By Mark Gleason
Plant Pathology Extension
Iowa State University
One of the most common problems of apples in Iowa begins to show up just about this time of year. The sooty blotch and flyspeck fungi have been showing up since late August, as they often do after moist growing seasons like 2009.
These tiny creatures live above the skin of apple 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.
Sooty blotch and flyspeck (SBFS) seems harmless enough – unless you are a commercial grower who wants to sell your apples as fresh fruit. Then SBFS becomes a disease problem, since it can prevent you from selling your apples.
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 sprayed-on 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 more effective organic fungicides appeared.
The names “sooty blotch” and “flyspeck” describe what these fungi look like on apples. Sooty blotch, as the name implies, shows up as dark brown to black smudges. 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.
Flyspeck also resembles its name. Groups of several to 50 or more, shiny black dots cover more or less circular areas, from less than 1/8 inch to more than an inch in diameter.
An interesting twist to the SBFS story is that at least 60 species of fungi can cause these spots and blotches in the U.S. alone. Over the last five years, our research group at Iowa State keeps finding more and more new fungi in these smudges and spots.
Since many of these species look alike, nobody could tell them apart in the past. Even a microscope was not help.
Then, a few years ago, along came molecular genetics. By going down even deeper than a microscope could manage, down into their genetic building blocks, a whole new, diverse world of fungi came into view. Even though the SBFS species looked alike, many behaved quite differently. Like a book, you can’t judge a SBFS fungus by its cover.
As a result of our research, we are re-thinking everything we thought we knew about these humble fungi.
Back to your backyard orchard. How can an apple grower cope with SBFS in a less chemical-dependent way? We know enough about the biology of SBFS to point out some simple tricks to reduce disease risk.
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 allowing 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. 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.
Mark Gleason, Plant Pathology, (515) 294-0579, firstname.lastname@example.org