NOTAS CIENTÍFICAS In vitro Determination of Fungicide Inhibitory Concentration for Phakopsora pachyrhizi isolates

In vitro assays were preformed to obtain the IC 50 of eight fungicides against Phakopsora pachyrhizi isolates from Passo Fundo, RS, Ponta Grossa, PR, and Primavera do Leste, MT. Different concentrations of the fungicides were added to Petri dishes containing soybean leaf extract agar medium. One milliliter of P. pachyrhizi uredospore suspension at the concentration Moura, B; Boller, W.; Deuner, C.C. In vitro Determination of Fungicide Inhibitory Concentration for Phakopsora pachyrhizi isolates. Summa Phytopathologica, v.42, n.2, p.170-171, 2016.

Asian soybean rust (ASR), caused by the fungus Phakopsora pachyrhizi Sydow & P. Sydow, is one of the most destructive plant diseases in Brazil and a great threat to the country's soybean production (8).Chemical management of ASR started in Brazil during the 2002-2003 season (6) and since then it has been the main disease management strategy.
Pathogen sensitivity to a fungicide is measured based on the IC 50 , which is the fungicide concentration (mg/L) that inhibits 50% of spore germination or mycelial growth (5).
According to Edgington & Klew (3), for a chemical to be considered a fungicide, its IC 50 for a specific pathogen should not be greater than 50 mg/L.Fungicides that are considered highly toxic have an IC 50 that does not exceed 1 mg/L.An IC 50 value between 1 and 50 mg/L would be defined as moderately toxic.We hypothesized that P. pachyrhizi sensitivity to fungicides has reduced along the seasons they were used.
The aim of this study was to determine the in vitro IC 50 values of eight fungicides against P. pachyhrizi isolates and to determine the sensitivity reduction factor (SRF) by comparing the new IC 50 values to those already reported in the literature.
Isolates of P. pachyrhizi were obtained from naturally infected plants during the 2013-2014 season from the cities Passo Fundo, Ponta Grossa and Primavera do Leste.
Dishes containing the culture medium received 1 mL of uredospore suspension at a concentration of 3.0 x 10 4 uredospores/mL.The dishes were incubated in a Biochemical Oxygen Demand (BOD) chamber, at 22°C, for six hours, in the dark.For each dish, 100 uredospores were counted and the percentage of germinating uredospores was calculated.These values were used in Probit regression procedure to estimate the IC 50 values.The sensitivity reduction factor (SRF) was calculated by dividing the estimated IC 50 values by the baseline IC 50 values obtained from the literature (2, 7).
Results show that pyraclostrobin and trifloxystrobin + prothioconazole were fungicides of high toxicity for all P. pachyrhizi isolates (Table 1).
only moderately toxic to Passo Fundo and Ponta Grossa isolates.This difference may be due to a fungicide resistance strategy used in Mato Grosso State since the 2007-2008 season, which replaced DMIs with mixtures of QoI + QoI (4).On the other hand, the mixture of QoI + DMI (pyraclostrobin + epoxiconazole) was moderately toxic to Primavera do Leste isolate but highly toxic to Passo Fundo and Ponta Grossa isolates.
The IC 50 compared to previously reported values, confirm an increase in the sensitivity reduction factor (SRF) (2) for epoxiconazole, azoxystrobin, pyraclostrobin and trifloxystrobin.The QoIs azoxystrobin and trifloxystrobin showed high SRF values.However, in comparison with the data from Reis et al. ( 7), the SRF of epoxiconazol (DMI) decreased for Primavera do Leste isolate.
Overall, the data show that no fungicide had sensitivity loss, but only reduction.IC 50 values varied among fungicides and regions.

Table 1 .
In vitro inhibitory concentration (IC 50 ) of fungicides to inhibit uredospore germination of Phakopsora pachyrhizi isolates, and sensitivity reduction factors (SRF) based on new IC 50 and baselines.