Preventive and conscious management is ideal for controlling leaf spots in the production system

Brazilian grain production faces major challenges due to climatic conditions that are favorable to crops, but which also allow for the good establishment and development of pests and diseases. In states where at least first and second crop cultivation is carried out, such as in Mato Grosso, considering the production system as a whole when it comes to management is even more relevant. This is because pathogens, such as the fungus that causes Target Spot (Corynespora cassiicola), has soybean and cotton crops as hosts, for example. And in recent harvests, in addition to this leaf spot, another pathogen has grown in importance in soybean crops: leaf blight of cercaspora (cercospora spp.).

Discussions about these two diseases in soybean cultivation will be one of the themes of the Soya Technical Meeting, held by the Mato Grosso Agricultural Research Support Foundation (Fundação MT), from April 26th to 29th, in Cuiabá-MT. The institution's Phytopathology researchers, Karla Kudlawiec and João Paulo Ascari, will bring research data from the 2021/22 harvest; the topic will also have the contribution of experts, Sergio Brommonschenkel, from the Federal University of Viçosa (UFV), Maurício Meyer, from Embrapa Soja, and Lucas Fantin, from Fundação Chapadão.

Target spot

The first reports of Target Spot in Mato Grosso date back to the 80s. Today, C. cassiicola is widespread throughout the State. The fungus that causes the disease can survive in crop residues, in infected soybean seeds, and still remain viable in the soil for years. In the soy/cotton production system or in the cultivation of sunn hemp species, there is a multiplication of the pathogen inoculum, which can occur from one harvest to another, as there are two host cultures for the fungus.

In the first harvest 21-22, a greater volume of precipitation and better distribution of rain throughout the period were observed. Favorable conditions for the development of the target spot occur during the reproductive phase of the crop, when the canopy is closed. And among the factors that increase the severity of the disease in the field are prolonged conditions of high humidity (> 85%), with leaf wetness periods of 48 hours and high temperatures. “With the presence of inoculum in the area, multiplied by the predecessor culture, and favorable environmental characteristics, we had the incidence of the pathogen and the evolution of the severity of the disease”, highlights Karla.

The expert explains that, in addition to monitoring climate forecasts for the harvest, the producer must consider in planning factors related to the choice of genetic material, taking into account the different levels of sensitivity to the occurrence of target spot. Also, chemical and biological control strategies must be used when defining the management of soybean crops.

“The inoculum of the fungus Corynespora cassicola, present in crop residues, reaches the first leaves of the lower third of the plant through rain splashes. From the initial infection, new spores are carried into the plant extracts, reaching the middle third and even the upper third. Therefore, it is important to protect the entire canopy, especially the leaves closest to the ground. To achieve this, we recommend that the application be carried out pre-closing between the lines.” explains the researcher.

The damage caused by this disease has an impact on final productivity and seed weight. These effects result from the early defoliation of plants in the field, which affects the translocation of photoassimilates to the grains. In particular, in the case of the soy/cotton or soy/crotalaria system, the producer needs to control the target spot in both crops. “The inoculum multiplied in one crop could cause disease in the next, therefore, it must be managed well and conscientiously, both in cotton and soybeans. Target Spot was once a disease that was not of great importance, but it has been evolving year after year”, reinforces the specialist.

Cercospora growth

The occurrence of Cercospora blight, caused by the fungus Cercospora spp., has attracted the attention of producers and specialists. In recent harvests, there has been an increase in the incidence and severity of the disease in soybean areas in Mato Grosso. In general, under field conditions, most materials are sensitive to occurrence and, among the species, Cercospora kikuchii, C. sigesbeckiae and C. flagellaris can be listed.

For researchers, the challenge is to understand how each species behaves and its predominance in each region of the State and Brazil. “We believe that there are differences between species, which can occur simultaneously under field conditions. Research institutions and universities have been working to understand the differences in pathogenicity of each one”, details the researcher from Fundação MT.

Cercospora infection can also occur when there is phytotoxicity of the leaf, triggered by the application of a chemical molecule. The fungus is opportunistic and takes advantage of the fragility of the tissue to colonize. “The visual identification of lesions on soybean plants is related to the effect of cercosporin, an exotoxin produced by the fungus and responsible for the reddish color of the lesions”, says Karla.

Furthermore, this fungus can attack all parts of the plant and be responsible for major losses in yield and seed quality, mainly by causing the so-called purple spot in soybean seeds.

Conclusions

The leaf spot complex in soybean crops can reduce grain productivity and quality. Among the control strategies for Target Spot, the predominant ones are the planting of cultivars that are less sensitive to the attack of the pathogen, the use of healthy seeds, the rotation and succession of crops with grasses, seed treatment and chemical control. As for Cercospora blight, the use of fungicides is an important management tool.

“It is necessary to consider all variables within the production system. The chance of success in controlling crop diseases is much greater when the producer considers the available strategies in his planning. In isolation, control measures may not have the desired effect. In the field, we can have the occurrence of both diseases together and also other pathogens, which together can reduce productivity and consequently the profitability of crops”, concludes the specialist.