Target spot management in soybean crops

The search for cultivars with a higher ceiling productive, shorter cycle and resistance to Asian rust and Phytonematoides has resulted in an increase in the severity of diseases such as target spot. Apply appropriate fungicides at the right time, make good use of application technology to cover the basswood leaves and use seeds good quality treatments are among the weapons to combat this disease, which has become a growing problem in crops.

The occurrence of diseases has been one of the main factors limiting the increase in productivity of soy in Brazil. The extensive cultivated area, which already approaches 34 million hectares, the hot and humid climate and the absence of crop rotation in most part of the crops are some of the aspects that explain this aggressiveness of diseases in soybeans in the country.

The target spot has presented increasing importance in several areas of production, especially in the states of Mato Grosso, Goiás, Minas Gerais, Tocantins, Maranhão and Piauí. In these areas, considerable losses have been observed in soybean crops, especially when using susceptible cultivars, where there are reports of reduction of up to 50% in soybean productivity due to the occurrence of the spot target.

The search for cultivars with higher production ceiling, shorter cycle and resistance to other problems phytosanitary diseases of great importance, such as Asian rust and phytonematodes, has resulted in an increase in the severity of diseases such as target spot. This fact It is possibly related to the scarcity of resistance sources and the narrow genetic basis of the cultivars used today.

The target spot is caused by the fungus Corynespora cassiicola, which, in addition to soybeans, can cause diseases in more than 70 genera of plants, many of economic importance, such as cotton and tomatoes. That fungus can be transmitted through seeds and also has the ability to survive in crop residues, remaining in soybean straw and other residues for more than two years. These are the main sources of spot inoculum target.

The ideal environment for The occurrence of the disease is characterized by temperatures between 18ºC and 21°C, minimum wetness period of 24 hours and relative humidity greater than 80%. Under under these conditions the fungus penetrates the plants and within 7 days to 10 days The first visible symptoms are usually observed. These conditions are obtained more frequently after closing the lines between lines and the consequent shading of the lower plants.

Symptoms can be observed in all parts of the soybean plant, including roots, stems, petioles, leaves, pods and seeds. However, the most characteristic symptom and of greatest importance occurs in the leaves, especially in the lower and median. The lesions are rounded and, as the name of the disease suggests, They resemble a target, generally reddish brown in color. The injuries are initially small, but can expand and reach up to 2 cm in diameter.

Stain control target can be achieved by the use of resistant cultivars, crop rotation non-hosts, treatment of soybean seeds with fungicides and application in aerial part. As there are no cultivars with complete resistance adapted to all regions and the cultivation system is carried out with soybean planting every years, disease control has been based on the application of fungicides in part aerial.

There are currently 43 fungicides registered with the Ministry of Agriculture, Livestock and Supply (Map) for the control of target spot in soybean crops, belonging to triazole groups, strobilurin, carboxamide, benzimidazole, dithiocarbamate, carboxanilide, dimethyldithiocarbamate and phenylpyridinylamine. Nonetheless, Typically, producers have opted to apply fungicides with mixtures formulated of triazole + strobilurin, carboxamide + strobilurin and carboxamide + strobilurin + triazole.

Even with abundance of registered fungicides, many of which have high efficiency, it is common to occurrence of failures in target spot control in commercial soybean crops. This occurs due to errors in the choice of fungicides, time of application, dose, very long intervals between applications and with application technology that does not allows adequate coverage of the basswood leaves with fungicide.

Within this context, one of the aspects that has most affected the efficiency of fungicides for Target spot control has been the delay in the first application. How does the fungus can be transmitted via seeds and survives in straw, normally Infection occurs early, still in the vegetative stage. When the closure of lines between lines and environmental conditions are more favorable conditions, the disease becomes severe and causes large levels of defoliation.

This can be observed in graph 1, where it can be seen that the efficiency of the same fungicide can drop drastically when the application is carried out after the first symptoms, compared with preventive application. In this example, the first symptoms of the target spot were detected at the R1 (flowering) stage of soybean. This occurs mainly due to the mode of action of available fungicides, most of which They are preventive, some are curative and practically none have an eradicating effect.

The application technology used and its ability to cover the leaves of lowland plants with fungicide Soybeans are essential for successful control of the disease. Therefore, even if application of the fungicide occurs before the onset of the disease, this may be inefficient if it is not possible to cover the plant as a whole, especially because the disease begins in the middle and lower thirds of the plants.

Other One aspect that cannot be left aside is the correct use of pesticides to avoid or delay the selection process of fungal isolates resistant to fungicides. In Brazil, the loss of sensitivity of some isolated from the fungus that causes the target spot for fungicides from the benzimidazole group. Carboxamide resistance problems have also been identified in cucurbits in Japan, showing that there is a risk of selection of isolates resistant to this group as well.

This information must serve as a warning, especially for regions where soybean is planted followed by cotton. Under these conditions, up to 4 applications of one same fungicide to control the target spot (2 applications on soybeans and 2 on cotton). Over the years and considering that the target spot inoculum most important survives in the straw, there may be selection of insensitive isolates or resistant to triazole and carboxamide fungicides, for example.

In this sense, the rotation of chemical groups with different mechanisms of action is important for preserve available fungicidal molecules. The association of fungicides with site-specific mechanism of action, such as triazoles, strobilurins and carboxamides to multi-site fungicides as well can be performed to delay the selection process of fungal isolates resistant or insensitive. This association can also improve the efficiency of some fungicides.

The occurrence every time most frequent and severe target spot is a threat to soybean crops in Brazil is yet another obstacle to be overcome in the search for greater productivity. In this sense, using available control techniques jointly is essential for obtaining satisfactory levels of control and maintenance of soybean profitability.

Use seeds from good quality, carry out good seed treatment with fungicides and protect the aerial part of the plant with foliar fungicides are very efficient practices to control the target spot. Additionally, the association of these practices with crop rotation, use of cultivars with some level of resistance and mulching the soil with straw can maximize control and gains productive for soybeans.

Article published in issue 223 of Cultivar Grandes Culturas, December 2017/January 2018.