Resistance of Asian rust to the application of pesticides

Favored by the climate, Asian rust arrived earlier in soybean crops in southern Brazil, with  increased disease pressure and a tendency for the crop to require more intense management and a lot of attention from producers.

At the same time, concern is growing about maintaining the efficiency of fungicides available on the market and the search for preventing resistance and less sensitivity of fungi to the application of these pesticides.

The soybean disease scenario is quite atypical in the 2015/16 harvest, mainly due to the irregular distribution of rainfall in the regions, influenced by the El Niño phenomenon. From the beginning of the harvest until December, above-average rains occurred in the southern half of the country, prolonging sowing and making cultural treatments difficult.

In the Central-West and North regions, the low rainfall until December made the establishment and development of crops difficult, with the need for replanting in some regions. The occurrence of diseases is directly linked to climatic factors, especially humidity, necessary for the infection of most fungi.

In the South region, excessive rainfall resulted in the death of seedlings during the crop establishment phase, sometimes as a result of the death of root cells due to excess humidity and the anaerobic environment of the soil, with subsequent colonization of dead tissues by fungi. , and also directly by pathogens such as Phytophthora soyae, which causes Phytophthora root rot. But the highlight in the South region was the early incidence of Asian rust, caused by the fungus Phakopsora pachyrhizi.

The mild winter of 2015, also under the influence of El Niño, combined with the increase in off-season soybean cultivation (January sowings), favored the survival of volunteer soybean plants (guaxas or tigueras) in southern Brazil (Figure 1). On the Antirust Consortium map, occurrences of voluntary soybeans were recorded in the states of Rio Grande de Sul and Santa Catarina during the months of July and August. These two states do not adopt the sanitary void because frosts in winter usually kill tiguera soybeans, which did not happen this year.

The first record of rust in commercial crops occurred on November 6, in Tibagi, Paraná, by Fundação ABC, part of the Antirust Consortium. Subsequent reports occurred in several municipalities in the state and also in Rio Grande do Sul, Santa Catarina and Mato Grosso do Sul (Figure 1).

Even with a climate more favorable to the occurrence of rust in the South region, crops sown shortly after the end of the sanitary vacuum had a low incidence, showing that this is still one of the main strategies to escape the disease. Although the producer was prepared to control rust with fungicides, in some cases there were delays in applications and reapplications due to rain. In some crops, sown after the second half of October, the delay in the first applications compromised control, even with the most efficient fungicides.

The State of Mato Grosso, which normally has greater rust pressure due to more regular rains, this year had its first report in commercial farming only on January 4th, in Primavera do Leste, by Ceres Consultoria, in farming in stadium R7, harvest is close. From then on, new outbreaks were detected almost daily, mainly due to the return of rain in January, causing an increase in rust pressure.

Another factor that contributed to the increase in the disease was the intensification of the harvest of the first crops, spreading spores in this operation. Close to maturation there is an increase in the production of fungal spores due to the loss of residual fungicides. All these factors, combined with the delay in sowing, caused by irregular rainfall at the beginning of the harvest, currently characterize a very serious scenario regarding the potential for losses that rust has in this harvest in Mato Grosso, since most areas were sown at from November. Based on current surveys, crops sown after October 31st are at greater risk of losses due to rust. This in itself demands greater attention to recommendations for efficient control.

In Western Bahia, soybean sowing in rainfed areas that begins at the end of October/beginning of November was also harmed in this harvest by the dry spells that occurred in the region during this period, and when associated with the initial pests, mainly the elasm caterpillar, compromised its establishment, resulting in replanting and delays in the installation of crops. This climatic situation continued throughout the month of December and thus, no high pressure of common diseases was observed in the region. Diseases were only observed with greater frequency and intensity in the month of January, which was rainy in the region, making management difficult and contributing to their appearance.

The rust was detected for the first time on January 25th, in the municipality of Barreiras, and was confirmed by the Bahia State Agricultural Defense Agency (ADAB) in soybeans at the R6 stage. From that date onwards, rust could be found in soybean crops, but its evolution ceased with the reduction in rainfall in February. The presence of spores in the region, maintained by dew, still threatens the crop, which is phenologically younger, requiring attention from producers with the return of rain, which may require more intense management of this disease.

Fungicides are an important control tool and have prevented losses from rust. However, despite there being 119 fungicides registered with the Ministry of Agriculture, Livestock and Supply (Mapa) for rust control, these have been losing their effectiveness in recent years due to the selection of isolates of the fungus P. pachyrhizi that are less sensitive to them.

The main site-specific fungicides registered for rust control belong to three chemical groups, acting on the biosynthesis of ergosterol, an important component of the fungal cell membrane (IDM, triazoles) and on mitochondrial respiration, such as quinone oxidase inhibitors (IQo, strobilurins) and succinate dehydrogenase (ISDH, carboxamides). Since 2003/04, fungicides have been evaluated in a network of cooperative trials, coordinated by Embrapa Soja, the Agricultural Technology company (Tagro) and the University of Rio Verde/GO (UniRV).

Of the 14 fungicides evaluated by the trial network in 2014/15, only five registered showed control efficiency above 50% (Table 1; treatments 6 to 10). The triple mixtures (triazole-strobilurin-carboxamide; treatments 14 and 15), still in the registration phase, also showed good control efficiency, but they are combinations of modes of action already in use in the field. Network trials for the 2015/16 harvest are in the field, but there are no new modes of action being tested. The fungicides being tested are new combinations or ready-mixes with multisite fungicides.

Among site-specific fungicides, the only mode of action that has not yet had its efficiency reduced are carboxamides, due to their recent entry into the market in Brazil (2013). However, cases of carboxamide resistance have been reported for 14 pathogens worldwide, including fungi such as Corynespora cassiicola (on cucumber) and Sclerotinia sclerotiorum (on canola), which are also pathogens of soybeans. Due to their greater efficiency, these products tend to be used more, resulting in greater selection pressure for resistance to these fungicides.

Multisite or protective fungicides had been evaluated when rust entered Brazil. However, due to the lower control efficiency compared to triazoles at the beginning of the tests, they were discarded for rust control. With the reduction in the efficiency of triazoles and mixtures of triazoles and strobilurins, multisite fungicides have been registered for disease control in soybeans and their use increased in the last harvest in association with systemic fungicides.

In the 2014/15 harvest, the testing network evaluated the efficiency of multisite fungicides for the first year. These fungicides are old, some with more than 60 years on the market, and have been reevaluated to increase disease management options. In network trials, five applications of isolated multisite fungicides were carried out, with an average interval of 10 days between reapplications.

The efficiency of the best multisite fungicides ranged from 59% to 69% (Technical Circular 113), much higher than isolated triazoles and strobilurins. Multisite fungicides have been seen as an important tool in soybean rust management programs, increasing the control efficiency of fungicides that already have resistance problems and can delay the appearance of those that do not yet have.

Fungal resistance to fungicides is a natural evolutionary response. Sequential applications of fungicides with the same mode of action can select resistant/less sensitive individuals, as occurred with triazoles. 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. Situations of soybeans sown close to maturing crops with high rust infestation can be observed in this harvest (Figure 2).

With the high amount of fungus inoculum and early incidence of rust, these areas end up requiring up to seven applications of fungicides, imposing high selection pressure for resistance to the fungus that comes from crops sown earlier, where around three applications have already occurred. 

Normative instructions have been proposed limiting the sowing date to reduce selection pressure for resistance. The states of Goiás and Mato Grosso have limited soybean sowing until December 31st and the same restriction comes into force in Paraná at the end of 2016. The objective is to reduce sowings that require a greater number of applications to delay resistance to carboxamides .

These sowings after January represented less than 1% of the soybean area in Brazil in 2014/15 (Figure 3), however, the intensive use of fungicides in these areas can accelerate the loss of fungicide efficiency. This measure will only be effective if adopted by all producing regions, since the fungus is efficiently spread by wind from one region to another.

Another way to delay the emergence of resistance is by adopting anti-resistance strategies in all sowings. General anti-resistance strategies for fungi include rotating and employing commercial mixtures of fungicides with different modes of action and no cross-resistance; use the dose and application interval recommended by the manufacturer, adjusted for the disease epidemic, avoiding a long interval between applications; apply preventively, monitoring the crop and monitoring the inoculum situation in the region, applying close to the closure of the soybean rows when rust has already been reported in the region. As for products with carboxamides, they should not be used in more than two applications per crop.

The big risk of losing currently available fungicides lies in the fact that there are no new modes of action expected to enter the market in the next few years. Because it is a natural process, it is almost certain that resistance to most new fungicides will occur.

However, the useful life can be extended with rational use and the adoption of good cultural practices. For rust, these good practices must include all available strategies such as the adoption of sanitary vacuum, the use of early cycle cultivars and sowing at the beginning of the recommended season, the reduction of the sowing window, the monitoring of the crop since the beginning of development of the crop, the use of fungicides when symptoms appear or preventively and the use of resistant cultivars. 

Cultivars with resistance genes have been released onto the market. These cultivars have lesions with fewer spores and do not eliminate the need for fungicides. They are important management tools and can help reduce resistance pressure to 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.

Although rust appears to be under control in recent years, failures due to the low curative efficiency of fungicides in areas where application delays occurred were observed this year in the South. These situations tend to repeat themselves in the Cerrado in years with regular rainfall distribution. The sustainability of Brazilian soybeans could be threatened if fungicides continue to have reduced efficiency due to the fungus' resistance and lower sensitivity to these products. 

By Claudia Vieira Godoy, Embrapa Soya; Maurício Conrado Meyer, Embrapa Soya; Fabiano Siqueri, Mato Grosso Foundation; Lucas Navarini, Federal Institute of Rio Grande do Sul; Monica C. Martins, Faculdade Arnaldo Horácio Ferreira/Círculo Verde Agricultural Advisory and Research

Article published in issue 202 of Cultivar Grandes Culturas magazine.