Figures 1a and 1b. Dusty conditions during harvest can be made worse by spores produced by common fungi growing on plants. (Photos courtesy of J. McNamara, Wiles Bros., Inc.)

August 30, 2012

As harvest continues this week, questions have been raised about unusually large amounts of "black dust" (Figures 1a and 1b) during harvest, particularly in non-irrigated fields. Many people are describing the clouds of dust as "black and sooty." They also noticed dusty, black discoloration on the surfaces of plants prior to harvest (Figure 2). Much of this black to gray dust-like material is made up of large quantities of microscopic spores (reproductive structures) produced by fungi, in addition to the normal dust created during harvest.

These fungi are not the "mushroom" fungi that most people may be familiar with. Instead of producing their spores in a mushroom, these fungi produce spores on the surfaces of plant material, giving it a sooty appearance (Figure 3) that can be rubbed off.

The most common fungus identified on samples submitted to the UNL Plant and Pest Diagnostic Clinic is Alternaria spp. This fungus is a common microorganism in the soil that acts as a common saprophyte (and sometimes a plant pathogen), growing on dead plant tissue to promote the necessary degradation of the material to be recycled into the soil. It is likely that recent rainfall events and dew have promoted the rapid and simultaneous production of spores that have been reported on the dried corn plants prior to and during harvest.
 

Other Late Season Corn Diseases

In addition, some other corn diseases are flaring up toward the end of the season that weren't evident earlier in the summer. For example, southern rust has become increasingly common in Nebraska corn fields and is caused by a fungus that is also capable of producing large quantities of orange and eventually black spores (Figure 4).
 

Figure 3. Under higher magnification, patches of dark fungal growth and sporulation can be observed and sometimes wiped off of leaf surfaces. (Photos courtesy of K. Korus, UNL Extension)

Figures 4a and 4b. The pathogenic fungus causing southern rust of corn can also produce large amounts of spores that are usually orange to tan in color, but eventually black. (Photos courtesy of C. Schleicher, UNL Extension)

Most of these fungi are not known to cause direct harm or disease in people. However, some fungi can cause illness in people with compromised immune systems or in those who are prone to respiratory irritation and illnesses. Large amounts of fungal spores, as are currently being observed, can behave similarly to other particulate matter, like dust, and cause a reaction in people. Individuals concerned about inhalation of these dust-like particles can reduce their exposure by wearing dust masks or respirators when working in these conditions.

If you are concerned about the identity of this or other plant problems, you can submit a sample for diagnosis to the UNL Plant and Pest Diagnostic clinic by printing and completing a sample submission form and shipping it and the sample in a plastic bag to the clinic.

Tamra Jackson-Ziems
UNL Extension Plant Pathologist
Kevin Korus
Plant and Pest Diagnostic Clinic