives after harvesting, which is why efficient controls are so important.
At the beginning of development, the symptoms are dark brown spots with an irregular shape on unifoliolate leaves, but under favorable conditions, it can also affect the first trefoils. In grain filling, it presents symptoms in trefoils, which begin with irregularly shaped brown spots and, over time, the spots become dark brown with a yellowish halo. At the end of the crop cycle, its main cause is related to defoliation, which can cause significant losses in productivity.
frog eye stain
Frog-eye spot disease is caused by the fungus Cercospora sojina and its main spread is through seeds. It is very common to occur during flowering and at the end of grain filling and can reach the stems and pods, which mainly affects the more advanced stages of the crop. This disease develops better in conditions of high temperatures and precipitation, and can cause up to 60% of productivity losses.
Symptoms at flowering are small dark spots that form brown spots with dark margins. In grain filling, its symptoms on the stems are elongated grayish lesions with reddish-brown edges, small dark brown circles are formed on the pods.
In more advanced stages, it shows symptoms on the surface of the leaves: on the upper part there are light brown spots, on the center and on the edges reddish brown spots, on the lower part, the central region is gray. Because it is a disease that can develop at various stages of the plant and cause significant losses, preventive control is recommended from the beginning of crop development.
Control
Thus, it is clear that it is essential to control these diseases from the moment of implantation of the crop. This can be done through the use of resistant cultivars, but there is still no record of cultivars that cover all these diseases. Therefore, the best strategy is to use those that are resistant to as many diseases as possible, especially taking into account the history of CFDs in the area. The use of certified seeds and their treatment also help to control these pathogens, considering that all these diseases can be disseminated via seeds.
Another very efficient management strategy is crop rotation. Since these fungi are necrotrophic and remain in the straw from one crop to another, we can use non-host species for these pathogens, reducing the chance of committing the next crop.
For better results, we can combine management practices with chemical control, through the application of fungicides. For this, it is important to plan the applications and carry out preventive management from the initial development of the crop, positioning preventive and curative fungicides based on triazoles, strobirulins, benzimidazoles and carboxamides, intensifying the application at the end of the crop cycle.
In chemical management, it is of paramount importance to rotate active ingredients, thus enabling greater longevity of these technologies. Finally, it is worth remembering that the Integrated Disease Management (MID) is considered one of the best strategies, that is, if we use all the available tools to be efficient and more assertive in the management of these CFDs: more resistant cultivars, sowing at the right time, adequate spacing and population, crop monitoring and more proactive decision-making.
Text written by Diogo Verzegnazzi and Eduarda Seger, students of the Agronomy course at the UFSM campus of Frederico Westphalen, members of the Tutorial Education Program (PET) Agricultural Sciences, under the supervision of the tutor, Professor Dr. Claudio Jose Basso.
Cover image: Maurício Meyer/Reproduction
