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The Hidden World of Sooty Blotch and Flyspeck

Sooty Blotch and Flyspeck


Fungi are easy to overlook. Few of them are beautiful, at least to our eyes. No fungus has ever appeared on a television talk show, flogging its latest movie or book.  Fungi do not get makeovers or implants. They lack Web sites, Web cams, chat rooms or blogs.  Image-wise, fungi are more or less permanently under the radar.

But some fungi are mysterious. Take the sooty blotch and flyspeck (SBFS) fungi, for example. These humble creatures eke out a meager existence on the waxy surfaces of many plants. Their name comes from the dark, cloudy smudges and speckles they form on the surface of apples. 

Sooty blotch and flyspeck fungi harm only the appearance of an apple, not its taste. But a few smudges and speckles are the difference between a saleable apple and one destined for the cider mill or compost pile.  So apple growers worry about losing money from SBFS, especially when these fungi appear suddenly on thousands of apples in late summer.

What’s mysterious here? Almost everything, as it turns out.  Sooty blotch and flyspeck fungi have probably grown on apples for the entire 5,000 or so years we’ve cultivated the crop, but we know almost nothing about these hitchhikers.

For starters, we don’t even know their names. After about 200 years of research, mycologists had managed to name four different SBFS fungi. 

But the story doesn’t end there.  Armed with the new weapons of molecular biology and genetics, ISU PhD student Jean Batzer recently found 30 additional SBFS fungi from apples in Iowa, Missouri, Illinois and Wisconsin orchards. Jean’s discovery was a startling plot twist in the SBFS mystery.

How could 30 different fungi have hidden from scientists for 200 years?  Ironically, they were hiding in plain sight, since they have been visible on apples all this time.

Their secret is that most sooty blotch and flyspeck fungi look nearly alike.  Even under the microscope, they shyly kept their identities to themselves. Only when Jean pried open their DNA did their differences become clear. 

Once Jean uncovered the genetic differences, other subtle distinctions began to emerge.  Some of the SBFS fungi preferred warm temperatures, others cooler temperatures.  Some were highly sensitive to the fungicides apple growers spray to control them, and others weren’t sensitive. The sooty blotch and flyspeck fungi began to seem like a fungal zoo rather than mere drab little microbes.

Another twist in the plot came when we started to look at SBFS fungi from other parts of the world.  It turns out that SBFS fungi from Shaanxi Province in central China are different from the ones on Midwest apples. With a Chinese collaborator, Dr. Sun Guangyu, we found at least a dozen additional SBFS species from China.

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Editors: A 300K photo suitable for publication is available