Fungicide sensitivity and monocyclic parameters related to the Phakopsora pachyrhizi–soybean pathosystem from organic and conventional soybean production systems

The failure of chemical control of soybean rust has been related to the selection of less sensitive isolates, and the infection capacity of such isolates could have implications for the management of the disease. The aims of the present study were to compare the sensitivity to tebuconazole and azoxystrobin and the monocycle of soybean rust using isolates of Phakopsora pachyrhizi from two soybean fields with different production systems (organic and conventional) in 2012/13 and 2013/14 seasons, and to monitor mutations in the CYP51 gene. To assess the sensitivity to tebuconazole and azoxystrobin, detached leaf tests and in vitro germination, respectively, were used. To evaluate the monocycle, detached leaves were inoculated with a urediniospore suspension and evaluated daily by counting the number of uredia. The occurrence of the mutations in CYP51 was investigated by a pyrosequencing assay. In both 2012/13 and 2013/14 seasons, the EC₅₀ to tebuconazole was lower for the population from the organic system (0.41 and 0.10 μg mL^?1, respectively) compared to the conventional system (1.60 and 4.44 μg mL^?1, respectively), while the EC₅₀ to azoxystrobin was similar for both populations. The lower sensitivity to tebuconazole and azoxystrobin was associated with F120L + Y131H mutations in CYP51, and the F129L mutation in CYTB, respectively. The monomolecular model fitted to monocycle data and parameters related to the maximum asymptote and the AUDPC were superior for organic than the conventional system.