Asian soybean rust advances with triple resistance in Paraguay

Asian soybean rust is circulating in Paraguay with multiple fungicide resistance profiles. This finding comes from a study with samples collected in soybean-producing areas in the east of the country. The authors identified reduced sensitivity to active ingredients from the DMI, SDHI, and QoI groups. They also mapped mutations associated with resistance in the CYP51, SDH-C, and CYTB genes.

The study evaluated 20 populations of Phakopsora pachyrhizi and 18 isolates obtained from a single population, PRP-1. Tests on detached leaves showed that prothioconazole lost effectiveness against Paraguayan populations, even at the highest concentration tested. Fluxapyroxad and azoxystrobin maintained good control at high doses, but lost performance when the concentration decreased. The 20 populations analyzed exhibited lower sensitivity than susceptible strains used as controls.

Genetic analysis

In the genetic analysis, all Paraguayan populations carried the F120L, V130A, and Y131F mutations in CYP51. The SDH-C gene presented the I86F mutation. The CYTB gene exhibited the F129L mutation at levels close to 100%. According to the authors, this set helps explain the reduced activity of fungicides in soybean areas of Paraguay. The study also did not find significant regional variation in mutation values, suggesting shared resistance mechanisms among the sampled areas.

Researchers advanced the analysis within a single population. Among 18 PRP-1 isolates, they detected three mutation patterns in CYP51 linked to resistance to DMI: F120L + Y131H; F120L + V130A + Y131F; and F120L + Y131H + I145V. The I86F mutation in SDH-C appeared in 14 of the 18 isolates. The F129L mutation in CYTB appeared in all isolates, with a frequency above 95%. The result reveals strong genetic and phenotypic variability even within a single plot.

The study also compared genomic DNA and cDNA. Mutations in CYP51 appeared with two to three times higher expression in cDNA. This data reinforces the importance of this gene in resistance to AMD. In SDH-C, mutation values ​​changed little between gDNA and cDNA. For CYTB, the high frequency of the mutation in gDNA already indicated strong selection pressure.

Selected isolates

In bioassays with selected isolates, prothioconazole better controlled strains with the F120L + Y131H mutation. The combinations F120L + V130A + Y131F and F120L + Y131H + I145V reduced the active ingredient's activity more significantly. Metconazole showed the opposite behavior. It better controlled isolates with F120L + V130A + Y131F and lost activity against F120L + Y131H and F120L + Y131H + I145V. According to the authors, this difference indicates distinct binding patterns at the target.

Molecular dynamics simulations reinforced this interpretation. In the wild type and the F120L + Y131H pattern, prothioconazole-desthio maintained a network of interactions between protein, water, and ligand. This network favored the coordination of the molecule. The F120L + V130A + Y131F pattern lost this network. The F120L + Y131H + I145V pattern showed the most unstable condition among the simulated arrangements. The authors point out that the I145V mutation increases the threat to chemical control with DMI, even without direct contact with the ligand.

According to the study, this is the first report of multiple resistance to DMI, SDHI, and QoI in soybean production areas in Paraguay. The authors advocate continuous molecular surveillance, strategic adjustment of applications, and integration of this information into the management of Asian soybean rust. The work also highlights the regional relevance of the topic.

Further information at doi.org/10.1002/ps.70434