Essential measures against Asian rust in soybeans

In a scenario in which the speed of resistance occurring in the field is greater than the discovery of new molecules and the time required to register new agrochemicals, the sanitary void and the sowing schedule become essential measures for the sustainability of production of soybeans in Brazil...

Soy is the main Brazilian commodity. In the 2019/20 harvest, it is estimated that Brazil produced around 124,8 million tons, a new record for grain production (Source Conab September/2020). The records associated with the soybean harvest in recent years in Brazil are mainly due to the adoption of new technologies in the field and mainly to integrated pest and disease management measures.

Soy is threatened by dozens of pests and diseases that are greatly favored by Brazil's tropical climate. One of the diseases most feared by farmers is Asian Soybean Rust caused by the fungus Phakopsora pachyrhizi. The disease was reported in Brazil in the 2001/02 harvest and demonstrated its full destructive potential for the crop over a few years. In just a few days, this disease can completely defoliate plantations and lead to major losses in grain productivity.

In South America, the disease began in Paraguay and spread through the state of Paraná, and is also found in volunteer soybean plants and in second-season soybeans in Brazil. In the following harvest (2002/03), losses from the disease were estimated at around 2 billion dollars. Still in the 2012/13 harvest, losses from rust were estimated at around 1,5 billion dollars (Aprosoja/MT, 2013).

Phakopsora pachyrhizi (causal agent of soybean rust) is an obligate biotrophic fungus, that is, it needs its living host (soybean) to complete its life cycle. In this way, off-season crops, or “off-season soy” are mainly responsible for keeping the rust inoculum alive in the field, which also benefits from the high occurrence of guax plants (voluntarily emerged) remaining from the seed harvest. These plants are the source of primary inoculum for the following harvest. In Brazil, there are still practically permanent sources of inoculum coming from neighboring countries such as Bolivia and Paraguay, since these countries do not have a planting calendar.

The best strategy for reducing inocula in the field is the absence of the host. This strategy also favors the reduction of insects, another concern that farmers face in their crops. The implementation of the soybean void (period of absence in the field) in Brazil occurred between 2006 and 2007, mainly due to the problems faced by soybean farmers with the fungus that causes Asian Soybean Rust, Phakopsora pachyrhizi.

This management measure in force to this day not only reduces rust inoculums but also other diseases that have been affecting production and increasing production costs, due to the greater need for fungicide applications. The minimum period without soybeans in the field is 60 days during the sanitary void. Thirteen Brazilian states and the Federal District adopted this measure through normative instructions.

But for this period to be efficient, producers must destroy live soybean plants on their farms, called volunteer plants (tigueras and guaxas). The producer must destroy the plants through the application of herbicides or by physical removal.

The FRAC-BR (Brazilian Fungicide Resistance Action Committee) has warned of another problem related to the presence of volunteer plants during the empty period: the presence of fungicide-resistant soybean rust inoculums.

The fungus that causes Asian Rust has developed several mutations in its genome over the years that confer the ability to survive even after the application of fungicides. Resistance arises due to a process called selection pressure, when successive applications of fungicides with the same mechanism of action are carried out on fungal populations and select individuals more adapted to the presence of fungicides that have the same mechanism of action.

At the beginning of the disease's emergence in Brazil, the fungicides most used to control it were those from the triazole and strobilurin groups. In 2007, approximately 5 years after the use of triazoles to control rust, the first reports emerged of the loss of effectiveness of these fungicides due to the resistance developed by the fungus. In 2012, strobilurins lost their effectiveness, and genetic resistance was also found in the fungus populations. 

The most recent and fastest case of this fungus adapting to fungicides was reported in the 2015/16 harvest. In three years of using fungicides from the carboxamide group on crops, populations of the Asian rust fungus showed resistance to fungicides of the same chemical class, through a genetic change in one of the genes that encode the carboxamide target protein in the fungal cells. The genetic adaptation occurred in the gene for the enzyme Succinate Dehydrogenase C, with the replacement of the amino acid isoleucine (I) by phenylalanine (F) at position 86 of the protein sequence (mutation sdhC -I86F). The mutant individuals quickly spread throughout the producing areas in Brazil.

In the guaxa soybean monitoring study carried out by FRAC-BR, the presence of resistant inoculum of Asian Rust with a high frequency of the I86F mutation was detected in the years 2017, 2018 and 2019 in practically all soybean regions in Brazil (Figures 1, 2 and 3).

These volunteer plants maintain the high frequency of mutants in the field. In the case of this study, only one mutation was studied (sdhC-I86F which confers loss of sensitivity to carboxamides), but these plants can carry mutant rust with reduced sensitivity also to triazoles and strobilurins. 

Figure 4 corroborates the data presented previously and illustrates the presence of volunteer soybeans (guaxa) with rust in several locations in Brazil. These plants, by hosting populations of the pathogen with a higher frequency of mutation, serve as a “green bridge” between planting times and contribute to populations less sensitive to fungicides multiplying with greater intensity and, consequently, infecting plants from new sowing. .

The data reinforces the importance of the sanitary void for reducing resistant populations and warns of the importance of another phytosanitary measure that has become fundamental for soybean production in Brazil: the sowing schedule.

The timing of soybean sowing or planting is the determination of a deadline for sowing soybeans in the harvest. This measure was established by state regulations only in seven soybean-producing states: Goiás, Mato Grosso, Paraná, Santa Catarina, Tocantins, Bahia and Mato Grosso do Sul. The main objective of the schedule is to reduce the number of fungicide applications throughout the harvest and, therefore, reduce the selection pressure for fungus resistance to fungicides.

Late soybean sowings can receive fungus inoculum from the vegetative stages, requiring fungicide application to be brought forward and requiring a greater number of fungicide applications. The greater the number of applications, the greater the exposure of the fungicides and the greater the chance of accelerating the process of selecting resistant populations. Graph 1 illustrates the maximum and median values ​​of the evolution of the frequency of the sdhC I86F mutation between the 2017 and 2020 harvests, whose percentage values ​​increase as plantings are carried out throughout the harvest beginning in September of each corresponding year.

Finding fungicides with new mechanisms of action capable of controlling resistant populations has become one of the biggest challenges for pesticide companies around the world. The speed at which resistance occurs in the field is greater than the speed of the difficult task of discovering new molecules with new mechanisms of action, as well as greater than the long process of registering new agrochemicals in Brazil. Within this context, the only alternative to guarantee the longevity of available technologies are management practices.

The sanitary void and the soybean sowing schedule are anti-resistance strategies with different objectives for the management of Asian soybean rust and that favor the control of other diseases and pests that affect the crop. These are powerful weapons that must be respected and promoted, and that contribute to the sustainability of Brazilian soybean farming.

There is a false feeling on the part of producers that rust is under control in Brazil and many movements have emerged questioning these phytosanitary measures. However, producers cannot and should not take risks. Rust can destroy crops and reduce productivity by 90%, especially in the current scenario, in which fungicides no longer exert their full control potential due to the resistance of the fungus.

The best way is to use good agricultural practices that have guaranteed Brazil's success in grain production. In other words, carry out good desiccation; respect the soybean fallow period and planting calendars; eliminate volunteer soybean plants around their properties; apply fungicides preventively, respecting the correct intervals; make use of good application technology; rotate and associate different chemical groups of fungicides, in addition to using multisite fungicides in the system. Only with these measures, soybean production in the country can be sustainable for more harvests and can maintain higher levels of productivity per planted area. 

By Frac - BR