Bacterial Leaf Spot in Peppers: How to Identify, Treat, and Prevent It

As we reach mid-July in Allegany County and surrounding areas, the weather turns warm, humid, and stormy, which are ideal conditions for bacterial leaf spot in home gardens. This disease is very common this time of year, especially in pepper plants, and can quickly spread if not addressed early. Bacterial leaf spot causes leaf drop, poor fruit development, and in severe cases, plant death. Fortunately, with the right identification, treatment, and prevention methods, you can protect your plants and enjoy a productive growing season.

What Is Bacterial Leaf Spot?

Bacterial leaf spot (BLS) is caused by the pathogen Xanthomonas campestris pv. vesicatoria. It primarily affects peppers and tomatoes, attacking the leaves, stems, and fruit. The bacteria thrive in warm, humid conditions with frequent rainfall or overhead watering, making mid-to-late summer in Western New York a prime time for outbreaks.

Symptoms to Watch For

• Small, water-soaked spots on lower leaves, spots become dark brown or black, often with yellow halos. Lesions may appear angular, limited by the leaf’s veins.
• Leaf drop or wilting in more severe cases
• Infected fruits may show sunken, scabby spots or cracking

These symptoms often look like fungal issues, but BLS is bacterial, which affects treatment choices.

How to Treat It?

To manage it effectively, you’ll want both preventative and curative measures. Chemical or organic, it depends on the severity of the infestation.

Mild Infestation (a few spots on lower leaves, plant still vigorous)

Go Organic First:

• Remove affected leaves immediately, then spray weekly with a biological product containing Bacillus subtilis or copper soap. As part of the Co-op extension office we don’t recommend specific brands; what is important is to look for the main ingredient containing Bacillus subtilis or copper soap (copper soap is usually cheaper and easier to find).
• Ensure no overhead watering, switch to drip or bottom watering.
• Boost airflow: Prune crowded foliage if possible.

Why organic first? Because the infection hasn’t spread aggressively, the plant’s immune system is still strong. Organic options can suppress the bacteria enough while preserving beneficial microbes.

Moderate Infestation (multiple leaves showing angular or expanding spots)

Combine Organic + Chemical:

• Start with a copper-based spray as soon as possible.
• Remove infected leaves and destroy them, or throw them in the garbage or far from your garden (don’t compost them).
• Apply Bacillus subtilis as a tank-mix or alternating weekly with copper.
• Watch for phytotoxicity on very hot days, spray early morning or late afternoon. (Phytotoxicity is when the plant is damaged by chemical substances, usually leaves show a yellow or brown color, curling, or deformed, after spraying on a very hot day, especially >85 degrees F).

Why combine? At this level, bacterial pressure is rising. Organic treatments alone may not hold it back fast enough, but they help support microbial balance and reduce copper resistance risk.

Severe Infestation (spots covering many leaves, some leaf drop, new growth affected)

Go Chemical Immediately:

• Copper hydroxide or copper sulfate sprayed ASAP, thoroughly coating all leaves.
• If the spread continues after 2 applications, add a product containing streptomycin, especially if you suspect resistant bacteria.
• Remove all infected plant debris from the base of the plant.
• Avoid touching wet plants and sanitize tools between each use.

Why chemical first? At this point, the infection is outpacing natural control. Fast-acting copper/strep treatments can stop it before it kills the plant or ruins the crop.

A tip for all levels of severity: add a sticker/spreader (any natural surfactant) to your spray to help it adhere to leaves better, especially before rain.

For the next seasons, to prevent bacterial leaf spot in peppers, choose resistant varieties labeled with resistance genes like Bs1, Bs2, or Bs3, often listed as X3R. Recommended options include bell peppers like Aristotle X3R, Paladin, Red Knight X3R, and hot peppers like Baron (poblano) and Anaheim X3R. These varieties offer strong resistance to bacterial leaf spot. 

Combining resistant plants with good garden hygiene, drip irrigation, and crop rotation offers the best protection when growing peppers in these areas. If you need help identifying this disease or having any other issues with your garden, contact your local Cooperative Extension office. We're always happy to help!

Last updated July 14, 2025