Using the same postharvest fungicide annually can lead to resistance of mold pathogens. Resistance of the blue mold pathogen to the postharvest fungicide Penbotec (pyrimethanil) was seen in Washington State within five years of the product being introduced, illustrating the need to use resistance management practices. Washington State University postharvest scientists observed resistance of blue mold (Penicillium expansum) to Penbotec for the first time in 2009 at a packing house where it had been used as a postharvest drench for apples annually since 2005. Dr. Chang-Lin Xiao, former extension plant pathologist based at WSU's Tree Fruit Research Center in Wenatchee, said that the two major decay-causing pathogens of apples and pears—blue mold and gray mold (Botrytis cinerea)—are at high risk of developing resistance^ to fungicides, so growers and packers must work to prevent development of resistance.
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