One of last week’s topic in the UF’s GreenHouse 101 was discovery and identification of plant diseases.  The teachers asked us to explore diseases in the plants we are growing.  I am growing basil.   I love basil.  I put it liberally on my salads, pizza, red sauces…. To meet my cravings, I try to grow as much basil as I can outside starting in early summer months.  

Sadly, one of my basil plants is sick.

The Goal

The goal of this post is to discuss a disease I am seeing on one of the basil plants.  I’ll start with observations and finish with a diagnosis.


Here is a picture of the sick basil plant:

basil in container

Here is the front of the leaf:

basil Front Leaf

the area around the dark spots has a wax-like “look and feel.”

…and the  back:

BasilBottomLeafwhen I first saw this picture of the back, I thought pests might be involved.  However, on closer inspection with a 10x magnifier, the areas that looked like there might be a webbing or small pest turned out to be dirt and/or dust.


The black spots are most likely due to either a bacteria or fungi.  Within the section on Plant Health (week 4) of the Greenhouse 101 course, the teacher (Brian J. Pearson) noted fungi on plants is typically dry whereas bacteria is typically waxy.  In this case, the black spots and area around had a waxy feel.  This leads me to believe the black spots are do to a bacteria infecting the plant.  While there are many posts on the Internet that discuss black spots, I found this post to help me.  This diagnosis makes sense not just because observations appear similar to images provided for black spots on plants that are infected with a fungi or bacteria, but also because it most often occurs in late Spring/early Summer when the plant leaves stay wet longer than they should.  We live in the Pacific Northwest of the US where it rains quite a bit during this period.  

The class material on diseases pointed out those based on bacteria or fungi will find it’s way to other plants since wind, rain, insects… will transmit the bacteria/fungi to other plants.  At this point, this was the only affected plant.  


Not every leaf had the black spots.  I removed those that did from the basil plant, leaving the healthy looking leaves alone.  For now I’ll keep an eye out for other plants that start showing black spots.  If the disease continues, I’ll consider a treatment such as a fungicide.  If that doesn’t work, I’ll assume the soil is contaminated with the disease and will remove the soil where the plants were growing that had leaves with black spots.  I also sterilized the scissors I used to clip off the infected leaves.




Ah yes.  Not a very exciting or technical post.  For me – fascinating and exciting to evolve my understanding of plant disease debugging as part of my seemingly endless effort to grow healthy vegetables and herbs.


Thanks for reading this far.  Please find many things to smile about.