PPDL Picture of the Week

August 20, 2018

Slime Mold

Dan Egel, Clinical Engagement Associate Professor-SWPAC, Botany and Plant Pathology, Purdue University

In graduate school, when we weren’t arguing about which pizza to order, we might argue about the definition of plant disease.  For example, does figure 1 represent a plant disease?  The growth on the turf grass shown here is from a slime mold.  My friends who argued that that the photo shows a plant disease would point out that the slime mold was shading the turf and thus perhaps slowing growth, a little bit.  My friends on the other side of the argument would point out that the slime mold wasn’t invading the inside the plant to remove nutrients as do most plant diseases.  

Slime molds that grow on turf grass don’t need nutrients from inside plants.  Instead, slime molds use turf as a growth structure and survive on nutrients on the surface of plants.  Slime molds on turf are more likely after wet weather and during humid conditions.  Turf with a lot of thatch may favor the growth of slime molds.  

Homeowners may have little interest in whether figure 1 is a plant disease.  However, homeowners may be interested in how to get rid of slime molds on turf.  Fortunately, the answer is easy.  Mow the grass.  In figure 1, slime molds are growing on turf that was somehow not cut during the last mowing.  When the turf is cut, the slime mold will fall to the ground.  Raking is another possible method to get rid of slime molds. 

I have never been very worried about slime molds on turf grass which is usually a minor problem.  Nor am I worried about the definition of plant disease.  Which pizza topping to order is much more important. 

Click image to enlarge

Figure 1:  A slime mold, probably in the Genus Physarum, is growing on this turf grass.