Bacterial Leaf Spot

Dark spots on leaves can mean many things.
 
It may be black spot. It may be bacterial brown spot. Or it may be bacterial leaf spot.

Bacterial leaf spot on peppers caused by Xanthomonas (Netty4299) Public Domain

Black spot is a fungal disease caused by Diplocarpon rosae that features round, black spots on leaf tops. Bacterial brown spot is caused by Pseudomonas syringae pv. syringae and features narrow light green borders and the centers tend to dry out and look tattered.

Black spot on rose leaves (LiquidGhoul) Public Domain

Bacterial brown spot on lilac (Jerzy Opiola) CC BY-SA 4.0

Bacterial leaf spot is generally caused by the Xanthomonas and Pseudomonas families of bacteria. These bacteria thrive in wet, cool soil, surrounding by plant debris and mulch. Rain and sprinklers splash these microscopic pathogens onto nearby plants. As temperatures reach 77°F to 86°F, these bacterium can start reproducing at astounding rates.
 
Bacterial leaf spot can kill leaves and weaken plants. This defoliation can also cause sunscald damage to fruit. Your basil, beets, eggplant, lettuce, peppers, and tomatoes are all susceptible to bacterial leaf spot. So are all your stone fruits, including almonds, cherries, and peaches.

Bacterial leaf spot caused by ‘Pseudomonas cichorii’ on hibiscus (Scot Nelson) Public Domain

Bacterial leaf spot symptoms
 
Symptoms first appear along leaf margins (edges) of older leaves as yellowish-brown spots, ¼” to ½” in diameter, that can be round or angular. These spots start out looking water-soaked and then dry. They may be seen on both the tops and bottoms of leaves and may form clusters. Regardless of the pathogen, these spots quickly turn black.
 
The Xanthomonas bacteria tend to produce small brown spots with yellow halos. The Pseudomonas bacteria produce reddish-brown spots. Stem streaking can also occur and you may see sunken areas in the fruit. These sunken areas are open invitations to other pests and pathogens.
 
Bacterial leaf spot control
 
Bacterial diseases can be devastating, but bacteria tend to be relatively weak pathogens. Unlike the more warrior-like viruses and fungi, bacteria generally need an opening to get inside a plant. Those openings can be caused by rubbing branches, wind damage, or insect and herbivore feeding. That sort of thing. By keeping your plants’ protective layers intact, you can significantly reduce the chance of many bacterial diseases taking hold.
 
That being said, there are other steps you can take to prevent bacterial leaf spot:

• use only certified disease-free seeds and seedlings
• plant resistant varieties
• provide good air circulation
• employ crop rotation
• avoid overhead watering
• remove infected plants and throw them in the trash

 
There are no effective chemical controls for bacterial leaf spot.
 
If you see spots on your leaves, take a closer look. See if you can determine the cause. And the cure.