Genetic resistance to Asian rust

In a scenario of increasing loss of sensitivity of the fungus that causes Asian rust, the genetic resistance gains even more importance, not as an isolated alternative of  control, but as a tool essential in the integrated management of this disease that challenges sustainability of soybeans in Brazil.

Since the first report of occurrence of Asian soybean rust in Brazil in 2001, the disease caused by the fungus Phakopsora pachyrhizi to be continued threatening the success of the soy production chain. Even after many years of “coexistence” with the rust, the production sector has not yet found an effective solution to combat it. The increase in the number of fungicide applications each harvest, with the current average close to three applications is a reflection of the problem with soybean rust. You costs of fungicides used to control rust are estimated at US$ 2 billion annually, which together with potential productivity losses make the disease a serious threat to the sustainability of soybean production in the Brazil.

Despite the variation in incidence of rust in soybean producing regions in the country, the disease dispersion map developed by the Antirust Consortium shows its wide occurrence (Figure 1). The disease recurs every year, with variations in its incidence influenced mainly by climatic conditions, efficiency of the sanitary void and the control of voluntary soybean plants in the off-season, which generally serve as a “green bridge” for the inoculum of the following harvest.

Despite the expectations of effectiveness of fungicides against the disease has been demonstrated by scientific community and agrochemical companies a significant loss of sensitivity of the fungus to active ingredients on the market. That occurs mainly through the selection process of resistant individuals present in populations of the fungus, which associated with its rapid ability to reproduce and dispersion contribute to the rapid loss of control effectiveness.

FRAC Brasil (Committee for Action on fungicide resistance: http://www.frac-br.org/soja) recently presented new recommendations for the management of Asian soybean rust, which are based mainly on the importance of delaying the rapid development of resistant populations of the fungus to the three main chemical groups used in controlling the disease. In addition to the loss of sensitivity of the pathogen to fungicides offered on the market, the concern of productive sector also exists due to the low expectation of product launches with high effectiveness against rust in the short and medium term.

Considering the bearish scenario effectiveness of the main chemical products available, the use of cultivars resistant becomes an important tool in rust management. INOX soybean cultivars are currently available on the market, with soybean rust resistance genes. The main resistance mechanism of these cultivars occurs due to a hypersensitivity reaction, which can be visually identified by the reddish-brown lesion type RB (from English, “reddish brown” – Figure 2). This type of reaction limits the development of fungus in the leaf tissue and significantly reduces sporulation, limiting the progress and spread of the disease. Susceptible cultivars show typical symptoms of the disease on the leaf with “TAN” type lesions, which have reproductive structures (uredia) with large amounts of uredospores viable. Uredospores are easily spread by wind, allowing rapid evolution of the rust epidemic. At first, symptoms appear as small punctures on the underside of the leaf, but under favorable conditions of temperature and high humidity, the severity can evolve quickly and result in yellowing and premature defoliation, consequently damaging the grain filling. 

Despite the resistance reaction to rust, in situations of high incidence of the disease, resistant cultivars can react with many RB lesions and this can result in loss of leaf area, if fungicide is not used to manage the disease. Thus, even with the use of cultivars INOX the application of fungicides continues to be fundamental, as in addition to reducing substantially the progress of the disease, according to results of field trials (Figure 3), there will always be a need to control other diseases, such as target spot and end-of-cycle diseases (CFDs).

Another role of the use of cultivars with rust resistance is the greatest flexibility for fungicide, as the genetic mechanism of action is continuous, from the establishment of the soybean crop until the reproductive. This allows greater control security in situations of periods periods of rain, which make fungicide applications impossible or when delays in applications. The contribution to greater security provided by INOX cultivars in commercial soybean planting areas can be observed in areas of high disease pressure (Figure 4).

Genetic resistance is conferred by genes known in the literature as Rpp (resistance to Phakopsora pachyrhizi). Currently, seven distinct genes are reported with the acronyms Rpp1-7 and for of each, there are different “versions” known as alleles. By analogy with products chemicals, each gene could be considered as a chemical group with a mode of different action, while alleles would be the different molecules within each group. The genes are found in sources (soybean lines – Figure 5) existing in germplasm banks, which in most cases are not adapted to the edaphoclimatic conditions of Brazil and, therefore, require a long genetic improvement work so that they can result in cultivars that combine genetic resistance with high productive potential.

The work to characterize the various sources of resistance show that there are very different hypersensitivity (Figure 6). The importance of the diversity of rust resistance genes is precisely due to the genetic variability existing in populations of the fungus, where There will always be some isolate or race that has the ability to “break” the resistance of a variety, as well as loss of sensitivity to fungicides. 

To avoid breakage of resistance and increase the longevity of genetic control of the disease, multiple genes can be combined into a single variety. To achieve this, it is essential to development of DNA analysis tools known as markers molecular, which allow the efficient selection of genes during the process of improvement. genetic improvement, however, it demands time, requiring an average of 7 years to develop a new variety.

Due to the high investment in development of resistant cultivars, the use of fungicides is also essential to preserving genetic technology. Fungicides help reducing selection pressure and eliminating individuals of the fungus that could potentially “break” the existing resistance in cultivars. From the In the same way, cultivars with genetic resistance can help control of individuals of the fungus less sensitive to fungicides, contributing to longevity chemical control in rust management. Therefore, it is recommended that management and number of fungicide applications on a resistant cultivar is similar to the of a susceptible variety.

Against Despite all the complexity in effective rust control, it is clear that the central management pillar is the reduction of the amount of pathogen inoculum (read reduction or elimination of viable uredospores) throughout the harvest and especially in the off-season. To this end, there is no single solution, but an integrated control proposal (Figure 7), which includes: (i) respecting the void with a minimum of 60 days and eliminate volunteer soybean plants in that period; (ii) give preference to early cycle cultivars that have good adaptation to planting at the beginning/opening of the harvest; (iii) use cultivars with genetic resistance to Asian rust, currently on the market with the INOX brand; (iv) carry out monitoring and rapid control with fungicides at the first symptoms or preventive in the field, with the adoption of protectors multisites and the alternation of fungicides, following the recommendations of manufacturers.

Finally, it is now clear that Asian soybean rust is a national security problem. The impacts socio-economic consequences will be catastrophic, according to surveys carried out by Brazilian Agribusiness Association (ABAG), if Brazil loses its ways of controlling rust. It is essential that the producer always continues to use the concept of integrated rust control, respecting and preserving the currently existing technologies.

Article published in issue 222 of Cultivar Grandes Culturas, November, 2017.