Chang-Lin Xiao recommends that use of Penbotec and Scholar alternate from year to year to preserve their effectiveness against fungal pathogens like this blue mold on a Gala apple.

Chang-Lin Xiao recommends that use of Penbotec and Scholar alternate from year to year to preserve their effectiveness against fungal pathogens like this blue mold on a Gala apple.

Chang-Lin Xiao

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.

“One key factor contributing to the development of fungicide resistance is selection pressure,” said Xiao, who is now working at the U.S. Department of Agriculture’s research laboratory in Parlier, California. “Exclusive or repeated use of the same class of fungicides creates a persistent selection pressure to the pathogens for resistance development.”

Since approval of Penbotec and Scholar (fludioxonil) for use at the packing house level in 2005, both have been effective fungicides. But even before the postharvest use was approved, extension educators were recommending that storage operators alternate Scholar and Penbotec from year to year to prevent subjecting P. expansum spore populations on fruit bins to selection pressure by using the same fungicide in successive years.

Emerging resistance

In one Washington packing house that had used Penbotec exclusively since 2005, Xiao found low levels of resistance to P. expansum (less than 1 percent) in 2009, but resistance jumped to 7 percent in the next year. By switching to Scholar in 2011, resistance dropped to 3 ­percent.

In another packing house that used Penbotec exclusively up to 2010, and then used both Scholar and Penbotec in 2011, the frequency of resistant isolates to P. expansum was more than 90 percent, he said.

A sampling of five packers in 2011 found only one with a high percentage (94) of Penbotec-resistant isolates to P. expansum. That packer, and another with 2.9 percent resistant isolates, used Penbotec exclusively as a drench until 2010, according to Xiao. The three other packers, (two with no resistant isolates and one with 1.75 percent resistant isolates) had not used Penbotec or Scholar widely for drench before 2010.

“Don’t panic,” he told packing house operators who were at the postharvest session of the Washington State ­Horticultural Association’s meeting in December. “Resistance is not a widespread problem in the industry. It is a packing house-specific issue that’s related to repeated annual use of the same fungicide as a postharvest drench.”

Drenching itself is not a problem, but contaminated bins contribute to the development of resistance, he noted.

“However, resistance management practice needs to be implemented industrywide.”

Xiao also encouraged packing house operators to pay attention to the fungicides used in the field, as they could compromise the effectiveness of fungicides used at the packing house level. Both the new preharvest fungicides of Vangard (cyprodinil) and Inspire Super (cyprodinil and difenoconazole) are in the same class of fungicides as Penbotec. Scholar also has a preharvest use that must be considered when using as a drench.

Resistance management steps include:

  • Avoid using the same fungicide as a postharvest drench year after year.
  • Alternate between Penbotec and Scholar on a yearly basis.
  • Avoid using the same postharvest fungicides twice on the same fruit (drench treatment and packing line treatment).
  • Sanitize bins before delivery to the field in a hot water treatment at 170°F for 20 seconds to kill spores, a ­treatment that also helps control codling moth.