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Bacterial Spot of Tomato

What is bacterial spot?  Bacterial spot of tomato is a potentially devastating disease that, in severe cases, can lead to unmarketable fruit and even plant death.  Bacterial spot can occur wherever tomatoes are grown, but is found most frequently in warm, wet climates, as well as in greenhouses.  The disease is often an issue in Wisconsin.

Sunken, scabby bacterial spot lesions on ripening tomato fruit. (Photo courtesy of Mary Ann Hansen, Virginia Polytechnic Institute and State University)
Sunken, scabby bacterial spot lesions on ripening tomato fruit. (Photo courtesy of Mary Ann Hansen, Virginia Polytechnic Institute and State University)

What does bacterial spot look like?  Bacterial spot can affect all above ground parts of a tomato plant, including the leaves, stems, and fruit.  Bacterial spot appears on leaves as small (less than ⅛ inch), sometimes water-soaked (i.e., wet-looking) circular areas.  Spots may initially be yellow-green, but darken to brownish-red as they age.  When the disease is severe, extensive leaf yellowing and leaf loss can also occur.  On green fruit, spots are typically small, raised and blister-like, and may have a yellowish halo.  As fruit mature, the spots enlarge (reaching a maximum size of ¼ inch) and turn brown, scabby and rough.  Mature spots may be raised, or sunken with raised edges.  Bacterial spot symptoms can be easily confused with symptoms of another tomato disease called bacterial speck.  For more information on this disease, see University of Wisconsin Garden Facts XHT1250.

Where does bacterial spot come from?  Bacterial spot of tomato is caused by Xanthomonas vesicatoria, Xanthomonas euvesicatoria, Xanthomonas gardneri, and Xanthomonas perforans.  These bacterial pathogens can be introduced into a garden on contaminated seed and transplants, which may or may not show symptoms.  The pathogens enter plants through natural openings (e.g., stomates), as well as through wounds.  Disease development is favored by warm (75° to 86°F), wet weather.  Wind-driven rain can contribute to more severe disease as the pathogens are splashed and spread to healthy leaves and fruit.  Bacterial spot pathogens can survive well in tomato debris, but they survive very poorly in soil when not associated with debris.

How do I save plants with bacterial spot?  A plant with bacterial spot cannot be cured.  Remove symptomatic plants from the field or greenhouse to prevent the spread of bacteria to healthy plants.  Burn, bury or hot compost the affected plants and DO NOT eat symptomatic fruit.  Although bacterial spot pathogens are not human pathogens, the fruit blemishes that they cause can provide entry points for human pathogens that could cause illness.

On tomato leaves, bacterial spot leads to small, angular (i.e., straight-edged) spots with yellow haloes. (Photo courtesy of Michelle Grabowski, University of Minnesota Extension)
On tomato leaves, bacterial spot leads to small, angular (i.e., straight-edged) spots with yellow haloes. (Photo courtesy of Michelle Grabowski, University of Minnesota Extension)

How can I prevent bacterial spot in the future?

Plant pathogen-free seed or transplants to prevent the introduction of bacterial spot pathogens on contaminated seed or seedlings.  If a clean seed source is not available or you suspect that your seed is contaminated, soak seeds in water at 122°F for 25 min. to kill the pathogens.  To keep leaves dry and to prevent the spread of the pathogens, avoid overhead watering (e.g., with a wand or sprinkler) of established plants and instead use a drip-tape or soaker-hose.  Also to prevent spread, DO NOT handle plants when they are wet (e.g., from dew) and routinely sterilize tools with either 10% bleach solution or (better) 70% alcohol (e.g., rubbing alcohol).  Where bacterial spot has been a recurring problem, consider using preventative applications of copper-based products registered for use on tomato, especially during warm, wet periods.  Keep in mind however, that if used excessively or for prolonged periods, copper may no longer control the disease.  Be sure to read and follow all label instructions of the product that you select to ensure that you use it in the safest and most effective manner possible.  Burn, bury or hot compost tomato debris at the end of the season.  Wait at least one year before planting tomatoes in a given location again, and remove and burn, bury or hot compost any volunteer tomatoes that come up in your garden.

For more information on bacterial spot of tomato:  Contact your county Extension agent.

Bacterial Speck of Tomato

What is bacterial speck?  Bacterial speck is a common disease of tomato that occurs worldwide wherever tomatoes are grown.  The disease can substantially reduce yield when it severely affects leaves early in the growing season.  The disease can have an even greater impact on quality (and marketability for commercial tomato producers) when symptoms occur on tomato fruit.

Small, brown/black spots on a green tomato characteristic of bacterial speck. (Photo courtesy of S. T. Koike)
Small, brown/black spots on a green tomato characteristic of bacterial speck. (Photo courtesy of S. T. Koike)

What does bacterial speck look like?  Leaf symptoms of bacterial speck consist of small black spots (approximately ⅛ to ¼ inch in diameter) that often are more prominent on the undersides of leaves.  As the spots age, a yellow halo may develop around the edge.  Spots on fruit are very small (almost pinpoint-like) and do not penetrate very deeply into the tissue.  The spots can be raised, flat or sunken, and range in color from brown to black.  On unripe, green fruits, the spots often have darker green haloes, while on ripe fruits the spots can have subtle, yellow haloes.  Leaf symptoms of bacterial speck can be hard to distinguish from other tomato diseases.  Bacterial spot, (see University of Wisconsin Garden Facts XHT1244, “Bacterial Spot of Tomato”) and tomato spotted wilt (a viral disease) may cause similar leaf symptoms.  Laboratory testing may be needed to determine which disease is affecting your tomatoes.

Where does bacterial speck come fromBacterial speck of tomato is caused by the bacterium Pseudomonas syringae pv. tomato.  The bacterium is typically brought into a garden on contaminated tomato seeds or transplants, and thrives in cool (63oF to 75oF), wet weather.  It can be moved from plant to plant via splashing water (e.g., from rain or overhead watering with a sprinkler) or on hands and gardening tools when working with contaminated, and then healthy plants.  The bacterium can overwinter in dead tomato debris or on porous materials such as wooden plant stakes or trellises.

How do I save tomatoes with bacterial speck?  Once tomatoes are infected, there is no cure.  You may be tempted to cut off affected leaves as symptoms develop, but this will likely not do much to minimize or slow disease development, and may actually promote spread of the pathogen.  Often the best course of action is to allow the disease to run its course and simply salvage any unblemished fruit as they ripen over the summer.  DO NOT eat symptomatic fruit.  Although the bacterial speck pathogen is not a human pathogen, the fruit blemishes that it causes can provide entry points for human pathogens that could cause illness.

On tomato leaves, bacterial speck leads to small, angular (i.e., straight-edged) spots with yellow haloes. (Photo courtesy of Alan Collmer, Cornell University)
On tomato leaves, bacterial speck leads to small, angular (i.e., straight-edged) spots with yellow haloes. (Photo courtesy of Alan Collmer, Cornell University)

How can I prevent bacterial speck in the future?  Start by using high quality, pathogen-free seed or transplants from a reputable seed supplier or garden center.  If you have seed that you believe is contaminated with the bacterial speck bacterium and would still like to use it (e.g., it’s a favorite variety with difficult-to-find seed), consider treating the seed in hot water prior to planting to eliminate the pathogen.  Treat seed with 122°F water for 25 minutes.

To prevent spread of the bacterial speck pathogen from plant to plant in your garden, DO NOT use a sprinkler to water; instead use a soaker or drip hose to water at the bases of plants.  Also, only work with tomato plants when they are dry and consider routinely disinfecting garden tools with 10% bleach or (better) 70% alcohol (e.g., rubbing alcohol).  Spray disinfectants that contain approximately 70% alcohol can also be used for this purpose.

If you have a problem with bacterial speck, remove contaminated tomato debris from your garden at the end of the growing season.  This material can be deep buried, burned (where allowed by local ordinance) or hot composted.  DO NOT replant tomatoes in the same area the following growing season; instead grow a nonsusceptible vegetable crop.  This approach is referred to as non-host crop rotation.  For more information on this technique, see University of Wisconsin Garden Facts XHT1210, “Using Crop Rotation in the Home Vegetable Garden”.

As a last resort, consider chemical treatments for bacterial speck control.  If you decide to go this route, use a product that is labeled for use on tomatoes and that contains copper as the active ingredient.  To be most effective, the first treatment must be applied before symptoms have developed.  Apply additional treatments every 10 to 14 days as long as cool, moist conditions continue.  Keep in mind however, that if used excessively or for prolonged periods, copper may no longer control the disease.  Be sure to read and follow all label instructions on the product that you select to make sure you use it in the safest, most effective manner possible.

For more information on bacterial speck of tomato:  Contact your county Extension agent.