No More Rotten Apples?

Fresh-fruit producers and distributors know the menace of Penicillium expansum—blue mold. Especially damaging to the apple industry, this post-harvest pathogen contributes to as much as 20 percent yield losses in developed countries, while threatening as much as half the total crops in developing nations. For that reason, a new discovery by University of British Columbia biologists could save a lot of apples—and money.

Rhiannon Wallace, PhD, candidate in Biology at the University of British Columbia’s Okanagan campus, explains that “The majority of post-harvest fungal pathogens are opportunistic. If a fruit is physically damaged, it is at an increased risk of rotting during storage.”

In practice, this means that even a tiny blemish incurred during harvest can allow for fungal pathogens to enter an apple and flourish as mold, and this has become an increasingly difficult circumstance to guard against.

“Traditionally postharvest decay of apples has been controlled with chemical fungicides such as Mertect and Scholar,” Dr. Wallace and her partner, UBC Professor Louise Nelson, PhD, tell Food Quality & Safety. “Consumer demand for fresh produce free of chemical residues and the development of resistance by the fungal pathogens to fungicides has driven the search to find alternative control strategies.”

That’s where Drs. Wallace and Nelson come. Their research, published in Postharvest Biology and Technology, has explored sustainable, non-toxic alternatives to fungicides growing ever more compromised by resilience. What Wallace and Nelson discovered is a very Canadian solution: Pseudomonas fluorescens, a bacterium native to the soil of the prairie province of Saskatchewan. Saskatchewan endures long and bitter winters, and because of that, P. fluorescens has developed the ability to survive cold storage. For potentially endangered fruit housed and transported in cold storage—especially apples—this is very good news.

In particular, Drs. Wallace and Nelson note, P. fluorescens improves upon traditionally used fungicides because those primarily attacked pathogens on one front alone.

“Our findings suggest that mechanisms of antagonism used by the bacterium, P. fluorescens, to inhibit the post harvest fungal pathogen, Penicillium expansum, may include competition for nutrients or space, ability to colonize the apple tissue and form a biofilm, and the production of inhibitory compounds that target spore germination and mycelial growth,” they explain.

In tests conducted over 15 weeks at a BC tree-fruit storage facility, Drs. Wallace and Nelson determined that P. fluorescens prevented the growth of blue mold on McIntosh and Spartan apples stored at 1 degree Celsius—and prevented blue-mold growth at a level roughly equivalent to two commercial methods in use, a biological control agent and a chemical fungicide. Drs. Wallace and Nelson’s published research used three P. fluorescens isolates, and they note that each isolate’s ability to inhibit blue mold differed according to types of apples. Only one isolate was able to colonize the spores of the blue mold pathogen, while all three attached to the P. expansum’s fungal structure and colonized apple wounds.