PPDL Picture of the Week

June 11, 2018



Black root rot of Watermelon

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


Many watermelon transplants destined for commercial fields start life in transplant trays.  Watermelon seeds are planted in a mix that might contain perlite, vermiculite, peat and rice hulls.  When the plant has filled the cell, it can be removed from the transplant tray and planted in the field. 


Occasionally, the transplants become diseased before they make it to the field.  The wilted and dead leaves of the watermelon transplants in Figure 1 could have several causes.  Above ground symptoms such as wilts and leaf death may be caused by problems underground.  When I investigated the plants in Figure 1, I found that many of the plants had a root rot.  The dark area at the base of the stem (technically, the hypocotyl) is caused by a fungus that is growing in the plant (Figure 2).  The fungus also can be found on the roots of the plant. 


This disease is known as black root rot of watermelon.  The fungus that causes this disease is Thielaviopsis basicola (Figure 3).  This fungus causes a similar disease on carrot, tobacco, pansies and many more crops.  The dark area on the base of the stem is actually a ‘sign’ of the disease since the fungus that causes the disease is visible.  The wilting and decline of the plant is a ‘symptom’ of the disease since it is a result of the infection.


This disease can best be controlled by good sanitation.  In this situation, the soil-like mix, the plastic transplant trays and/or a piece of equipment became contaminated with the fungus.  When the watermelon were seeded in the trays, the fungus began to attack the plants.  To avoid black root rot, start with clean soil-like mix and clean or well sanitized equipment and trays. 


​Click image to enlarge


Figure 1:  The dead leaves on this watermelon transplant are caused by black root rot.

Figure 2:  The dark area at the base of the stem is a result of infection by the fungus Thielaviopsis basicola, causal agent of black root rot of watermelon. 

Figure 3:  Chlamydospores of the fungus Thielaviopsis basicola can be seen here growing in the root tissue of a watermelon seedling.