PPDL Picture of the Week

September 21, 2015

Marigold Mystery

Tom Creswell, Plant Disease Diagnostician, Director PPDL, Purdue University

When I noticed a single marigold plant in a planting of 10 or so looking a little off color last year I didn’t think much about it but wondered what was wrong. Maybe an insect had attacked the roots? Was something buried in the soil? The plant turned red, became stunted and eventually died toward the end of the summer. This year when the same thing happened in the same location I had to find out why. 


Figure 1 shows the stunting and red discoloration before the plant gradually died.

By bringing the entire plant into the lab I was able to examine the root system and main stem.  Figure 2.

Dissection of the main stem showed the presence of a dark streak in the water conducting vessels; a symptom we often see with vascular wilt diseases. As the fungus clogs the vessels the tissue dies and dark streaks form. Eventually the whole plant dies. Small pieces of the stem were placed on growth medium and Fusarium grew out of each segment, providing evidence that the main problem is Fusarium wilt.  Figure 3.

The fungus will stay in the soil in that spot year after year so next year I’ll plant something other than marigold there. The Fusarium that attacks one species of plant does not usually attack others so chances are I’ll get away with having a successful planting of other annuals.

​Click image to enlarge

Figure 1

Figure 2

Figure 3