Production Blog: Why Crops Turn Black

DECATUR, Ill. (DTN) -- You may have noticed harvest conditions have taken on a dark, foreboding look. The black soot currently decorating machines, hands and nearly everything it touches is most likely the result of saprophytic fungi and not any cause for alarm.

It is common and nature's way of degrading plant tissue at the end of the season.

Iowa State University Plant Pathologist Alison Robertson said wet weather followed by warm, humid weather and morning dews favor growth of these organisms. They are not known to produce toxins, and the harvested grain should look relatively clean. Since they are not pathogens of crops, this black mold will not be a source of inoculum for diseases next year.

Individuals with allergies or respiratory problems are encouraged to wear dust masks to reduce breathing in masses of spores. Saprophytic fungi are a big contributor to the mold portion of the pollen and mold counts. It's also important to keep combine engines and can filters clean. If your family is picky, laundering your clothes separately is advised.

Robertson said corn is typically the worst for containing this kind of black sooty dust. But other crops such as soybean and milo can also be colonized by saprophytic fungi.

ORANGE AS A CORN CURL

If southern rust invaded your fields earlier this year, you probably noticed an orange color that came with it. At the end of the growing season, southern rust stops producing orange spores and starts producing black spores. However, Robertson told DTN that southern rust shouldn't be much of a contributor to the black dust currently being experienced.

"The orange spores burst through the leaf epidermis and are very loose and escape easily. Ask anyone who has walked through a field of southern rust and come out looking like a Cheetos," Robertson said. "The black spores are produced in structures that remain mostly covered by the leaf epidermis, so they don't escape as easily."

The black dust currently in question is also different than tar spot, a late-season disease known to infest cornfields. Tar spot is characterized by distinct irregular-shaped raised spots that are scattered across the leaf surface -- think road tar sprinkled on the leaf. They don't rub off.

Saprophytic fungal growth looks fuzzy or out-of-focus on the corn leaf, Robertson said. If you are still uncertain, just wear a white shirt into the field you are harvesting and watch it turn black.

Read Robertson's blog about saprophytic fungi here: https://crops.extension.iastate.edu/….

Pamela Smith can be reached at pamela.smith@dtn.com

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