Reviewed August 2026 against USDA NASS, UC Statewide IPM Program, University of Florida IFAS Extension, and North Carolina State Extension.

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Bacterial spot is a seedborne disease caused by four Xanthomonas species that produce small, water-soaked, dark lesions with yellow halos on tomato and pepper leaves and fruit — it spreads by rain-splash and contaminated transplants, and University of Florida/IFAS field data puts yield loss as high as 50% in Florida tomato when it goes untreated. If you searched “bacterial leaf spot on tomatoes” or “treating bacterial leaf spot,” this article gives you the diagnosis, the exact copper and mancozeb rates growers use, and where “yellow spots on cucumber” fits into a different but related picture. Bacterial spot is not the same organism as bacterial speck, and treating one as the other wastes a spray pass — the distinction below matters more than it looks.

What Bacterial Spot Is — and Isn’t

Bacterial spot of tomato and pepper is caused by four distinct Xanthomonas species — a fact North Carolina State Extension calls out specifically because it explains why resistance breeding and chemical control both struggle to keep up: a treatment or resistant variety effective against one species may not hold against another (North Carolina State Extension). The pathogen moves into a field three ways: seedborne infection, contaminated transplants, and rain-splash dispersal once it’s established (UC Statewide IPM Program). There is no single “cure” — every university extension source treats bacterial spot as a disease you suppress with timing and sanitation, not one you eliminate with a single spray.

“Bacterial leaf spot” is the common-name version of the same disease and is used interchangeably with “bacterial spot” in most US extension literature, including the sources cited throughout this article. If your plant shows small dark spots with a yellow halo on the leaves, fruit, or both, and you’re searching either term, you’re looking at the same pathogen group.

Identification: Bacterial Spot vs. Bacterial Speck vs. Cucumber Yellow Spots

Three different search terms point at three related-but-distinct problems, and mixing them up is the single most common reason a treatment doesn’t work. Here is how to tell them apart before you spray anything.

Bacterial spot (Xanthomonas spp.)

• Lesions: Small, water-soaked spots that turn dark brown to black, often with a yellow halo, on leaves, stems, and fruit.
• On fruit: Raised, scabby spots — this is the symptom that costs growers at the packing line, since affected fruit gets downgraded or rejected.
• Host crops: Tomato and pepper.
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Bacterial speck (Pseudomonas syringae pv. tomato)

• Lesions: Tiny, pinpoint dark specks, smaller and more uniform than bacterial spot lesions, usually with a narrow yellow halo.
• Temperature preference: Speck favors cooler, wet conditions; bacterial spot favors warm, humid weather. If your outbreak follows a cold snap with rain, speck is more likely than spot.
• Host crops: Tomato only (not pepper).

Cucumber leaves with yellow spots

This symptom description covers several possible causes that are not bacterial spot at all, since bacterial spot does not infect cucurbits. Yellow spotting on cucumber leaves is more commonly associated with angular leaf spot (a different bacterial pathogen, Pseudomonas syringae pv. lachrymans), downy mildew, or nutrient stress. The research base cited in this article — UC IPM, IFAS, and NC State — documents bacterial spot specifically on tomato and pepper; none of these sources publish cucumber-specific rates for the Xanthomonas species covered here. If your cucumber crop is showing yellow spotting, the correct diagnostic step is to look for the accompanying pattern: angular, vein-limited lesions with a wet or greasy look point to angular leaf spot, while spots that follow leaf veins in a mosaic pattern point toward downy mildew. A local extension diagnostic lab or your state’s cooperative extension office can confirm the pathogen from a leaf sample — that identification, not a generic “yellow spot” treatment, determines which product and rate to use.

“Peck bacteria” appears to be a shorthand or misspelling reaching for either “bacterial speck” or general bacterial leaf infection information — the identification table above and the speck section covers what that search is most likely looking for.

How Big a Problem Is Bacterial Spot in US Tomato Production?

US tomato production was valued at $1.9 billion in gross production value in 2023, grown across roughly 330,000 acres nationwide, according to USDA NASS (USDA NASS). Bacterial spot doesn’t carry a published national loss estimate specific to the disease — USDA NASS does not break out disease-specific crop loss for bacterial spot in its publicly available summaries — but regional field data gives a concrete sense of what’s at stake. In Florida, one of the country’s largest fresh-market tomato states, University of Florida/IFAS Extension documents yield losses of up to 50% in fields where bacterial spot is not actively managed (University of Florida IFAS Extension). Applied against a $1.9 billion crop, a 50% loss in an unmanaged field is the difference between a marketable harvest and a write-off — which is why every extension source treats spray timing and sanitation as non-negotiable rather than optional.

There is no published incidence or prevalence percentage for bacterial spot broken out by US region outside Florida in the sources reviewed for this article. If you farm outside Florida and want a region-specific estimate, your state’s cooperative extension plant disease clinic is the right first call — they track regional outbreak patterns that don’t make it into national datasets.

Treating Bacterial Leaf Spot: Rates, Timing, Products

This is the section most searches for “treating bacterial leaf spot” are actually looking for, so here are the numbers directly, from the University of California Statewide IPM Program’s current tomato bacterial spot guidance:

• Copper hydroxide: 0.75–1.75 lb/acre, applied as part of a rotation program (UC Statewide IPM Program).
• Mancozeb: 1–1.5 lb/acre for one common formulation, typically tank-mixed with copper rather than applied alone — the combination slows the development of copper resistance in the bacterial population (UC Statewide IPM Program).

Two things about these rates that matter more than the numbers themselves. First, copper alone has documented resistance issues in Xanthomonas populations in several US growing regions — this is exactly why UC IPM recommends the copper/mancozeb combination rather than straight copper. Second, rates are reviewed and can be revised each growing season; UC IPM updates its guidance annually, so before you order product, check the UC IPM tomato bacterial spot page directly for the current-season rate rather than relying on a number printed in an article.

On cost per acre and labor cost for a full spray program, none of the sources reviewed for this article publish a figure — product pricing varies by supplier and region and is not tracked in the university extension literature cited here. Contact your regional ag retailer or extension agronomist for a current quote; the calculator below at least gets you from a per-acre rate to a total product volume so you can price it out yourself.

On timing: sprays go on preventively, before symptoms are widespread, and continue on a schedule through the period when rain-splash conditions favor spread — not as a one-time reactive treatment after an outbreak is already visible. By the time lesions cover a field, a spray program manages further spread; it does not reverse existing damage.

Bacterial Leaf Spot on Tomatoes: Field Management Beyond the Spray

Chemical control is one piece of managing bacterial spot on tomatoes — sanitation and sourcing matter as much because the disease is primarily seedborne. UC IPM and NC State Extension both identify seedborne infection and contaminated transplants as primary entry points into a field, ahead of rain-splash spread once the pathogen is established (UC Statewide IPM Program). That means the highest-leverage intervention often happens before the first spray:

• Source certified, disease-tested seed and transplants. Since bacterial spot is seedborne, an infected seed lot or transplant tray can introduce the pathogen into a field that had no prior history of it.
• Avoid overhead irrigation where possible. Rain-splash is a documented spread mechanism; overhead irrigation recreates the same splash dynamic on a schedule you control. Drip irrigation reduces leaf wetness and splash-driven spread.
• Rotate away from tomato and pepper for a season where field history includes bacterial spot, since crop debris can carry inoculum forward.
• Scout early and often during warm, humid stretches — the conditions bacterial spot favors — so a copper/mancozeb program starts before lesions are already widespread across the field.

For growers tracking disease pressure across multiple fields or blocks, that scouting cadence is where remote monitoring earns its keep — see the monitoring section below for what satellite-based tools can and can’t tell you about an outbreak in progress.

Cucumber Leaves With Yellow Spots: A Different Diagnosis Path

If you landed here searching “cucumber leaves yellow spots” or “yellow spots on cucumber plant leaves,” it’s worth being direct: the bacterial spot pathogen and application rates covered above are documented for tomato and pepper, not cucumber, in the sources this article draws on. Cucumber foliar yellow spotting most often traces to one of three causes, and telling them apart before treating saves a wasted spray pass:

None of the university sources cited in this article — UC IPM, IFAS, or NC State — publish cucumber-specific data for the Xanthomonas species that cause bacterial spot on tomato and pepper, because that pathogen group does not infect cucurbits. A confirmed diagnosis is the correct next step: send a leaf sample to your state’s cooperative extension plant diagnostic clinic, most of which offer this service for a modest fee and turn results around within a week or two.

Copper Spray Calculator

Use the UC IPM copper hydroxide rate (0.75–1.75 lb/acre) against your own field size and a rate you choose within that range to estimate total product needed for one application.

Assumptions: uses UC Statewide IPM Program’s published per-acre rate ranges for copper hydroxide and mancozeb on tomato bacterial spot as of this review. Does not include water carrier volume, adjuvants, product cost, or application labor — check current-season rates and tank-mix compatibility with your local extension office or product label before applying.

Catching Bacterial Spot Before It Spreads: Remote Monitoring

Because bacterial spot sprays work preventively rather than curatively, the value of any monitoring approach is entirely in how early it flags a developing outbreak. Multispectral satellite imagery — the kind Farmonaut’s platform provides — tracks vegetation health indices like NDVI across a field over time, which can surface stress patterns (uneven canopy vigor, localized decline) before lesions are visible to a scout walking the rows. This doesn’t replace a diagnostic leaf sample or a confirmed identification from your extension lab — satellite indices flag “something changed here,” not “this is bacterial spot” — but for growers managing many acres or multiple blocks, it narrows down where a scout should walk first.

Farmonaut’s large-scale farm management tools extend this to multi-field operations, giving agronomists a way to prioritize scouting routes based on which blocks show the earliest stress signals rather than covering every acre on the same fixed schedule. For growers who also carry crop insurance, having a documented history of field monitoring and intervention timing can support crop loan and insurance claims tied to disease losses. And for operations managing spray equipment and crews across several fields during a bacterial spot outbreak, fleet management tools help coordinate the application timing that copper/mancozeb programs depend on.

Businesses and developers building disease-monitoring workflows into their own systems can pull real-time field data through Farmonaut’s API, with implementation details in the API developer documentation. For supply chains that need to document where and when a disease intervention occurred — relevant for buyers requiring traceability on produce sourcing — Farmonaut’s product traceability tools log field-level events. Growers also managing sustainability reporting alongside disease control can track input use through carbon footprinting, since copper application volume is one of the sustainability metrics increasingly asked about by processors and retailers.

Comparison Table: Bacterial Spot Control Options

Farmonaut’s Subscription Plans

Choose a Farmonaut plan that fits your operation — from field-level satellite monitoring to multi-block scouting prioritization for growers managing bacterial spot risk across a season.

FAQ

Conclusion: A Verifiable Approach to Bacterial Spot

Bacterial spot on tomato and pepper is a seedborne, rain-splash-spread disease with documented copper hydroxide (0.75–1.75 lb/acre) and mancozeb (1–1.5 lb/acre) treatment rates from UC Statewide IPM Program, and yield losses up to 50% in unmanaged Florida fields per University of Florida/IFAS Extension. The durable method here doesn’t expire with next season’s numbers: identify correctly first (spot vs. speck vs. a cucurbit disease entirely), source clean seed and transplants, apply preventively rather than reactively, and confirm current rates against UC IPM’s page before every season rather than a fixed figure from any one article.

Farmonaut supports that workflow with satellite-based field monitoring that flags stress patterns early, helping growers prioritize scouting before an outbreak is visible from the ground. Begin monitoring your fields with Farmonaut to build a documented, season-over-season record of disease pressure and intervention timing.

Related reading: bacterial leaf spot control treatment tips and Clavibacter michiganensis control methods for related tomato bacterial diseases.