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Bacterial Leaf Spot

Xanthomonas spp.

Bacterial Leaf Spot, Xanthomonas Leaf Spot, Bacterial Canker

A hand holding a green leaf with leaf spot
Xanthomonas is a genus of bacterial plant pathogens. Species in the group demonstrate high specificity to different plant species. Many important Xanthomonas pathogens are split into groups called pathovars (abbreviation pv.). A pathovar is a bacterial strain or group of strains which share similar characteristics, (i.e. produce similar symptoms) but use different plants as hosts. Xanthomonas bacteria are common plant pathogens, infecting a wide range of plant species. Symptoms may vary slightly depending on the pathovar, although they'll often involve leaf spots and cankers. Some of the most well-known and devastating diseases caused by Xanthomonas include Citrus Canker, Bacterial Leaf Spot, and Bacterial Blight of Rice.
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Growth factors

Xanthomonas pathovars often spread during wet weather.

Symptoms

Open lesions in trees, otherwise known as canker.
Black spots on leaves, fruits and stems.
Black spots appear between the leaf veins.
Black spots are dead plant cells, these can fall out causing 'shothole'.
In Roses, dark spots are encircled with yellow halo.

Biological treatment

If you suspect a plant is infected, remove and destroy plants (or diseased-parts) promptly. Ensure all plant matter is cleared and removed.

Lifecycle

Bacteria can spread around plants through water, so infections tend to become worse following wet or humid weather. Initially, the bacteria begin to multiply on the surfaces of leaves, fruits and other external plant tissues. The second stage involves bacteria entering the plant through wounds or natural openings, such as the leaf stomata (pores). From here, the disease spreads via fallen plant matter and seed. The latter can be transported further via wind, rain and agricultural machinery. Animal and insects can sometimes be vectors, but this is less likely.

Prevention

Use filters when collecting rainwater to avoid infected plant matter contaminating supply. Avoid overwatering plants. Water should be able to evaporate quickly so that bacteria doesn't have a medium to travel through. Maintain good tool hygiene. Be sure to clean tools, boots and vehicles if you treat a plant which shows symptoms. Always opt to grow resistant varieties, where possible.

Affected plants

Leopard Lily

Dieffenbachia spp.

A close up of some green Pilea depressa leaves

Depressed Clearweed

Pilea depressa

Passion Flower

Passiflora incarnata

Tobacco Plant

Nicotiana spp.

A red rose on a Rosa plant

Rose

Rosa spp.

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