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1/19/04

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Yard and Garden Column for the Week Beginning Jan. 23, 2004

Refrigerator Fright

Barbara Ambruzs
Plant Pathology
Iowa State University Extension

During the 1860s Louis Pasteur refuted the spontaneous generation theory by proving that micro-organisms arise only from pre-existing microorganisms. Therefore, the fungus growing on the forgotten zucchini in the caverns of the refrigerator didn’t just appear on its own.

We bring home sacks filled with nutritious fruits and vegetables. If you’re like me, the produce gets stashed in the depths of the crisper drawers. Despite our good intentions, weeks go by without a thought of a healthy diet. Then the day comes when the idea arises to eat a balanced meal. Low and behold, the nice, fresh bounty transformed into a soupy, slimy, stinky, rotten mess in the vegetable bin.

Rot tends to start in bruised, scratched, punctured or otherwise damaged produce. During and after harvest, great care is taken to minimize injury to fruits and vegetables. But with high-volume production, some damage is unavoidable.

Sometimes plant parts are diseased before picking. Symptoms may go unnoticed since storage conditions, such as low temperatures, may delay disease progression. Conditions differ from the grocery store and home. Under the new conditions, decay can be rapid, often occurring within a few days.

Several of the more common refrigerator fungi are species of Penicillium, Fusarium, Alternaria and Botrytis. Cutting away the moldy section and eating the remainder isn’t considered safe since harmful compounds produced by some fungi can penetrate the rest of the tissue.

Penicillium is probably the most frequently occurring fungus of the group. Penicillium infections usually begin as small, soft, watery-looking sunken spots that rapidly expand. Dusty blue and green mold surrounded by a white ring soon covers the rotted area. The white area is newly forming microscopic threads of the fungus. The pigmented centers are filled with spores that can float through the air. Penicillium enters produce through wounded areas but can also spread by contact between diseased and undamaged produce.

Even though Penicillium can be annoying at times, some species can be used to our benefit. In 1928, Alexander Fleming discovered that Penicillium notatum prohibited bacterial growth. Several years later this fungus was used in the development of the antibiotic drug Penicillin. Penicillium is also used in the production of cheeses, such as blue cheese, Camembert and Brie. Blue cheese is ripened by Penicillium roquefortii, while the white coating on the outside of Brie and Camembert is the fungal threads (mycelium) of Penicillium camembertii.

Fusarium tends to be pinkish or salmon colored. It is a common soil fungus, so produce that grows close to the ground or under it will be more prone to this type of infection. Examples are cucumbers, squash and potatoes. It’s best to discard fruits and veggies with pink fuzz.

Alternaria can cause dark spots on just about any fruit or vegetable. This fungus may cause rot beneath the surface before the fuzz appears on the skin or rind.

Since Botrytis is known as gray mold, you can probably guess what color its moldy layer is. Grapes, onions and berries are commonly attacked by Botrytis. This fungus likes sugary items.

The previously mentioned refrigerator invaders are fungi. However, there’s a nasty, foul smelling after-harvest bacterial problem that you may be acquainted with. Several species of bacteria can decay plant material into slimy, oozing, putrid, disintegrated flesh. Carrots, cabbage, onions and potatoes, among others, are vulnerable to this disease known as bacterial soft rot. Soft rot bacteria are one celled organisms that rapidly multiply within diseased plants or plant parts. They produce enzymes that break down plant cell structure, hence the soupy mess.

Keeping the refrigerator clean eliminates most unwanted organisms, and regular washing with soapy water can be very effective. Be sure the refrigerator is cool enough. Temperatures below 40 degrees Fahrenheit are unfavorable for the growth of most decay organisms. Some fungal or bacterial growth may occur, but it will be very slow. Inspect items for damage, bad odor and fuzz when making grocery selections.

Please don’t get angry at these decay organisms for lunching on your cache. They’re just doing what is natural to them. Without them, the world would be piled high with garbage. There’d be no compost for our gardens. Blue cheese, Brie and the antibiotic Penicillin would be unheard of.

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Editors: No photos are available for this week’s column.

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