Management of spontaneous soybeans or guax

Disease management in soybean crops has reached levels of complexity that require combined actions throughout the year. As soybean monoculture is a permanent activity, the pressure of necrotrophic diseases has been increasingly high, given the vast opportunity for these pathogens to survive in “soil/straw” conditions. Another additional factor are biotrophic diseases such as rust and powdery mildew, which have also required expanded knowledge from agronomists, technicians and producers. In this case, they have found great availability for survival, even in the off-season, by lodging on spontaneous soybeans or guax.

Spontaneous soybeans or guaxa are the result of the germination and emergence of seeds resulting from loss during harvest or even during transport via highways and railways. After the summer harvest, these plants establish themselves in fallow areas, pastures or even in winter crops that do not carry out correct weed management. In this way, soybeans become a preferred host of diseases and insects during the off-season.

Anyone who observes these guava soybean plants can identify problems such as Asian rust (Phakopsora pachyrhizi), powdery mildew (Diffuse Microsphaera), septoria (Septoria glycones) and cercosporiosis (cercospora spp.), as well as stem flies (Melanagromyza sojae).

The damage caused by these diseases in the normal harvest can reach worrying magnitudes. In the case of necrotrophic diseases, high severities of septoria or cerposporiosis can compromise more than 30% each of the final yield (Guimarães, 2008; Godoy et al., 2016). For biotrophic plants, productivity losses can reach up to 90% for rust (Godoy et al., 2020), and 35% for powdery mildew (Godoy et al., 2021).

With all these conditions available to host these pathogens and insects, the off-season period, which should be absent of soybeans to reduce the viability of these individuals, becomes a fertile field for their permanence and multiplication. Thus, the beginning of the following soybean harvest, which should have a low availability of disease inoculum, is now targeted by a great pressure of pathogens, given that they remain in the off-season in adequate and uncontrolled conditions.

The presence of disease installed in the crop already in the first stages of soybean will overwhelm the satisfactory control effectiveness capacity of any fungicide. Therefore, the need for a greater number and quantity of applications will be necessary, not related to the success of the control. This fact will be directly related to productivity losses, greater deposition of products in the environment, low control effectiveness and greater financial expenses on the part of the producer, reducing their profitability.

Another factor closely linked to this is the ability of pathogens to overcome controls through the directional selection of resistant individuals, through the use of fungicide. At this point, individuals in the Phakopsora pachyrhizi (soybean rust) have great abilities to avoid being killed by chemical controls, through different strategies. In this case, we are placing all the responsibility for controlling this disease on chemical fungicides and it is no surprise that their effectiveness has been decreasing with each harvest. Therefore, it is essential that integrated management strategies are observed so that chemical control is not overloaded but rather a partner.

Given these facts, efforts from several public and private institutions have been combined to develop strategies to mitigate this wear and tear. In addition to recommendations for the correct use of fungicides, two tactics have been important, such as sanitary vacuuming and the sowing calendar. The latter came through Ordinance SDA/Mapa nº 840, which was modified by nº 886, which aims to concentrate soybean sowing in a restricted period, to try to reduce the exposure of plants in conditions that are possibly more favorable to infection by the pathogen.

On the other hand, technically it is more important than the soybean sowing calendar to use and respect the sanitary void. Ordinance No. 306 explains the defined and continuous period, of at least 90 days, in which live soybean plants cannot be sown or maintained in the field. During this period volunteer plants must be eliminated. The objective is to reduce the population of the fungus in the environment during the off-season and thus delay the occurrence of the disease during the harvest (Embrapa, 2022).

However, these same states in the south of the country have not done enough homework to reduce soybeans in their fields, resulting in worrying situations (Figure 10). Many producers have relied on the possible effect of frost to control these plants, but this effect is becoming less and less reliable.

Therefore, agronomists, technicians and producers need to focus on the off-season as a way to reduce spending on chemical or biological controls during the harvest, given the ability of diseases and pests to survive in the green bridge. Disease control in the current soybean harvest is directly related to what occurs in the off-season. For producers to spend less on fungicides and avoid damage from diseases, year-round management is essential.

By Marcelo Gripa Madalosso e Leonardo Gollo, Madalosso Research

Article published in issue 293 of Cultivar Grandes Culturas Magazine