Question
Dear Dr. Vann,
I have seven large mature Crown of Thorn plants in a very large planter in the patio of my Florida home.  They have been in the same spot for 12 years.  They were blooming beautifully until about 3 months ago when I noticed that some of the stalks were turning almost black after the leaves turned yellow and then brown.  We do get a lot of rain in Florida, specially during the summer months. Part of the plants still look healthy, but then another section has this problem.  Could they have fungus or root rot?  Any way to prevent this or alleviate this situation?  Thank you so much for your help!!

Answer
Vivian:

These black lesions, especially if at the bases of the plants is a good indicator of a root/crown rot.  This disease may be caused by multiple fungal organisms and a few bacteria. Often, these diseases are associated with poorly drained or soils that stay wet for extended periods.  For plants with these dark brown to black areas at the bases, best to remove entire plant and discard.  Be sure to always use a commercial pasturized soil that drains good.  Soils with lots of peat or peat moss will retain moisture, thus careful attention needs to be paid when watering.  For plants with these dark lesions on aerial portions, prune out those portions but making your cut 2-3 inches below the dark area. Clean off your pruner blades with rubbing alcohol between each cut in order to sanitize the metal blades.

Soil drainage needs to be addressed for these root rot diseases.  Excessive wetness aggravates these types of diseases.  Sanitation is important too. Remove infected plants and plant parts and destroy.  If feasible, water your plants from below, that is avoid getting the stems and leaves wet.

Regards
Steve