Bacterial spot on tomatoes — symptoms and treatment

Severity: medium

What is Bacterial spot

Bacterial spot of tomato is caused by Xanthomonas species. It causes leaf spots and fruit blemishes, and the word bacterial shapes everything on this page: this is not a fungus, so the tools are different and prevention carries far more weight than treatment. Severity is rated medium — the plant is not usually killed, but blemished fruit is unsellable and premature defoliation costs the crop that would have ripened on those leaves.

Symptoms

The disease shows up first as small dark raised spots on the leaves and stems. The raised texture is the detail worth catching: run a thumb over them and they are palpably bumpy rather than flat, which separates them from most fungal leaf spots. On the fruit the damage looks different and matters more — water-soaked fruit lesions, spots that look wet, sunken and glassy as though soaked from underneath, which later roughen into scabby blemishes. Premature defoliation follows as the leaf spots multiply, and fruit left on a bare plant ripens badly.

• Early: small dark raised spots on leaves and stems
• Progressing: water-soaked lesions on the fruit
• Advanced: premature defoliation and blemished, unmarketable fruit

Early blight makes larger flat lesions with concentric rings, not small raised spots on stems.

Causes and conditions

Bacteria have no spores to drift on the wind — they need to be carried and they need a way in. Splashing water does the carrying: rain, overhead irrigation and the spray off a hose move the bacteria from infected tissue and debris onto clean leaves and fruit. Warm wet weather is the driver, and each wet spell is a fresh dispersal event — which is exactly why treatment is aimed at wet weather. Entry comes through wounds and natural leaf openings, so handling wet plants and pruning help the bacteria in. The other route is the one growers underestimate: the bacteria travel on seed and survive on infected crop debris, so an outbreak often arrives with the planting material itself.

Treatment

Clean seed and rotation — cultural

Pre-season. Use pathogen-free seed. Rotate away from solanaceous crops for 3 years. Avoid overhead irrigation. These are the decisions that actually control bacterial spot, and all are made before a plant is in the ground. Clean seed closes the commonest way in; rotation for 3 years starves the bacteria in the debris; keeping irrigation off the foliage removes the splash that spreads them.

Copper hydroxide + Mancozeb — chemical

At transplanting through harvest. Apply the copper-mancozeb mix every 7-10 days during wet weather. This is protective cover on healthy tissue during the wet spells when spread happens, not a cure — copper does not eradicate an established infection, and spots already on the fruit stay. Pre-harvest interval: 14 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Start with pathogen-free seed; it is the single highest-value step against this disease.
• Rotate away from tomatoes, peppers, eggplant and potatoes for 3 years.
• Irrigate at the base with drip and never overhead.
• Stay out of the crop while the foliage is wet — handling wet plants spreads bacteria.
• Remove and destroy infected crop debris rather than tilling it into the bed.

Frequently asked questions

Can I eat tomatoes with bacterial spot? The lesions are blemishes, not a food-safety hazard — cut them out and the fruit underneath is fine, though badly spotted fruit is not marketable. Observe the 14-day pre-harvest interval after spraying.

Is it contagious to my peppers? Treat it as such — it is why rotation goes away from solanaceous crops as a group for 3 years, not just tomatoes. Splashing water moves the bacteria between neighbouring plants.

I sprayed copper and it is still spreading — why? Because copper protects healthy tissue; it does not cure infected tissue. If it is going in wet weather with overhead irrigation, or the seed was infected, spraying cannot keep up. The wet-weather schedule, drip irrigation and clean seed have to work together.