Management of downy mildew in soybeans

Downy mildew has shown an increasing incidence in recent soybean harvests in Brazil, with significant production losses. Among the factors that cause predisposition to the disease is nutritional imbalance, which affects the ability of plants to defend themselves. Therefore, the supply of phosphites plays an important role in activating defense mechanisms and consequently in managing this pathogen.

Soy is the annual crop that has shown the greatest growth in recent years, both in productivity and in hectares sown, with Santa Catarina growing by 5,9% in sown area and 4,2% in production. However, there are many obstacles that limit the exploitation of the cultivar's maximum productive potential. Among the various limitations are diseases that cause a lot of losses, such as rust, which can cause losses of up to 100%.

Downy mildew is a disease caused by the fungus Peronospora manshurica (group of Oomycetes) and despite being considered secondary, it has been occurring more frequently in recent harvests, with production losses that can reach 14%. The initial symptoms of downy mildew are light green spots, which evolve into yellow on the upper page of the leaf, and later into necrotic tissue (Figure 1). On the back of the leaf, the spots are yellowish and fruiting structures of the fungus appear, with a cottony appearance and a slightly pink to gray color.

Fungicides that are normally used to control other end-of-cycle diseases in soybean crops have no effect on downy mildew control, making it necessary to use alternative means or new strategies to control and manage this disease.

Among the main causes for the occurrence of the disease and the predisposition of plants to pathogens is nutritional imbalance, which can be considered one of the main factors responsible for triggering defense mechanisms. Phosphites are an important management strategy, as they have the ability to activate plant defense mechanisms and reduce the severity of diseases.

The currently most accepted function of phosphite is to be an excellent inducer of plant defense mechanisms, increasing the amount of phytoalexins and proteins related to pathogenesis, which prevent infection by pathogens such as bacteria and fungi.

Phosphites can act directly, as they have a fungicidal action on some invasive fungi inside the plant tissue, causing death or inhibition of the growth of the fungus, proving to be effective in controlling diseases from the Oomycete group, such as downy mildew, or act indirectly through the activation of plant defense systems.

Phosphites are the only rapidly absorbed systemic antifungal chemical compounds that, in addition to being mobilized through the xylem, also do so through the phloem.

Research carried out by Silva (2011) in corn crops concluded that the foliar application of phosphite reduced the severity of diseases in decreasing order: cercospora spot, white spot, common helminthosporiosis and rusts. Furthermore, potassium phosphite provided an increase in corn productivity ranging between 6,5% and 7% in relation to the control, due to the control of foliar diseases.

In soybean crops, potassium phosphite reduced the severity of Asian rust when a single application was made, applied twice and after the application of fungicides (pyraclostrobin + epoxiconazole, methyl thiophanate + flutriafol and tebuconazole), showing that the phosphite had effect on disease severity, however, there was no difference in productivity (Neves, 2006).

With the aim of verifying the effect of using phosphites associated or not with fungicide on downy mildew in soybean crops in Campos Novos, the University of the West of Santa Catarina (Unoesc) conducted a field experiment in the 2012/2013 harvest (Figure 2) where we sought to evaluate the effect of this product on controlling the severity of the disease and the productivity of soybeans.

Daily data on precipitation, temperature and relative humidity for the cultivation site were obtained from Epagri/Ciram - Campos Novos, where up to 40 days after sowing the climatic conditions were favorable for the development of downy mildew, with 316,5 records being recorded during this period. 40mm of rainfall. From 60 to 3 days after sowing there was no rain (Figure XNUMX).

Even with this irregular rainfall, a variation in disease severity was observed between 4,73% and 14,03% between the phenological stage V7 and R5 (Figure 4). The first symptoms of soybean downy mildew were observed at stage V7 and maximum severity occurred at stage R5 in treatment with fungicide alone.

In the evaluation carried out at stage R1 and stage R5, treatments that included the use of phosphite (T1, T2 and T3), both alone and associated with fungicide, showed lower mildew severity compared to treatment with only fungicide (T4) (Figure 4 ), proving the product's efficiency in controlling the severity of mildew.

In addition to the soybean downy mildew severity assessments, crop productivity was also assessed (Figure 5). The data obtained demonstrated that the application of phosphites in conjunction with fungicide provided greater yield, justifying the use of this technology in the municipality of Campos Novos.

The fungicide + phosphite treatment recommended dose (T3) increased 264,4kg/ha and the fungicide + phosphite treatment doubled dose (T4) had an increase of 350kg/ha in relation to the fungicide treatment (T1), and both were superior to the treatment fungicide (T1). The phosphite treatment (T2) was significantly lower than the fungicide treatment at 559,4kg/ha. The use of phosphites alone, although it had effects on the severity of the mildew, was lower in productivity when compared to the fungicide treatment alone. This was likely due to the inability of phosphites to control other diseases. 

Grain mass is an important yield component for the crop, and is related to the final productivity of the crop. As with productivity, treatments using phosphite combined with fungicide (T3 and T4) were those that achieved the highest mass of 100 grains (Figure 6).

In the Fungicide treatment (T1), which is normally used by producers in the Campos Novos region, Santa Catarina, productivity was 2.864kg/ha, below the region's average of 3.300kg/ha. This probably occurred due to the lack of rain between 40 days and 60 days after sowing.

The use of phosphites, alone and associated with fungicide, provided control over the severity of soybean downy mildew. The application of phosphites associated with the fungicide provided a higher grain yield, when compared to the fungicide-only control. The results obtained recommend the use of phosphites to control soybean downy mildew, bearing in mind that it must be part of an integrated management program combining phosphites with fungicide.

Sandra Dalla Pasqua, Erivelton Zanon – Academics from the Agronomy course at Unoesc; Tamara Pereira Felicio – Professor of the Agronomy course at Unoesc

Article published in issue 193 of Cultivar Grandes Culturas.