| Pesticides

Target spot evolves rapidly in the Cerrado

Weather conditions in the coming weeks tend to be quite conducive to the development of the fungus

21.12.2024 | 05:55 (UTC -3)

Cultivar Magazine

In recent years, target spot, a disease caused by the fungus Corynespora cassiicola, has become an increasingly important problem in Brazilian soybean crops.

This season, it has already arrived. Producers in the Central-West region have frequently reported classic symptoms of the disease in plants less than 50 days after emergence (DAE).

And the weather conditions over the next few weeks are likely to be quite conducive to the development of the fungus.

The fungus cycle

The fungus that causes the disease has a wide range of hosts, being capable of infecting several species of weeds, as well as economically important crops in the region, especially cotton, explains researcher Maurício Stefanelo, from Ceres Consultoria Agronômica.

It survives in crop residues, infected seeds and soil, making it a perennial threat.

The initial symptoms, which can still be confused with symptoms of other diseases, or even phytotoxicity, are characterized by small dark brown dots with a yellowish halo that evolve into the classic symptom of the disease, manifesting as circular spots with concentric rings that give the appearance of a "target".

Ideal conditions for the development of the disease include temperatures between 20°C and 30°C, relative humidity above 80% and prolonged periods of leaf wetness. Under these circumstances, the first symptoms appear between 5 and 10 days after infection, says Stefanelo.

Impact on productivity

Susceptible cultivars can suffer losses of up to 40% in productivity.

Studies show that the fungus reduces photosynthetic efficiency not only in damaged areas, but also in apparently healthy tissues, reducing the plant's ability to assimilate carbon. This is due to the production of toxins that induce necrosis in leaf tissues.

The impact on the photosynthetic process results in reduced grain filling, a significant drop in productivity and poorer quality of the final product.

Control and management

Although there are no cultivars with resistance to target spot, an integrated management approach can mitigate damage, Stefanelo explains. Recommended practices include:

Crop rotation: alternating soybeans with non-host crops, especially grasses, reduces the inoculum present in crop residues.

Good agronomic practices: balanced fertilization, adequate spacing, adjusted plant population and use of certified seeds are measures that contribute to the overall health of the crop.

Foliar fungicides: The use of mixtures, especially containing carboxamides and triazoles, has shown efficacy, especially when applied preventively, always in association with multisites. Alternating the modes of action of fungicides combined with the use of multisites helps to avoid the selection of resistant isolates. It is essential to ensure adequate leaf coverage, especially in the lower areas of the plant.