Chances are, you’ve run into a bacterial leaf spot at some point, as it affects a wide range of plants.
Gardeners may find bacterial leaf spots on a wide range of edible plants, including beans, cereals, cucurbits (gourds), peppers, Prunus (almond, apricot, cherry, peach, and plum trees), strawberries, and tomatoes.
At a glance, it may be indistinguishable from a fungal leaf spot, which has its own range of affected crops but has different treatment methods.
Bacterial leaf spot doesn’t stop at food crops, however, and also affects a wide range of ornamental plants.
The following plants are known to be susceptible to bacterial diseases:
- English Daisy
- English Ivy
What Causes Bacterial Conditions?
One of the most common bacterial infections your plants might face, bacterial leaf spot is actually an umbrella term for infections caused by members of the Comamonadaceae, Pseudomonadaceae, and Xanthomonadaceae families.
The disease of leaf spots is mainly caused by three genera:
NOTE: Acidovorax was recently distinguished from Pseudomonas rRNA Homology Group III and added to a new family, Comamonadaceae.
Be aware there are variations in product labels which may or may not include Acidovorax as part of the Pseudomonas genera based upon this recent change.
Overcrowding and wet conditions allow bacterial leaf spots to spread quickly from plant to plant.
It can overwinter in plant debris such as fruit, leaves, stems, or twig cankers; making cool conditions an easy way for it to hide until conditions are more appealing.
Overhead watering and warm weather lead to the most active periods, and droplets containing the pathogen on infected plants may be carried via air circulation.
The good news is the bacterial leaf spot disease requires fairly specific conditions to thrive.
The disease has a short lifespan in water or soil lacking plant matter.
Additionally, the bacteria causing this plant disease are weak and they require lesions (usually caused by insect damage) to infect a plant.
Transmission is seed-borne, and older leaves are more likely to offer entry points.
What Damage Does It Cause?
The symptoms of bacterial leaf spot are hard to distinguish from fungal leaf spot, but there are a few differences.
The foliage of a host plant infected by Xanthomonas will appear to have black or brown spots.
These may or may not include a yellow halo and tend to be uniform in size.
When wet, the spots will expand and often run together but will have a more speckled appearance when dried out.
Acidovorax and Pseudomonas may appear across the leaf or be confined to the margins or between the leaf veins.
These spots are generally more angular and have a reddish-brown appearance.
These spots will dry out, turning entry lesions black soon after the infection sets in.
The next phase of disease development happens when the spots grow more numerous.
This can cause leaf withering, yellowing, and shedding.
Infected fruit may take on a spotty appearance or display brown sunken spots.
Sun scalding, defoliation, fresh lesions, and nutrient deficiencies are all potential side effects of this disease.
How To Control Bacterial Diseases?
There is no current cure for bacterial leaf spot, so prevention (cultural control) is the best approach.
Cultural control helps reduce the risk of infections, while chemical controls will reduce but not eliminate existing infections.
Cultural Control Methods
Check to ensure any seeds are disease-free and sanitize your tools before and after working with vulnerable plants.
Xanthomonas is especially notorious for infecting seeds, and treating infected seeds with bleach is a time-honored tradition in food crops.
Pest management is effective against possible infection by reducing the risk of lesions often leading to water spots.
Moisture is a real problem when fighting bacteria, so be sure to space any vulnerable species properly.
This, combined with avoiding overhead irrigation (such as sprinkler irrigation or exposure to rainfall), are two good ways to protect the leaves.
Meanwhile, ensure good drainage so plants aren’t left in standing water.
In the event you encounter infected plants, remove them to prevent the spread of infection, and consider using crop rotation to introduce more resistant varieties of plants to deprive this or other bacterial diseases of viable hosts.
Chemical Control Methods
When dealing with an existing outbreak, your first reaction may be to reach for fungicides.
These will have no effect on bacterial pathogens.
Instead, you will need to invest in bactericides designed to target bacterial leaf spot.
Be sure to read the labels carefully and follow the instructions to avoid causing further harm to your garden.
Streptomycin is an antibiotic popularly used to treat tuberculosis and other human diseases.
However, Streptomycin (Agri-mycin 17 – agricultural streptomycin) has proven useful in plant disease management, especially against bacterial leaf spot.