Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
9/18/00
Contacts:
Mark Gleason, Extension Plant Pathology, (515) 294-0579,
mgleason@iastate.edu
Elaine Edwards, Extension Communication Systems, (515)
294-5168, eedwards@iastate.edu
Yard and Garden column for the Week Beginning Sept. 22
Secrets of Cloudy Apples
By Mark Gleason
Extension Plant Pathologist
Iowa State University Extension
All living creatures need a home. This statement will stimulate the following reaction from a 13-year-old: "Duh." But sometimes a creature's home borders on the bizarre, at least from a human perspective. So if you are interested in bizarre things (and what 13-year-old isn't), keep reading.
Consider the strange case of fungi living on the surface of an apple. These tiny creatures live above the skin of the fruit, in the protective layer of wax. The wax layer is what makes an apple shiny after you rub it on your shirtsleeve. The fungi hunker down in the wax, sipping dribbles of apple juice that seep through the skin.
Before the 13-year-old person can comment again, here's why wax-inhabiting fungi are interesting: they affect whether or not an apple can be sold. A group of these fungi cause diseases known as sooty blotch and flyspeck.
Sooty blotch, as the name implies, shows up as dark brown to black smudges on the apple surface. The blotches range in size from half-inch-diameter circles to smears that can cover half the apple surface. Some blotches are so faint they are barely visible, while others appear as dark and intense as certain 13-year-olds.
One reason for this polyglot appearance is that sooty blotch has multiple personalities. Any similarity to 13-year-olds is merely coincidental. At least three fungi are known to belong to the sooty blotch complex, and now it appears there may be others.
Flyspeck also resembles its name. Groups of several to 50 or more shiny black dots cover more or less circular areas, ranging from less than 1/8 inch to more than an inch in diameter. Flyspeck is blamed on a single fungus, but others may also be involved. Color pictures of sooty blotch and flyspeck are in an ISU bulletin entitled "Tree Fruits: Insect and Disease Management," IDEA 3, available from your local ISU Extension office.
An interesting twist to the sooty blotch/flyspeck story is that these fungi cause mainly cosmetic damage. They don't affect the shape, size, taste or texture of an apple, merely its eye appeal. In other words, they are "cosmetic" diseases.
Sooty blotch and flyspeck may be as old as apples themselves. Drawings of apple varieties from the 1820s clearly show sooty blotch on every fruit. It wasn't until inorganic pesticides, such as lime sulfur and lead arsenate, became popular around 1900 that consumers began to expect to buy apples without a heavy coating of sooty blotch and flyspeck. The cosmetically perfect apples found in today's supermarkets weren't common until the middle of the 20th century, when highly effective organic fungicides appeared.
If you're the produce manager of a supermarket, this seems like a happy ending. But lately consumers have begun to worry about the health hazards of pesticide residues on apples. These worries resulted in new federal restrictions on pesticides, especially the 1996 Food Quality Protection Act. As the food-safety bar rises, commercial apple growers are under pressure to use less pesticide.
Back to sooty blotch and flyspeck. How can an apple grower, whether commercial or backyard, cope with theses diseases in a less chemical-dependent way? We know enough about the biology of sooty blotch and flyspeck to point out some simple tricks to reduce disease risk. Luckily, sooty blotch and flyspeck fungi behave similarly, so the same defenses work against both problems.
One good principle is to keep the air moving. Give apple trees plenty of clearance, in full sun, and cut down any trees whose branches touch the apples' canopies. Every year, sometime between January and the end of March, prune your apple trees.
Pruning is as much an art as a science, but it's vital to allow wind and sun to penetrate the foliage. Air currents dry the apples and leaves after a rain or dew, and dryness discourages the sooty blotch/flyspeck fungi. If you're unsure about how to prune, go to your local extension office or public library for helpful resources. Mowing the grass and controlling weeds under apple trees also helps the apples to stay dry.
Another key to good air movement is to thin the fruit load early in the season. Within a few weeks after bloom, when the apples are still smaller than grapes, remove excess apples so that only one remains per six inches of branch. Apples on a properly thinned tree not only dry faster, but also get the right amount of sunlight to develop large size and good flavor.
The presence of raspberries or blackberries adds risk of sooty blotch and flyspeck. The sooty blotch/flyspeck fungi use the brambles to multiply. During a rainstorm, spores from the brambles float through the air and infect nearby apples. Since growing raspberries or blackberries near apple trees increases the risk of sooty blotch and flyspeck, try to keep apples and brambles as far apart as possible.
At the high-tech end of the management spectrum are disease-warning systems. These systems allow you to use the weather to figure the risk of disease, so you only spray a fungicide when you absolutely need to - not just because it's Tuesday (the traditional way). At the ISU Horticulture Farm near Gilbert, a warning system for sooty blotch and flyspeck has consistently saved two to three fungicide sprays per season, with no loss of disease control. If you're interested in trying this system, please contact me (phone: 515-294-0579; email: mgleason@iastate.edu) for details.
I hope I've dispelled some of the mysteries surrounding cloudy apples. And it's important to remember that, like most 13-year-olds, apples coated with sooty blotch and flyspeck have a great deal to offer despite their rough-and-ready appearance.
ml: isugarden
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