Abstract
This review focus on the sensitivity reduction of Phakopsora pachyrhizi to site specific fungicides in the Brazilian state of Mato Grosso.The soybean grown area in this state in the 2018/19 growing season was 9,756,668 hectares. The main crop disease is Asian soybean rust that depending on disease severity may cause 80% yield reduction. The rust chemical control started in the 2002/003 season with site-specific fungicides applied solo and with 3.6 sprayings/ha/season. For these reasons, the fungus had sensitivity reduction to the fungicides DMIs, QoIs and SDHIs resulting in a short effective life. Anti-resistance strategies were not adopted to prevent or delay the development of soybean rust resistant population to site-specific fungicides. Reduced sensitivity to the target site has been observed for all site-specific fungicides involving cross and multiple resistance resulting in ineffective control. So far, eight mutations in Phakopsora pachyrhizi have been identified conferring reduced sensitivity to DMIs, QoIs and SDHIs but so far specific mutations are not considered in chemical control. The directional selection has occurred and has been aggravated season-after-season by the continuous use of site-specific fungicides in a large area, for 18 season and with more than three sprayings/area/season. The presence of soybean weed in one million hectares of cotton crop infected by rust aggravates the directional selection. Anti-resistance strategies should include sowing at the beginning of the recommended season, avoid the December season and replacing it by February, use of scientific criteria to time the first application and the use of multisite fungicides in all applications and in the entire area cultivated with soybean.
Highlights
In Brazil, soybean [Glycine max (L.) Merr.] is grown from Rio Grande do Sul to the far north, in Roraima, from the West in Rondônia, to the East in Maranhão, extending the growing season and the period of the green plants presence in the field throughout the years
Failure in disease control has been reported in the group of carboxamide fungicides, the succinate dehydrogenase inhibitors (SDHIs) (Simões et al, 2008; FRAC, 2017). These reports confirm the occurrence of cross and multiple resistance of P. pachyrhizi to the site-specific mechanism of action (MOA) fungicides used to Asian soybean rust (ASR) control (DMI, quinone outside inhibitor (QoI), SDHI)
The first mutation linked to reduced sensitivity of P. pachyrhizi to SDHI fungicides was detected in the 2015/2016 season
Summary
In Brazil, soybean [Glycine max (L.) Merr.] is grown from Rio Grande do Sul to the far north, in Roraima, from the West in Rondônia, to the East in Maranhão, extending the growing season and the period of the green plants presence in the field throughout the years. Due to the large number of sprayings of the same active ingredient, researchers, and farmers mainly from the Midwestern states, Mato Grosso and Goiás, already in the 2007/08 season, observed ASR control failure (Silva et al, 2008) This fact was reported in other soybean producing states and regions, with the DMI fungicides and soon after with the quinone outside inhibitor (QoI) and their mixtures (Reis et al, 2015). Failure in disease control has been reported in the group of carboxamide fungicides, the succinate dehydrogenase inhibitors (SDHIs) (Simões et al, 2008; FRAC, 2017) These reports confirm the occurrence of cross and multiple resistance of P. pachyrhizi to the site-specific mechanism of action (MOA) fungicides used to ASR control (DMI, QoI, SDHI). The rapid adaptation of P. pachyrhizi to DMIs in less than a decade from the beginning of its use for rust control, is attributed to the selection pressure due to its use in large area, a high number of sprayings/area, as well as the clonal dikaryotic fungus life cycle and short generation (Schmitz et al, 2014)
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