Asian rust: How much it rusted

With soybean crops in the final harvest phase of the 2017/18 harvest, estimates released in mid-April show that Brazilian soybean production should total 114,962 million tons, an increase of 0,8% over the season's harvest. previous and average productivity of 3.276 kg/ha (Conab, 2018). The good productivity reflects the climate conditions that, in general, favored the crop and is shaping a new “super harvest” of soybeans in Brazil, with states such as Mato Grosso do Sul, Goiás, Tocantins, Maranhão, Piauí, Bahia, Minas Gerais and São Paulo, recording productivity records. In this context, the occurrence of Asian rust, caused by the fungus Phakopsora pachyrhizi, continues to play an important role, as its control affects the production cost of crops and the achievement of good productivity.

Asian rust control is carried out through the adoption of various management practices, but the pathosystem surrounding the disease has been shown to be dynamic, changing with each harvest, resulting in the need for constant improvement of these practices as new knowledge is gained. acquired.
The sanitary void remains an important measure and, although issues such as duration, effective dates and efficiency of the measure are frequently debated, the general consensus is that it should be maintained in states where it already exists and adopted where it does not yet exist. As a result, the state of Rio Grande do Sul, as the last large producer without a sanitary void, is discussing the creation of the measure. In turn, the states of Santa Catarina, in 2017, and Piauí, in 2018, established sanitary emptiness and Bahia increased its period from 60 to 99 days (Figure 1).
Another legislative measure that has been adopted more recently is the scheduling of the soybean sowing period. This measure aims to restrict very late sowings, which tend to require a greater number of fungicide applications, due to the early occurrence of Asian rust, which can consequently worsen the problem of the fungus' resistance to fungicides. To date, seven states have adopted the calendar to limit the soybean sowing period and, in some, the harvest limit (Figure 2). Even so, cases of late sowing, violating state regulations, were verified.
Sowing soybeans at the beginning of the recommended season is extremely important because it allows “escape” from the period of the most severe occurrence of Asian rust. This harvest was no different, as despite the delay in rain in some regions, most crops were sown early and, when the disease occurred, the crops were already in the grain filling phase (R5) or more advanced, at 86%. of occurrences recorded on the Antirust Consortium website (Figure 3). This resulted in good control of the disease with fungicides and made it possible for a large part of the crops to not experience loss of productivity caused by the disease.
Chemical control remains the main tool for combating Asian rust, and the fungus' resistance to fungicides remains the biggest concern regarding this control method. The occurrence of resistance is a fait accompli, but the complexity of its distribution and frequency appears to have increased in the last two harvests. The use of different chemical groups and more elaborate management programs, in addition to the high natural variability of the fungus, seem to be contributing to varying situations of greater or lesser efficiency of the fungicides used.
The offer of cultivars with resistance gene(s) was an advance, although there is still a small number of materials on the market. Although they do not eliminate the need for the use of fungicides, the disease progresses more slowly in these cultivars, which can provide greater security to producers, for example, in a situation where the climatic condition does not allow the application of fungicide at the correct time. It is important to highlight that the continuous use of the same cultivar can result in a loss of effectiveness of the resistance gene that the cultivar possesses, which is why cultivars with different resistance genes must be rotated. 
Below is a summary of how the 2017/18 soybean harvest progressed in the main producing states in Brazil, with a focus on the occurrence of Asian rust. The information was obtained from the Antirust Consortium, the 2018 Harvest Rally, the National Supply Company (Conab), state agencies linked to agriculture and communications received from technicians and farmers linked to the soybean production chain.

Harvest x-ray

Northeast Region (Bahia, Maranhão and Piauí)

In this region, in general, it rained regularly during the harvest, providing good plant development. Keep reading...

Rafael M. Soares

 Claudia V. Godoy

 Claudine D.S. Seixas

 Maurício C. Meyer

Embrapa Soy