Welcome to You Ask Andy

Dick Schaffer, age 9, of Boise, Idaho, for his question:

What causes mildew?

Mildew is usually rated as an uninvited nuisance. It sets up housekeeping in damp cloths and neglected leather. It claims squatter's rights on our food, covers the cheese with a misty mold and makes blue­grey blotches in the bread. And mildew destroys the things it attacks. True mildewed cloth falls apart, the mildewed food is not fit to be eaten.

You can watch a slice of fresh, moist bread day and night. But you never see a patch of mildew arrive. The first thing you notice is a small, bluegrey blotch which grows hour by hour. Actually, it started long before the first small blotch appeared. It could start right under our eyes and we would not see it. This is because the beginnings of mildew are too small for our eyes to see.

Mildew is a plant, a member of the fungi family. This makes it a small relative of the mushroom. Being a plant, we expect it to spread by seeding. And so it does. The seeds are spores, too small for our eyes to see and almost too small for us to imagine.

These little mildew spores float in the air. They are wafted on every breeze all over the world. Certain spores have been found in the air miles above the ocean. The number of mildew spores in the air is countless. They are present in the air outdoors and in the air throughout the whole house. There is no way to shut them out of the kitchen or damp closets.

Mildew spores settle with the dust. Those that fall on hard, dry surfaces come to nothing. The lucky ones land on warm, damp organic materials, such as clothes or food. Unlike the green plants, the fungi cannot make their food from air, water and sunshine. They need prefabricated food made by the more advanced green plants. For this reason the mildew needs a slice of bread, or a bit of cheese, a little warmth and a little moisture.

When these conditions are right, the mildew spores claim squatters' rights. They start to grow. Our eyes behold a patch of fuzzy mold. Under a magnifying glass, the mildew looks like a velvety carpet. The pile of the carpet is an army of cobweb spikes, each wearing a small crown.

The microscope enlarges the view still more. The velvety carpet becomes a dainty garden, a fairy wonderland. The cobweb spikes resemble tall grasses. Each bears a crown of some delicate shape. Some mildews are topped with small, lacey fans. Others wear pour‑pours or crowns that look for all the world like dainty leaves.

The crowns of the mildew spikes, however, are not leaves. They are fruiting bodies. Each bears a host of tiny spores. At the proper time the spore cases burst and the midget seedlets are launched into the air. They will float and float and finally come to rest. But this part of their life story is too small for our eyes to see. We notice only after they land, say on a slice of bread, and grow a now garden of mildew.

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