Three soybean root diseases that deserve attention

In recent years, we have followed several reports of problems in soybean crops regarding the occurrence of early plant death or reduced productivity due to root rot, such as charcoal rot, red root rot and rhizoctoniosis.

Root diseases are generally considered difficult to control, since: a) the causal agents have the ability to produce specialized resistance structures that can survive in the soil for several years; b) have a wide range of host plants; c) soybean cultivars do not present a high level of genetic resistance; and d) the use of chemical fungicides is restricted to seed treatment. Furthermore, it is very common to observe more than one disease affecting the soybean plant, which indicates that a complex of pathogens is present in the field (Figure 1).

Root charcoal rot

Charcoal root rot or gray rot is mainly caused by the fungus Macrophomina phaseolina. However, two other species, M. pseudophaseolin e M. tecta, were recently reported in Brazil.

Soybean plants affected by root rot generally die prematurely and the leaves remain attached to the stem. To confirm the occurrence of the disease, it is necessary to pull the plants from the ground and observe the roots. Commonly, the root epidermis detaches easily, with just light pressure from the fingers. If this does not happen, the epidermis must be removed with the help of a pocket knife, to examine the innermost tissues of the root. The symptoms of the disease are characterized by the presence of a grayish color on the soybean root with “black dots”, which are the microsclerotia produced by the fungus (Figure 2).

Microsclerotia are specialized resistance structures that can survive in the soil for up to 15 years and are considered one of the main sources of inoculum. Additionally, the fungus can be spread by seeds. Soybean plants can be infected at any stage of development, however, when there are periods of summer and water stress, the disease tends to be more severe.

the fungus M. phaseolina it can affect more than 500 plant species including crops such as beans, corn, sorghum and sunflower. Therefore, crop rotation is not considered a completely effective practice for reducing charcoal rot problems. However, crop rotation is interesting from the point of view of adding biological diversity to the production system, as well as organic matter, which can contribute to improving soil conditions and assisting in disease management.

Red root rot

Red root rot (PVR) or sudden death syndrome (SDS) in soybeans can be caused by different species of Fusarium which are part of the complex of Fusarium solani FSSC. In Brazil, there are currently reports of the species Fusarium tucumaniae, Fusarium brasiliense, Fusarium crassistipitatum e Fusarium paranaense as causal agents. The disease can interfere with both the pod formation process and grain formation and weight.

Symptoms of red root rot are characterized by reddish coloring just below the root epidermis (Figure 2). To observe the symptoms, the outermost tissues of the root must be removed with the help of a pocket knife. Symptoms of “carijó leaf” can sometimes be seen in the aerial part of soybean plants (Figure 3), which occur due to the production of a toxin by the fungus. But be careful: the occurrence of “carijó leaf” is not a rule! For example, it was found that F. crassistipitatum e F. paranaense did not induce symptoms on the leaves.

The “carijó leaf” symptom caused by red root rot is very similar to the symptoms of phytotoxicity after the application of fungicides such as prothioconazole. However, the distribution pattern of symptoms in the field and the speed of appearance are different. In the case of phytotoxicity, generally, the entire field (or a large part of it) will show symptoms on the leaves and this occurs a few days after the application of the fungicide, while in the case of red root rot, the symptoms are more sparse in the field and slower to appear. It is worth remembering that the symptom of “carijó leaf” is not exclusive to red root rot; other diseases such as brown stem rot (Phialophora gregata), stem canker (Diaporthe phaseolorum f. sp. meridionalis) and some nematodes, can also cause the appearance of “carijó leaf”.

The survival of species of Fusarium that cause red root rot occurs through cultural debris or chlamydospores which are the specialized resistance structures of the fungus. Chlamydospores can survive in the soil for several years and conditions of high soil humidity favor the occurrence of the disease.

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