Bacterial leaf spot
Also known as Bacterial blight, Xanthomonas leaf spot, Pseudomonas leaf spot
In short
Bacterial leaf spot is a group of leaf diseases caused mainly by Xanthomonas, Pseudomonas and Acidovorax bacteria. It shows as dark, water-soaked spots, often with a yellow halo, and spreads in splashing water. There is no cure for infected leaves, so prevention and early removal matter most.
How it looks
Spots start small and look water-soaked or greasy, frequently appearing first on the underside of the leaf. They darken to brown or nearly black and are often ringed by a yellow halo. Spots may be angular (held in by the leaf veins) or rounded, and can merge into larger blighted patches. On begonias and geraniums, Xanthomonas often makes V-shaped lesions that run inward from the leaf edge. On Dieffenbachia, Xanthomonas spots are very dark, almost black, with a yellow halo, and start at the margins; if they reach the main veins the leaf can collapse.
Signs
Water-soaked spots that turn brown or black; yellow halos around spots; spots merging into blighted areas; yellowing and dropping of badly spotted leaves; rapid spread to neighbouring plants after overhead watering or rain.
Life cycle
The bacteria usually arrive on infected seed, cuttings or young plants. Once present they spread in splashed water from rain or watering, on wet hands and on tools, and enter leaves through natural openings and wounds.
What favours it
Warm, humid conditions with water sitting on the leaves. North Carolina State Extension gives optimum temperatures of about 25–30 °C (77–86 °F) for Pseudomonas and Xanthomonas, which matches the warm, wet weather common in the tropics. Poor nutrition makes Dieffenbachia more susceptible. Overhead watering and handling wet plants spread it.
Prevention
- Buy clean plants and cuttings, and quarantine new arrivals before placing them near your collection.
- Water the potting mix rather than the leaves, and water early in the day so any wet foliage dries quickly.
- Space plants for air movement and avoid misting plants that have had leaf spots.
- Do not handle or prune plants while the leaves are wet.
- Clean and sanitise tools, benches and reused pots.
Treatment
Start with the first step and move down only if the problem persists.
- Remove spotted leaves promptly and bag or bin them; discard badly affected plants and check the plants next to them.
- Keep the leaves dry from now on and improve ventilation.
- Isolate the plant until no new spots appear.
- Copper-based bactericides (and Bacillus-based biological products) can protect healthy tissue when used preventively on a regular schedule, but they do not cure existing spots and some bacterial strains are resistant; follow the product label.
Pros and cons
Cons
- Infected tissue cannot be cured.
- Spreads quickly through a collection by splashing water and handling.
Look-alikes
- Fungal leaf spot
- Fungal spots are usually dry and often show rings or tiny dark dots (spore bodies); bacterial spots start water-soaked and are often angular with a yellow halo.
- Anthracnose
- A fungal disease producing sunken, tan-to-dark lesions, often with fruiting bodies; not water-soaked at first.
- Edema
- Edema forms raised, corky blisters on the leaf underside and does not spread from plant to plant.
- Cold damage
- Chilling causes water-soaked patches too, but they appear all at once after a cold spell, usually on the youngest leaves, and do not keep spreading.
Affected plants
A wide range of ornamentals, including begonia, geranium (Pelargonium), poinsettia, chrysanthemum, gerbera, canna, petunia, vinca, salvia, zinnia, mandevilla, Dieffenbachia, hibiscus, English ivy (Hedera) and Phalaenopsis orchids (bacterial brown spot).
Related
Sources
- Bacterial Leaf Spot on Greenhouse Ornamentals — NC State Extension
- PP286: Dieffenbachia Diseases: Identification and Control — UF/IFAS
- Houseplant Diseases & Disorders — Clemson HGIC
- Bacteria — American Orchid Society
Written in our own words from these sources. Check a product label before using any treatment.