PPDL Picture of the Week
September 21, 2015
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
By bringing the entire plant into the lab I was
able to examine the root system and main stem.
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.