Use of fungicides against Asian rust

Over the years, the fungus Phakopsora pachyrhizi, which causes Asian rust, has shown changes in relation to the application of fungicides, with the selection of less sensitive isolates. With this unfavorable scenario, extending the useful life of these products through rational use and the adoption of good cultural practices becomes increasingly essential. Limiting the sowing date is one of the measures to help delay resistance and loss of efficiency.

The occurrence of Asian soybean rust (Phakopsora pachyrhizi) in Brazil brought several changes to the crop's production system, the most pronounced being the intensification of the use of fungicides. Before rust, these products were recommended for the control of powdery mildew (Microsphaera diffusa), when symptoms were observed on plants in the field, and for end-of-cycle diseases (Septoria glycines and Cercospora kikuchii), during the formation and filling phase of grains. Powdery mildew occurs sporadically, in conditions of mild temperatures (20oC - 22oC) and low relative humidity, with differences in resistance between cultivars. End-of-cycle diseases cause early defoliation and early harvest, with greater damage in humid and hot periods. The maximum damage attributed to these diseases is 20%, in situations favorable to their development. 

The change in scenario that occurred with the entry of the fungus P. pachyrhizi is attributed to the high damage potential of this disease, which also causes early defoliation, but can cause up to 80% reduction in productivity. In addition to the high damage potential, the fungus occurs in practically all soybean-producing regions, with efficient spread by wind and an increase in inoculum as sowings are successive. For diseases with high damage potential such as rust, there is a high correlation between fungicide efficiency and productivity. Since 2003/04, network trials have been carried out by public and private institutions with the aim of comparing the efficiency of fungicides for different biological targets. Tests of products to control the fungus P. pachyrhizi have been conducted on all harvests since the beginning. 

In the trials carried out to control Asian rust, in the 2015/16 harvest, 17 fungicides were evaluated, three of which were included to monitor the sensitivity of the fungus (T2 to T4), six are still in the registration phase (T12 to T15, T17 and T18) and the others (T5 to T11 and T16) are ready-made mixtures of different modes of action (Table 1). Sequential applications of the products were carried out, starting at pre-closing and reapplying at intervals of 21 days and 14 days. The areas for conducting the trials are sown late to increase the probability of the disease occurring, due to the increase in fungus inoculum throughout the harvest. The main registered products evaluated are mixtures of site-specific fungicides, that is, they act at specific points in the fungus' metabolism, such as demethylation inhibitor fungicides (IDM, “triazoles”), which act by inhibiting the formation of the fungus' cell membrane and represent around 70% of fungicides registered for rust; quinone oxidase inhibitors (IQo, “strobilurins”) and succinate hydrogenase inhibitors (ISDH, “carboxamides”), which act by inhibiting fungal respiration.

In addition to efficiency, network tests have made it possible to monitor the change in sensitivity of the fungus P. pachyrhizi to fungicides that has occurred over the years. This change in sensitivity is attributed to the selection of less sensitive isolates, as a result of the intensive use of fungicides.

In trials carried out in 2015/16, the highest control percentages were observed for mixtures of picoxystrobin and azoxystrobin with benzovindiflupyr (T18 - 78% and T11, 76%), followed by triple mixtures of bixafen + prothioconazole + trifloxystrobin (T17 - 71 %) and pyraclostrobin + epoxiconazole + fluxapyroxad (T16 – 69%), and the mixture of trifloxystrobin + prothioconazole (T8 – 68%) (Table 1). The ready-made mixtures of the fungicides picoxystrobin + tebuconazole with the multisite mancozeb (T13) and azoxystrobin + tebuconazole + mancozeb (T14 and T15) showed control ranging from 55% to 64%, with a significant reduction in severity observed with an increase in the dose of the azoxystrobin mixture. + tebuconazole + mancozeb (T14 and T15). The lowest severity among these mixtures was observed for picoxystrobin + tebuconazole + mancozeb (T13).

Treatment with tebuconazole showed the lowest percentage of control (T2 - 19%), followed by azoxystrobin (T4 - 24%) and cyproconazole (T3 - 26%). The lower sensitivity of the fungus P. pachyrhizi to the fungicides IDM (T2 and T3) and IQo (T4), confirmed through bioassays and molecular studies, compromised the efficiency of the isolated active ingredients (Figure 1) and mixtures of fungicides with these two modes. of action.

In network trials, fungicides are evaluated individually, in sequential applications, to determine control efficiency. This information must be used to determine control programs, always prioritizing the rotation of fungicides with different modes of action and adapting the programs to the sowing season. Sequential applications of fungicides with the same mode of action can select resistant/less sensitive individuals, as happened with IDM and IQo.

Multisite fungicides were evaluated when rust was reported in Brazil. However, due to the lower control efficiency compared to IDM fungicides and mixtures of IDM + IQo in these tests, they were disregarded for rust control. With the reduction in efficiency of IDM and mixtures of IDM + IQo, multisite fungicides have been registered for disease control in soybeans and their use increased in the last harvest in association with site-specific fungicides. In the 2014/15 harvest, the evaluation of multisite fungicides for rust control began in network trials, alone and in combinations with site-specific fungicides.

In trials with isolated multisites, five applications are carried out, starting at pre-closure and repeating with an average interval of 10 days between applications, to determine the individual efficiency of each fungicide. In the network trials of the 2015/16 harvest, the control percentage of the best multisite fungicides ranged from 56% to 67% in relation to the control treatment (Table 2), being lower than the treatment with the site-specific fungicide azoxystrobin + benzovindiflupyr, with three applications. Multisite fungicides have been considered an important tool in soybean rust management programs, as they increase the control efficiency of fungicides with resistance problems, and may delay the appearance of those that do not yet have them.

In addition to trials with isolated products, others with multisite fungicides in combination with site-specific fungicides were carried out in 2014/15 and 2015/16. In general, the increase in efficiency of the site-specific fungicide in combination with the multisite is greater the lower the efficiency of the specific site alone, but not always sufficient for efficient control of the disease. The dose of the multisite fungicide is important to obtain the results observed in network trials and there is an interaction between the site-specific fungicide and the multisite fungicide, and generalizations should be avoided. Multisite fungicides can be an important tool in Asian rust management programs in soybeans, requiring registration with the Ministry of Agriculture, Livestock and Supply (Mapa) for their use.

Although fungicides represent an important control tool, all management strategies must be used to avoid reductions in productivity due to rust, involving the use of early cycle cultivars and sowing at the beginning of the recommended season; the elimination of voluntary soybean plants and the absence of soybean cultivation in the off-season (sanitary void); monitoring the crop from the beginning of crop development; the use of fungicides preventively or when symptoms appear and the use of cultivars with resistance gene(s). Cultivars with resistance gene(s) present lesions with fewer spores and do not require the use of fungicides. They are important management tools and can help reduce selection pressure on fungicides, but, as they have one or at most two resistance genes, the fungus can overcome this resistance in a similar way to what happens with fungicides. Sowing at the beginning of the recommended season is one of the main strategies to escape the disease, since the fungus begins to multiply after the period of lack of health.

From the results of network trials, it can be seen that there are a limited number of fungicides with good rust control efficiency and the new fungicides, in the registration phase, are new combinations of modes of action already available on the market or associations with multisite fungicides. . The big risk is that there is no new mode of action to enter the market in the next few years. As it is a natural process, there is a high probability that resistance to most new fungicides (ISDH) will occur. However, the useful life of these products can be extended with rational use and the adoption of good cultural practices.

One of the ways to reduce selection pressure for resistance is to limit the number of fungicide applications to the crop. As the need to use fungicides increases as the sowing season advances, defining sowing deadlines can contribute to reducing the number of applications. Normative instructions have been proposed limiting the sowing date to reduce selection pressure for resistance. The states of Goiás, Mato Grosso and Paraná limited soybean sowing until December 31st. The objective is to reduce sowings that require a greater number of applications to delay resistance to ISDH fungicides. These sowings after January represented less than 1% of the soybean area in Brazil. However, intensive use of fungicides in these areas can accelerate the loss of fungicide effectiveness.

The results presented were only possible with the joint work of several research institutions, which are part of the Antirust Consortium. The complete results of the network tests can be accessed on the Embrapa Soja website (www.embrapa.br/soja).

Cláudia V. Godoy, Maurício C. Meyer, Embrapa Soya; Carlos M. Utiamada, Tagro; Hercules D. Campos, UniRV

 

Article published in issue 208 of Cultivar Grandes Culturas.