Tomato Diseases: How To Fight Bacterial Speck

by Matt Gibson

About Bacterial Speck

Bacterial speck of tomato plant is not a very common tomato disease, but it does occasionally occur in modern home gardens in the right conditions. Bacterial speck is similar to bacterial spot and bacterial canker, but is easy to distinguish based on the symptoms. The culprit of the disease is the bacteria Pseudomonas syringae pv. The symptoms are small spots that appear on the leaves of the tomato plant, which are distinctly smaller than the spots that occur with bacterial spot and bacterial canker.

The spots of bacterial speck are brown in the middle and surrounded by a yellow outer ring. In severe infections, the specks may overlap, which make them look larger and more irregular than they actually are. In very extreme cases, the spots can spread from the leaves to the fruit of the tomato plant.

To tell the difference between bacterial speck, spot, and canker, look closely at the lesions themselves. Bacterial speck has the smallest spots of the three, and also the least damaging. Though bacterial speck can be very unsightly, it is never fatal to the tomato plant like speck and canker can be. Bacterial speck only ever affects the leaves and fruit of the plant, never the stems, which is not the case with bacterial canker. Also, bacterial speck appears on tomato plants exclusively, while bacterial spot and bacterial canker also appear on pepper plants. 

Causes And Symptoms of Bacterial Speck

Bacterial speck is caused by pseudomonas syringae pv. tomato, a bacterium that is spread by rains which thrives in cool, damp conditions. Often, the cause of bacterial speck is infected seeds. If the seeds being used to plant your tomato plants are infected or damaged, the bacteria can easily infect the resulting plants. Bacterial speck and damage both the foliage and fruit of the tomato plant.

The lesions of bacterial speck start off as small black spots, most often surrounded by a yellow or golden ring. The infected leaves will also curl up on the edges and tip. On the fruit itself, bacterial speck often forms small, raised lesions (another distinguishing trait of bacterial speck, as the spots of bacterial spot and canker are sunken, not raised). The spots sometimes resemble freckles or make the fruit of your tomato plants appear bruised.

Treatments and Control of Bacterial Speck

Bacterial speck can be treated in mild cases, by just scraping the lesions off of the fruit, as the specks are only present on the surface level of the fruit. Copper fungicidal preparations should then be used to treat the plants after the unsightly specks are removed from the fruit surface on the fruit of all infected plants.

Copper sprays are the most effective fungicides to use to protect your tomato plants, as copper is used to kill many different organisms that attack the tomato plant, including both fungi and bacteria, and is very effective at doing so. Though bacterial speck is a bacterial disease, not a fungal disease, copper fungicides are still highly effective for treating the bacterial infection. Copper preparations should be sprayed directly on the plants as soon as you notice any symptoms of bacterial speck. Spraying should be repeated every seven days to help minimize crop loss and damage from the disease.

There are a few other things that you can do to prevent and control bacterial speck outbreaks on your tomato plants:

• Only purchase seeds, seedlings, or transplants from reputable merchants and make sure that your seeds or small plants are free from any signs of disease.
• Most respectable nurseries and garden supply centers practice sanitary treatments of seeds to insure that they are free from disease. Seeds should be treated with hot water to eliminate any bacteria before planting into the ground.
• Rotate your crops and plant your tomatoes in a brand new location every year, only planting tomatoes in a location where tomatoes have been previously grown every four years to limit exposure to the same plant debris.
• Remove any weeds from the area that you plan to use for planting tomatoes, as weeds can house the bacteria that causes bacterial speck
• Do what you can to promote good air circulation, making sure that each of your tomato plants have enough exposure to the air in order to prevent bacterial speck. This involves proper spacing of each plant, as well as staking or providing support to increase air circulation.
• Buy varieties that are resistant to developing bacterial speck and other diseases. Using these varieties will also help to prevent the spread of the disease.

Common Questions and Answers About Bacterial Speck

Can you eat tomatoes with bacterial speck?

While bacterial speck is not contagious to humans, experts from the Wisconsin Horticulture Division of Extension recommend not eating tomatoes that show signs of bacterial speck. The blemishes caused by bacterial speck give pathogens that are contagious to humans an entry point to invade and infect the fruit. However, tomatoes growing on plants with bacterial speck that do not have blemishes are safe to eat.

How do you treat bacterial speck?

Often, the best option is to simply allow bacterial speck to run its course, focus on minimizing spread, and harvest the tomatoes that are unblemished. (Do not eat fruit with blemishes, as the spots can give pathogens an entry point into the tomatoes.) Removing foliage with bacterial speck is ineffective at stopping progression of the disease, and there is no way to cure or treat tomatoes once they show signs.

There are several ways to reduce the spread of bacterial speck and prevent it happening in the future. Begin with seeds or young plants that are pathogen free and come from a trusted source. If you must plant seeds that you suspect may be contaminated, first soak them in hot water (at least 122 degrees Fahrenheit) for 25 minutes.

Disinfect your gardening tools with bleach or rubbing alcohol before and after using them and when moving from infected to disease-free plants. Do not use a sprinkler to hydrate your plants. Instead, water plants from the base with a drip hose or soaker. Refrain from working in the garden when plants are wet from rain, morning dew, or being watered.

At the end of the season, remove all plant debris from the infected area and bury it deeply, burn it, or use the debris in hot compost. (If you choose to burn your plant debris, ensure it is legal to do so in your area.)

Next season, do not grow tomatoes in an area where your plants have previously had bacterial speck. In fact, don’t plant any crop susceptible to bacterial speck in a part of the garden that’s struggled with it the previous season. Instead, use non-host crop rotation to place plants that aren’t prone to bacterial speck in the affected area.

How does bacterial speck spread?

The bacteria that causes bacterial speck spreads by movement of water due to rain, runoff, irrigation, or generally wet conditions. It can also enter gardens via contaminated seeds or transplants as well as being spread on gardening tools, equipment, clothing, hands, or gloves. Bacterial speck can hide out over the winter in infected soil, plant debris, or porous material, such as stakes, trellises, or other wooden supplies in the garden.

Is bacterial speck contagious?

Bacterial speck is contagious and spreads from plant to plant in splashing water, whether it comes from rainfall, runoff, or irrigation. The bacteria responsible for bacterial speck can overwinter in your garden’s soil, debris from the previous season’s plants, or on wooden supplies like stakes and trellises. Infected gardening tools, clothing, gloves, gardener’s hands, and contaminated seeds or transplants can also introduce bacterial speck to your garden.

What are the symptoms of bacterial speck?

Bacterial speck causes small black spots on plant leaves that happen most frequently on the underside of foliage. The spots measure between an eighth and a quarter of an inch across and may develop a yellow halo as they grow. On tomatoes, spots may be black or brown, and on unripe tomatoes, the normally yellow halo might be dark green. Lesions may be flat, sunken, or raised and only penetrate a few cells deep into fruit tissue.

Bacterial speck looks a lot like bacterial spot, bacterial canker, and tomato spotted wilt. If you have trouble discerning which disease your plants have and need help to diagnose it, you can contact your nearest Extension office. To find your local Extension office, choose your state from this map from the National Pesticide Information Center.

What causes bacterial speck?

Bacterial speck is caused by the bacterium Xanthomonas campestris pv. Vesicatoria. It is worst in wet, cool conditions and spreads via the movement of water, whether through rainfall, irrigation, or runoff. The bacteria can hitch a ride into gardens on infected seeds or transplants as well as contaminated gardening tools, supplies, gloves, clothing, or gardener’s hands.

What does bacterial speck look like?

The most noticeable symptom of bacterial speck are small black spots on foliage. The spots tend to happen most frequently on the underside of leaves and range from one eighth to one quarter of an inch wide. The black spots may gradually develop a yellow ring around the outside. Spots may also appear on tomatoes, but are likely to be as small as a pinpoint, and they rarely penetrate the tomatoes very deeply. In addition to black, spots on tomatoes may be brown, and on unripe fruits the outer ring may be dark green instead of yellow. Spots may be flat, sunken, or raised.

It can be difficult to tell bacterial speck apart from bacterial spot, bacterial canker, or tomato spotted wilt. You can contact your local Extension office if testing is needed to diagnose what is infecting your plants. To find your nearest Extension office, click on your state on this map from the National Pesticide Information Center.

Have you seen bacterial speck on your tomatoes? If you have pictures, please send them our way.

Want to learn more about bacterial speck?

Cornell University covers Bacterial Speck of Tomato

Gardening Know How covers Tomato Bacterial Speck

University of Wisconsin-Madison covers Bacterial Speck of Tomato

University of Wisconsin-Madison covers Bacterial Speck of Tomato

Iowa State University covers Bacterial Speck and Spot of Tomato

University of California covers Bacterial Speck

Plantwise Knowledge Bank covers Bacterial Speck

Cornell University covers Bacterial Diseases of Tomatoes

Bugwoodwiki from University of Georgia covers Bacterial Speck of Tomato