Does your basil have striking black spots on the
leaves? The likely cause is bacterial
leaf spot caused by Pseudomonas cichorii. A number of occurrences are popping up in
Connecticut this summer, following favorable wet weather.
Where does this pathogen come from? It is a seed borne bacterium so contaminated
seed is a possibility. This bacterium
has a wide host range and can survive on the surfaces of plants without causing
visible symptoms until conditions are favorable for infection. These populations can spread to new plants by
wind-driven rain. The bacteria can also be spread from one plant to another during handling, transplanting or harvesting of leaves, especially when the plant surfaces are wet.
What to do about it?
When plants are dry, infected leaves or whole plants should be removed
and discarded. Promote leaf dryness by
spacing and mulching to allow for good airflow around each plant. If overhead irrigation is used, water early
in the morning so the sun will dry the leaves during the day. Do not work among the plants when they are
wet. Copper-based products labeled for
bacterial leaf spot on herbs may offer some/minimal protection.
If you do have infected basil plants, don’t save the seed
for starting new plants next year.
More information on this disease and pathogen can be found at these links.
J Allen
Uconnladybug's Blog 
July 9, 2011 at 11:10 am
I’ve been able to control and avoid this by simply moving my 6 pots of basil into the garage before it rains and never getting the leaves wet when I water.
May 5, 2014 at 8:43 pm
[…] See Black Spots on Basil Leaves […]
October 5, 2017 at 5:18 pm
I am watering at the bottom and in the morning but everytime I water I see the next day few leaves have black spots. I have been discarding the respective leaves everytime I see them. but Do I need to discard to whole plant? Can I make a new plant from its stem? Please suggest.
October 5, 2017 at 7:05 pm
Hello Puja Paul,
If the plant is planted outside in the garden, the cooler weather of October is going to kill the plant off shortly. If the basil is in a pot indoors, keep removing the leaves and cut back the plant. Let the soil dry out a bit, but not to the point where the soil pulls away from the pot. If the plant does not produce new, fresh leaves after about two weeks, discard the plant.
Regards,
UConn Home and Garden Center