Used as a pre-harvest fungicide since 2004, Boscalid has been used to control gray mold disease caused by pome fruit pathogens, especially Botrytis cinerea. Besides Boscalid, other SDHIs (Fluopyram and Fluxapyroxad) have been registered to control theses pathogens. Resistance to Boscalid in B. cinerea was first reported in the Washington State, in 2010, and even though they were registered recently, Fluopyram and Fluxapyroxad, there is already data showing resistance in pome fruit orchards in WA.
In the Spring of 2016, decaying apples from 65 packing houses in WA were collected. These apples were cut in half, and with a sterile scalp, plugs measuring 3×3 mm were taken from lesion to be put in acidified potato dextrose agar (APDA). Mycelial plugs were transferred to malt extract agar (MEA) and incubated at 200 C until sporulation was seen. After they were single spored, a total of 1,600 isolates were collected and tested for sensitivity to Boscalid, Fluopyram, and Fluxapyroxad using yeast peptone acetate agar (YBA) at concentrations of 5,1 and 2,5μg/ml. A total of 16.3, 3.2 and 2.1% were the percentage of resistance to Boscalid, Fluopyram, and Fluxapyroxad respectively.
Experiments were also run using detached apples. Eight isolates were used – 2 isolates sensitive to Fluxapyroxad (FluxR), and 2 isolates sensitive to Fluxapyroxad and Fluopyram (Flux-FlupR). Eight organic grown apples were sterilized and wounded twice (5mm wide x 3mm deep) near the stem-end zone. They were immersed in Fluopyram and Fluxapyroxad suspensions at 0.54 and 0.22ml/liter respectively for 30 seconds. Apples immersed only in water were used as control. After 4 hours, apples were inoculated with 25μg/ml of a spore suapension at 105 spores/mlfrom each isolate. They were incubated for 6 days at 200C in sealed boxes containing 200 ml of sterile water. They found that all controls developed lesions, and that the average disease when compared to control were 93.5% for FluxR and 97% for Flux-FlupR .
There is a need for caution when using these fungicides to avoid a rapid selection of B. cinerea population cross-resistance to all SDHIs in the orchards.