What Does Bacterial Canker Look Like on Florida Tomatoes?

Bacterial canker is a destructive disease of tomato caused by the bacterium Clavibacter michiganensis subsp. michiganensis (Cmm). In Florida’s warm, humid climate this pathogen can spread rapidly, causing yield loss, unmarketable fruit, and long-term contamination of fields and greenhouse facilities. This article describes what bacterial canker looks like on tomato plants, how to tell it apart from other diseases, how to confirm a diagnosis, and concrete management steps Florida growers can take to reduce risk and impact.

How bacterial canker behaves in Florida

Tomato bacterial canker thrives in warm temperatures and is spread primarily by contaminated seed, infected transplants, splashing water, tools, hands, and occasionally insects. Florida’s combination of high temperatures, heavy rains, and frequent overhead irrigation creates conditions favorable for spread. Once established in a greenhouse or field, the bacterium can persist on plant debris, on stakes and trellises, and in contaminated seed lots, making prevention and strict sanitation essential.

Typical visual symptoms on tomato plants

Symptoms vary depending on whether the infection is systemic or localized. Early detection is possible if you know where to look and what to look for.

Leaves

• Leaves may show marginal necrosis and a scorched appearance, often starting at leaflet edges and progressing inward.
• Look for irregular, water-soaked spots or streaks along veins. Leaves can curl, turn yellow between veins, then brown and die.
• Wilting may occur on individual shoots or entire branches even when soil moisture is adequate. This “sudden” wilting is a hallmark of systemic infection.

Stems and cankers

• Stems often develop sunken or raised cankers: tan to brown lesions that may girdle stems and petioles.
• Under the bark, vascular browning (dark streaking in the stem) is common when you split the stem lengthwise.
• A diagnostic sign is gummy or sticky ooze from cut stems, petioles, or fruit peduncles in moist conditions. The exudate can dry to a crusty residue.
• Cankered areas can crack, collapse, and give the plant a “blighted” appearance.

Fruit

• Fruit may show the characteristic “bird’s-eye” spot: small, hard, raised lesions often surrounded by a yellow halo. The center may be a darker, slightly depressed spot.
• Surface scarring, pitting, or concentric corky areas may develop. Affected fruits are often unmarketable.
• Internal fruit symptoms can include discoloration around vascular bundles beneath the calyx or pedicel.

Blossoms, pedicels and seeds

• Blossom infections can result in flower drop or small necrotic spots on petals and pedicels.
• The pedicel (fruit stem) is often infected and can be a primary site that spreads the pathogen into the fruit or plant.
• Seed contamination is a critical pathway for long-distance spread; infected seeds or contaminated seed lots can start outbreaks.

How to distinguish bacterial canker from similar problems

Misidentification is common because several diseases and physiological disorders produce similar symptoms. Diagnostic clues and comparisons:

Bacterial spot and bacterial speck (Xanthomonas and Pseudomonas)

• Both produce small leaf and fruit spots. Bacterial spot often has dark, greasy lesions on leaves and scabby fruit lesions.
• Unlike bacterial canker, these pathogens rarely produce systemic vascular browning or the prominent gummy stem exudate typical of canker.
• Fruit lesions from bacterial spot/speck are usually more superficial and evenly scattered than the bird’s-eye spots of canker.

Early blight and Septoria leaf spot (fungal)

• Early blight (Alternaria) shows concentric rings in leaf lesions, giving a “target” appearance. Septoria produces many small, circular tan spots with dark borders.
• Fungal leaf spots do not cause the vascular browning or stem cankers associated with canker, and fungal infections generally start lower in the canopy and spread upward.

Fusarium and Verticillium wilt (vascular fungi)

• Both cause wilting and yellowing, often of one side of the plant, and vascular browning can be observed in stem cross-sections.
• Fusarium and Verticillium wilts do not create leaf spots or stem exudates and usually have a more gradual, seasonal pattern.

Physiological disorders and abiotic damage

• Heat stress, herbicide injury, and nutrient imbalances can cause leaf scorch and wilting. However, abiotic causes do not produce cankers, stunted pedicels, or bacterial ooze.

Field tip: split suspicious stems longitudinally and inspect vascular tissue. A brown to dark streak within the stem, especially when combined with stem cankers or gummy exudate, strongly suggests bacterial canker.

How to confirm a diagnosis

Field diagnosis is possible with careful observation, but laboratory confirmation is recommended before making major management decisions.

• Field checks: look for the combination of leaf scorch, stem cankers, vascular browning, and fruit bird’s-eye spots. Note spread pattern: if symptoms occur on plants associated with a common seed lot or transplant batch, suspect seed-borne inoculum.
• Collect samples: choose recently symptomatic tissue (stem sections, pedicel, symptomatic fruit). Place samples in paper bags or clean containers, keep cool, and label with location and date.
• Lab tests: county extension services and diagnostic labs can perform culture, serological tests, or PCR to confirm Cmm. Confirmed positives are important for regulatory reporting and specific control recommendations.

Management and control strategies for Florida growers

There is no single cure for bacterial canker. Integrated management emphasizes exclusion, sanitation, and minimization of spread.

Cultural controls

• Start with certified disease-free seed and transplants. Avoid reusing seed saved from suspect plants.
• Use transplants from reputable greenhouses with strict sanitation programs.
• Avoid overhead irrigation when practical. Use drip irrigation to reduce leaf wetness and splashing.
• Space plants to improve air flow and accelerate drying.
• Practice strict weed control; some weeds can harbor pathogens or prolong leaf wetness.

Sanitation and hygiene

• Remove and destroy infected plants immediately. Remove entire plants including root systems to reduce inoculum.
• Do not compost symptomatic plant material. Instead, bag and dispose according to local regulations or burn where allowed.
• Disinfect tools, stakes, and hands frequently. A common disinfectant is a freshly prepared 10% household bleach solution (one part household bleach to nine parts water) or commercial sanitizers labeled for agricultural use. Allow tools to remain wet with disinfectant for at least 30 seconds to one minute when practical.
• Clean greenhouse surfaces, benches, and containers between crops.

Seed and transplant measures

• Use seed from reputable suppliers with disease testing and sanitation practices.
• Some commercial seed lots are treated to reduce seed-borne bacteria; consult extension or seed suppliers for validated seed treatment options and verified protocols.
• Inspect transplants on arrival; quarantine and monitor new transplants before putting them into production.

Chemical and biological options

• Copper-based bactericides can reduce spread and suppress symptoms when applied preventively, but they will not eliminate systemic infections once established.
• Multiple applications may be needed; follow label instructions and rotate products to reduce phytotoxicity risk and resistance issues.
• Biological products may provide some suppression in certain systems, but results are inconsistent and should not replace sanitation and exclusion.

Crop rotation and resistant varieties

• Rotate away from tomato and related solanaceous crops for at least one season when possible to reduce local inoculum levels.
• There are few tomato varieties with strong resistance to bacterial canker. Check current variety descriptions and seed company information for any available tolerance. Even tolerant varieties can become infected and should be managed defensively.

When to remove plants and how to dispose

• Remove any plant showing systemic symptoms (wilting, extensive stem cankers, severe fruit spotting). Partial removal of a single infected shoot may not be sufficient because the bacterium can move systemically.
• Place removed plants in sealed bags; do not leave infected material in the field or greenhouse.
• Burn or discard according to regulations. If composting is the only option, use high-temperature, properly managed composting systems that reach temperatures sufficient to kill pathogens, but note that composting is not guaranteed to eliminate Cmm in all systems.

Practical monitoring and record-keeping

• Map plantings and keep records of seed lots, transplant sources, irrigation methods, and any symptoms observed with dates and locations.
• Monitor plants at least weekly during warm, wet periods when disease risk is highest.
• Record weather conditions, irrigation events, and any cultural or chemical interventions to help correlate practices with disease outbreaks.

Quick practical takeaways for Florida growers

• Vigilance matters: inspect seedlings, transplants, and fields routinely for wilting, stem cankers, vascular browning, and bird’s-eye fruit spots.
• Prevention is the best tool: use disease-free seed and transplants and practice strict sanitation.
• Control spread: avoid overhead irrigation when possible, disinfect tools and hands, and remove symptomatic plants promptly.
• Manage expectations: copper sprays can suppress but not cure systemic infections; combine chemical tools with cultural measures.
• Get confirmation: submit samples to an extension diagnostic lab to confirm bacterial canker before making major production changes.

Final notes

Bacterial canker is a serious but manageable disease when detected early and addressed with disciplined sanitation and cultural practices. In Florida’s environment, proactive measures–especially sourcing clean seed and transplants, preventing splashing and leaf wetness, and maintaining rigorous hygiene–are the most effective ways to protect your crop. If you suspect bacterial canker in your tomato crop, collect representative samples and consult your local extension or diagnostic laboratory for confirmation and tailored management advice.