Chemical control of Asian rust in soybeans

 

Despite widespread expansion, soybean farming systems, in general, are still not stable. Among the various factors that contribute to variations in yield, diseases deserve to be highlighted. Approximately 40 diseases caused by fungi, bacteria, nematodes and viruses have already been identified in Brazil.

In this context, Asian rust is the main foliar disease of soybean crops worldwide. The first report of the presence of the fungus on the American continent occurred in Paraguay in 2001, followed by Argentina and Brazil in the 2001/02 harvest, and it is currently present in all soybean-producing states.

Symptoms begin on the lower leaves and are characterized by the appearance of gray-green spots that are darker than the healthy tissue of the leaf. On the abaxial side of the leaf, in the corresponding location, protuberances are observed, which are the uredia, where the propagative structures of the fungus, the uredospores, are found. These require six to ten hours of leaf wetness at temperatures between 15ºC and 30°C, with 22°C being optimal to penetrate the plant. Uredia form after the fungus' incubation period of approximately nine days, while uredospores are formed within three weeks.

The most efficient, long-lasting and economical control of plant diseases is obtained through the sum of available management measures and never through an isolated practice, and for this disease, the recommended strategies are: the use of early cycle cultivars, sowing in beginning of the recommended season, the elimination of volunteer soybean plants, the absence of soybean cultivation in the off-season (sanitary void), the planting of resistant cultivars and the use of fungicides preventively or at the onset of symptoms.

Chemical control of Asian rust began in Brazil in the 2002/03 agricultural harvest, using a mixture of triazole + strobilurin or triazole alone. The mixture of fungicides, in general, had better control efficiency and higher crop yield when compared to isolated fungicides.

From the 2007/08 harvest onwards, lower efficiency of triazoles was observed in trials carried out during late sowing in the state of Mato Grosso. This occurred due to the selection of a less sensitive population of the fungus to this chemical group, which was probably caused by the use of isolated fungicides, reduction of the fungicide dose and application with high disease severity.

The loss of sensitivity of the fungus that causes Asian rust has also been confirmed for strobilurins in the 2013/14 harvest, and this can be monitored by determining the Inhibitory Concentration (IC50) of the fungicide, which is the amount in mg/L capable of inhibiting 50% of spore germination or mycelial growth of a fungus. Comparing the IC50 value of the fungicide azoxystrobin and azoxystrobin + benzovindiflupyr (Figure 1), it was found that the former presented a value 39 times higher than that for the fungicide containing carboxamide. This is quite worrying, since many registered fungicides are mixed with the chemical group strobilurin, so the rational use of fungicides is essential.

Currently, Brazil has 114 fungicides for commercial use registered for Asian rust, 68% of which are isolated triazoles, 35% mixtures of triazole + strobilurin, 3% isolated strobilurins, 2% a mixture of strobilurin + carboxamide and the The others are a mixture of triazole + strobilurin + benzimidazole, triazole + benzimidazole. Although recently registered by the Ministry of Agriculture, Livestock and Supply (Mapa), the chemical group carboxamide (ISDH) is not new, having been commercialized since the 1960s.

However, the molecules registered for soybean cultivation are new and, in recent years, research has devoted special attention to them, due to the urgent need for new chemical groups. This need occurs mainly due to the loss of sensitivity of the fungus that causes Asian rust to the chemical groups of fungicides used for decades.

Regarding the mechanism of action of fungicides, the triazole chemical group (IDM) acts on the integrity of the plasma membrane, altering its permeability, being known as an inhibitor of C-14 demethylation (IDM). Similarly, triazolinthione inhibits the biosynthesis of ergosterol in the intrinsic property of membranes, as it inhibits the synthesis of ergosterol and has in its composition a sulfur molecule in a double bond to the aromatic ring.

Fungicides from the strobilurin and carboxamide groups interfere with different respiratory processes. Strobilurin acts on complex III of the electron chain, this occurs through the inhibition of the external quinone of the mitochondria (IQe), thus blocking mitochondrial respiration, paralyzing the production of ATP. Carboxamide (ISDH) inhibits the oxidation of the succinate dehydrogenase enzyme, making respiration unavailable, however, it acts on complex II of the electron chain in the mitochondria.

In recent years, in experiments at the Federal University of Passo Fundo (UPF), traditional Asian rust control programs (containing only triazole + strobilurins) and new ones (containing triazole + strobilurins alternated with strobilurins + carboxamide) were evaluated (Table 1).

Due to the absence of chemical groups to rotate with triazole and strobilurin, the producer could only choose to use one of these chemical groups or a mixture of the two in all applications. With the registration of carboxamide, the producer now has the option of using a mixture of triazole + strobilurin alternating with strobilurin + carboxamide, being an excellent anti-resistance strategy, which can prevent or delay the loss of sensitivity of fungi to fungicides, similar to what occurred with triazole and strobilurin.

Regarding the severity of the disease (Figure 2) evaluated at stage R.5.5 (pods between 76% and 100% granulation), it was found that the lowest severity of the disease was for treatment 9, in which two of the four applications were composed of the carboxamide benzovindiflupyr.

To control the disease, treatments containing two applications of carboxamide alternating with triazole + strobilurin, 9 (benzovindiflupyr) and 8 (fluxapyroxad) stood out from the others with the highest control values.

For yield (Figure 3), it was found that treatment 9 (benzovindiflupyr) was statistically superior to all the others, followed by treatment 8 (fluxapyroxad) which did not differ statistically from the other fungicide treatments.

The lowest value for fungicide treatments was 2 (four applications of pyraclostrobin + epoxiconazole), a fungicide in use since 2001. 

Therefore, due to the low IC50 values, high control values ​​and soybean yield, new chemical control programs for Asian rust must contain the carboxamide chemical group alternating with the triazole + strobilurin chemical groups, as, in addition to having Optimal control of the disease is an excellent anti-resistance strategy and can prevent or delay the loss of sensitivity of fungi to fungicides.

By Carolina Cardoso Deuner, Gustavo Visintin e Giovani Pastre, University of Passo Fundo

Article published in issue 189 of Cultivar Grandes Culturas magazine.