NOVEMBER 2017 • COUNTRY LIFE IN BC Penicillium is an insidious
Blue Meany for fruit growers Research aims to break the mold, which can cost producers millions each year
You’ve all seen it. Apples
you’ve harvested from your own tree or purchased from the store start to develop some questionable spots and
Research by MARGARET EVANS
soon you’ve got full-on mold through to the core.
Once an apple is damaged,
it’s at immediate risk of rotting in storage. A tiny blemish can open the door for an opportunistic fungus to invade the fruit and ultimately destroy it. That post-harvest infection is blue mold and it is one of the molds caused by a fungal pathogen called Penicillium expansum. It is responsible for the loss of millions of apples a year. Yield losses in Canada and other developed countries can be as much as 20% of the harvest. In developing countries, the loss can be up to half the crop. At the University of British
Columbia’s Okanagan campus, PhD candidate Rhiannon Wallace and Dr. Louise Nelson have developed a method to stop – or at least control – blue mold by exposing apples to a bacterial solution prior to the fruit being stored. The bacteria used in the trials was Pseudomonas fluorescens which was isolated from cold Saskatchewan soils. “The bacterial biocontrol agents are applied to the fruit by drenching or dipping whole apples into the inocula,” says Wallace. “The bacterial solution is diluted in tap water before the apples are submersed in it. Our findings suggest that [the] mechanisms of inhibition or antagonism used by the bacteria to inhibit the post- harvest fungal pathogen may include competition for nutrients or space, and the production of inhibitory compounds.” She says that preliminary antagonism experiments in the lab showed that P. fluorescens had strong inhibitory activity against the fungus which led to the commercial storage trials on apples. “P. fluorescens has been studied extensively as a biocontrol agent for plant diseases in the rhizosphere [soil environment], but little work had been done to assess the bacteria's potential to
control common postharvest pathogens of apple.” The tests were done at the
BC Tree Fruits Co-operative storage facility in the Okanagan and the apples used in the experiments were McIntosh, Spartan and Gala. The bacteria provided control of blue mold and appeared to
be as effective as a commercially available biocontrol agent and a chemical fungicide. “To date, we have
performed tests on several other apple varieties,” says Wallace. “In general, the level of disease control varies depending on the bacterial isolate and variety of apple. In our studies, we have seen some apple varieties look to be inherently more resistant to fungal decay, such as Gala and Spartan apples.”
Soft fruits at risk The blue mold pathogen
has many hosts. As much as apples are susceptible, Wallace says that pears, peaches, nectarines, apricots, plums, cherries, grapes, strawberries, raspberries, mango, passion fruit, avocado, carrots, onions and tomatoes are also at risk. As a
soil inhabitant, the fungus is commonly present on decaying vegetation such as ripened and windfall fruits. Throughout the growing season, the fungus can accumulate on fruit surfaces as airborne spores. But in recent years, chemical fungicides have become less effective as the target pathogens have developed a resistance to them. And, today, there is a discerning consumer push- back to the use of chemical applications. While Wallace and her team are dedicated to providing sustainable alternatives, would the public feel comfortable with biological control? “A question we are commonly asked is what will happen if someone eats an apple treated with the bacteria?” says Wallace. “Presently, we are conducting storage trials to assess how much of the bacteria is actually left on the fruit after months of cold storage. Also, most people wash their fruit before eating it which should remove a lot of the bacteria on the surface. Traditionally, post-harvest decay of apples is controlled with chemical fungicides. There has been work done (not in our lab)
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Rhiannon Wallace performs a starch test on apples in the lab to check for blue mold. UNIVERSITY OF BRITISH COLUMBIA OKANAGAN PHOTO
showing fungicides get absorbed into the apple (sometimes centimetres deep). I would consume an apple treated with a commercially registered biological control bacteria over a chemical fungicide any day, but being a
microbiologist, I am slightly biased!”
She adds, however, that the use of a bacterial agent is not a stand-alone solution. Minimizing fruit decay starts in the orchard with good
See MOLD on next page o
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