Bacterial Leaf Spot

Dark spots on leaves can mean many things.

It may be black spot, bacterial brown spot, or 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 (Pseudomonas syringae pv. syringae) features narrow light green borders with centers that tend to dry out and look tattered. But bacterial leaf spot is something else.

Black spot on rose leaves (LiquidGhoul)

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

Bacterial leaf spot refers to infection by two bacterial families. Pseudomonas and Xanthomonas are pathogens that live in damp soil, plant debris, and mulch and reproduce at an alarming rate when temperatures are in the 77°F to 86°F range. Sprinkler spray and raindrops often splash these microscopic pathogens onto nearby plants, spreading the disease.

Bacterial leaf spot can kill leaves and weaken plants, and resulting defoliation can cause sunscald damage to fruit. Basil, beets, eggplant, lettuce, peppers, and tomatoes are susceptible to bacterial leaf spot, as are almond, cherry, peach, and other stone fruits trees.

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 and dented areas on the fruit can also occur. These 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 warrior-like viruses and fungi, bacteria generally need an opening to access the inside of a plant. Those vulnerable openings may result from rubbing branches, wind damage, or insect and herbivore feeding. By keeping your plants’ protective layers intact, you can significantly reduce the chance of many bacterial diseases taking hold.

These are other steps you can take to help 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 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 then the cure.