Warning for the appropriate use of fungicides to control soybean rust

Caused by the fearsome biotrophic fungus Phakopsora pachyrhizi, Asian rust has been alarming technicians and soybean producers in the vast majority of the country's producing regions for more than a decade, due to its aggressiveness and changes in behavior regarding sensitivity to (systemic) fungicides.

In this 2017/18 harvest, more than a hundred occurrences of outbreaks of the disease have already been reported throughout Brazil, evolving significantly in recent days, when the plants in most crops are in the reproductive phase. This period is considered quite critical in terms of plant susceptibility and what this can have on their productive potential.

Combined with the critical stage of the crop (reproductive), the climatic factor has greatly contributed to further aggravating the risks of epidemics in this harvest. All care must be taken with regard to preventive and curative applications of fungicides to obtain their best performance, without forgetting good practices related to application technology aimed at good coverage and plant protection.

Reinforce the importance of reaching the lower parts of plants, where the first symptoms of the disease appear. Correct diagnosis is essential for decision-making and the need to adequately reach the target, especially for the first application of the crop, preferably carried out before closing the lines. For other applications, the interval between them must be strictly observed in accordance with current recommendations, generally 14 to 15 days.

The use of fungicides is practically the only alternative for disease management when soybean crops are established. Particularly for rust, the use of systemic fungicides associated with multisites is mandatory to minimize damage and risks of fungal resistance, which is already widely known for the three main chemical groups used (DMIs, QoIs and SDHIs).

For this harvest, especially in regions where there has been greater rainfall, it is important to be alert to choosing the correct time for applications and to choosing products registered and evaluated by research institutions, to avoid problems of failure to control diseases or unnecessary investments. It is also important to adopt a diversified program regarding groups of fungicides, in addition to the correct use of registration doses, the interval between applications and adjuvants recommended for each product.

Therefore, in this harvest scenario with a significant presence of inoculum (uredospores) distributed in the vast majority of soybean crops in the country and a favorable climate for the development of the disease, the impact of rust could be even greater if failures of different natures occur. before and during fungicide applications, resulting in a drop in crop efficiency and yield.

Finally, the Biological Institute has been continuing studies on monitoring the sensitivity of populations of P.pachyrhizi to fungicides from different chemical groups, using the detached leaf methodology (according to FRAC), in addition to being developed during the 2107/18 harvest, national field trials also aiming to monitor the resistance of the fungus in the State of São Paul.