Directional Selection of Phakopsora pachyrhizi Towards Site-Specific Fungicides in Mato Grosso State

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.

Asian Rust Severity in Soybean Sown in December and February in Mato Grosso State

“…To fulfill this objective in Mato Grosso, has been imposed by legislation limiting the soybean sowing time. However, more importante than sowing time limitation is the highest frequency of sprayings reaching P. pachyrhizi, performed for several seasons and in a large area, on soybean weeds amid cotton crop where, so far, nothing has been done to be mitigated (Zambolim, 2019;Reis et al, 2021b). A second strategy is the application of co-formulation allways containing site-specific + multissite fungicides as performed in our work (Ishii & Hollomon, 2015).…”

“…Soybean fields for the complementary commertial and own growers seed production have been established in December, as a result of IN 002/15. Despite being changed by IN 001/21, nothing has changed in relation to the seeding date and continues to lead to high rust severity as has been recorded, demanding a great number of site-specific fungicide applications in double or triple mixtures (DMI + QoI or DMI + QoI + SDHIs) (Reis et al, 2021b). This practice has reduced the growers profit and with high potencial for environmental damage.…”

Minimizing the Selection Pressure of Site-Specific Fungicides Towards Phakopsora pachyrhizi in Mato Grosso State: A Review

“…Selection pressure is also a function of the number of site-specific applications per area (van den Bosch et al, 2014). The average in the MT state, in soybean, is three sprayings, but considering the soybean plants as weed amid the cotton crop (Reis et al, 2021b) and attacked by rust, it is six-seven, totaling 10 per season/area; certainly this is another problem to be solved.…”