An analysis of soybean diseases in the 2017/18 harvest

The soybean harvest in southern Brazil had different climate scenarios, which influenced diseases and crop productivity. There was the presence of initial diseases, affecting soybean establishment, later root rot, powdery mildew, white mold and Asian rust. Its impact on the crop's grain yield varied in intensity depending on the region, which is why a regionalized analysis is necessary.

In Rio Grande do Sul, there was excess rainfall in October, delaying the implementation of soybeans. Soggy soil conditions, associated with soil compaction, resulted in the death of soybean seedlings shortly after emergence, which was mainly associated with the Phytophtora fungus, usually present in the soil. These same conditions also favored red rot, caused by Fusarium species. Both diseases varied in intensity depending on the cultivars used, so it is important to know their behavior. At the end of the cycle, especially between February and March, there was a more prolonged drought, with higher temperatures, which favored root rot caused by another soil fungus, Macrophomina phaseolina.

Between the months of December and March there were dry periods, accompanied by mild night temperatures, favorable to powdery mildew, caused by Erysiphe diffusa. This fungus does not require leaf wetness to infect plants, and its preference is for maximum temperatures lower than 29 oC, which is why its presence was greater in this harvest. The intensity varied between cultivars and locations, but reached severity levels exceeding 50% in some of them, accentuating plant defoliation. Powdery mildew manifests itself from the vegetative stage of soybeans, which is why the first applications must include fungicides that are effective in controlling it.

In some experiments conducted at the University of Passo Fundo and in commercial crops in the Middle Plateau of RS, the absence of the first application of fungicide in the pre-closing of the rows resulted in differences of -3 to -6,8 sc/ha in final productivity. Such differences are associated with the control of powdery mildew, leaf spots, anthracnose and rust prevention.

It is also important not to delay this first treatment. In three years of comparison, at UPF, between managements with four applications, the first starting before or after closing the rows (average difference of 7 to 9 days), variations in final soybean productivity reached 8 sc/ha (2016) , 4,5 sc/ha (2017) and 5 sc/ha (2018) in favor of pre-closure.

Asian rust occurred later this harvest. The presence of guax soybean plants, responsible for maintaining the disease agent (Phakopsora pachiryzzi) in the off-season, was minimal after the frosts that occurred in July. In these situations, the onset of the disease depends on spores released from crops in other regions. The plant infection probably occurred in the second half of January, with rainfall frequency being almost daily. Its perception at the beginning is difficult due to the emission of new leaves by the plant, the latency of the disease (time between infection and the appearance of the first symptoms and signs), and the limitation of the human eye in finding the disease when the incidence rate is below 5%. in leaflets.

The evolution of rust occurred between February and March, therefore, in the final phase of the crop. Its severity reached between 20 and 70% depending on the sowing time and cultivar. The compromise in productivity was smaller, but still significant, especially in later plantings. In an experiment conducted at Cotripal, in Condor-RS, the difference in productivity between four fungicide applications and none varied from 6,9 sc/ha (sowing in September) to 17,9 sc/ha (October), 17,7 sc/ha (November) and 20,0 sc/ha (December).

Control of Asian rust in the 2017-18 harvest achieved better results than in the previous year. Several factors can be associated with this superior performance. Due to the later occurrence of rust, the first applications were more preventive, producers were able to maintain regular intervals of 14-15 days between applications, there was greater insertion of reinforcements with other fungicides (triazoles, morpholines and, mainly, multisites) and, possibly, a lower frequency of fungicide-resistant mutants. Their ability to survive in the off-season is being evaluated, but it is possible that they were more affected by the elimination of guaxa soybeans due to the cold. As a consequence, especially in relation to fungicides from the carboxamide chemical group, their performance in the 2017/18 harvest was much better than the previous year.

On the contrary, in the state of Paraná, especially in the West region, there was greater difficulty in managing rust. This harvest saw a delay in the implementation of the crop due to lack of rain, exposing the soybeans to a longer period of contact with the rust pathogen. The frequency of rain in some periods was almost daily, favoring infection. The proximity to Paraguay may also have contributed, as the cultivation of soybeans in the second harvest favors the perennialization of rust and the selection of individuals more resistant to fungicides.

The state of Paraná, in previous harvests, presented averages ≤ 3 in fungicide applications in soybeans. In some regions of the state, possibly even less, just as the insertion of reinforcements with multisites was also smaller. With the worsening of the rust situation in the 2017/18 harvest, the management programs that were being practiced were probably insufficient.

A very important disease in the 2017/18 harvest, in several locations in southern Brazil, was white mold, caused by the fungus Sclerotinia sclerotiorum. It benefits from periods with frequent rain and average temperatures lower than 21oC during soybean flowering, which occurred in December (PR) and January (PR and RS). The damage to productivity is proportional to the incidence of white mold among plants, which reached up to 30 to 40% of plants in some locations. In these areas, the disease may appear again in the next harvests, as long as the favorable combination of rain and mild temperatures during flowering is repeated. Therefore, it is important to adopt preventive measures (covering the soil with straw, less dense arrangement of plants, cultivars with less branching,...), leaving the decision to apply fungicides specific to white mold conditioned on the forecast of a favorable environment. to disease at flowering.

In summary, the 2017/18 soybean harvest was quite variable, between regions, in terms of the occurrence of diseases, with the main presence of powdery mildew, rust and white mold. Controlling such diseases requires fungicides and different management, which is why technical updating, crop monitoring and attention to climatic conditions are essential. Preventative control measures are safer and more effective, which is why they should be prioritized when planning and managing soybean crops, especially in relation to diseases.