by Matt Gibson
About Bacterial Spot
Bacterial spot is among the most devastating tomato diseases due to its ability to spread quickly and its resistance to control methods. Caused by four species of Xanthomonas, bacterial spot occurs all over the world wherever tomatoes are cultivated.
The disease causes leaf and fruit spots as well as defoliation, which leads to sun-scalded fruit and crop loss. Because of the diversity between the bacterial spot pathogens, the disease can occur at various temperatures, but is generally favored by temperatures in the range of 75 to 86 degrees F, as well as high precipitation.
It’s important not to eat fruit or vegetables that are infected with bacterial spot. Although people can’t catch bacterial spot, the lesions provide an opening for all kinds of pathogens that we can catch to enter the produce. That’s why you unfortunately must dispose of all infected crops by burning them or burying them so that it does not spread.
Causes And Symptoms of Bacterial Spot
Bacterial spot is the result of the bacteria xanthomonas campestris pv. vesicatoria, which lives on infected tomato seeds, peppers, and specific trees and weeds. It can be spread through irrigation, rain, or wet plants, and plants can be infected through their pores or through wounds. Tomato plants are especially susceptible to bacterial spot in warm, moist weather conditions.
The first signs of bacterial spot infestation is the appearance of small, irregular, oily marks on the underside of the tomato plant’s leaves. The lesions start as dark green spots and gradually shift to purple and eventually turn grey with black centers, sometimes with a white or yellowish outer ring. Bacterial spot symptoms also include thin and cracked leaf tissue and possible defoliation. The thinning and eventual disappearance of the leaves result in damaged and sun-scalded fruit. The spots that appear on the tomatoes themselves are small brown bumpy lesions that sink in as the fruit matures, making the fruit appear scabbed.
Treatment and Control of Bacterial Spot
One of the best ways to avoid bringing bacterial spot into the garden is to purchase certified disease-free tomato seeds. Also, gardeners should always use either a sterilized soil medium or one that is commercially made. If purchasing disease-free tomato seeds is not an option, it is important to try to sterilize the seeds as best you can, eliminating any bacteria that may exist on the seed’s surface and interior. To do this, fully submerge your tomato seeds in 1.3% sodium hypochlorite for one minute. A slightly more risky technique exists which could affect germination but helps to sterilize both the outer surface and interior of the tomato seed by submerging the seeds in 122 degree F water for 25 minutes.
Practicing crop rotation will help lessen the spread of bacterial spot. Because moist conditions and humidity attract bacterial spot, watering should be done in the early morning hours to allow plants plenty of time to dry out before the afternoon heat comes along. Gardeners should also be careful not to give their tomato plants too much water, and should try to keep leaves dry when watering their plants, focusing on watering the soil instead of the plant itself. Drip irrigation is recommended over standard overhead watering techniques.
Aside from the cultural control preventative methods, copper fungicides that are designed to counter tomato diseases are highly recommended. Copper fungicides will deny bacterial spot growth for two to four weeks after application. Use this type of fungicide after planting the seeds but before moving the plants into the garden or field.
Copper sprays are most effective when used before any signs of the disease are present as a preventative treatment. Sprays should be applied before and after rains but not during downfall. If bacterial spots are noticed, spray the plants for seven to 10 days, and again the day before transplanting. Once placed in the garden or field, plants with bacterial spot should be sprayed for an additional week after being planted in the ground, every five to seven days when weather conditions are wet, and every ten days when the weather is dry.
Commonly Asked Questions About Bacterial Spot
Can you eat tomatoes with bacterial spot?
It is not safe to eat tomatoes with bacterial spot, and doing so could make you sick. Although humans can’t catch bacterial spot from tomatoes, the bacterial spot lesions on the tomato provide a convenient spot for other pathogens that humans can contract to make their way inside the fruit. Dispose of infected tomatoes properly: burn them, bury them, or discard them in hot compost. (You can learn about the difference between hot, cold, and warm composting in our article on the topic if you aren’t sure.)
How do you treat bacterial leaf spots?
Copper fungicides are the most commonly recommended treatment for bacterial leaf spot. Use copper fungicide as a preventive measure after you’ve planted your seeds but before you’ve moved the plants into their permanent homes. You can use copper fungicide spray before or after a rain, but don’t treat with copper fungicide while it is raining. If you’re seeing signs of bacterial leaf spot, spray with copper fungicide for a seven- to 10-day period, then spray again for one week after plants are moved into the field. Perform maintenance treatments every 10 days in dry weather and every five to seven days in rainy weather.
An ounce of prevention is worth a pound of cure, so in addition to preventive copper fungicide treatments, always use certified disease-free seeds and either sterilize your soil or use commercial soils. If your seeds are not sterile, you can sterilize them yourself by soaking the seeds in 1.3% sodium hypochlorite for one minute.
Crop rotation is another best practice to help gardeners prevent bacterial leaf spot. You can read all about crop rotation in our article on the topic. Finally, avoiding too much moisture and humidity is another way to prevent bacterial leaf spot. Water in the morning instead of later in the day, and if you can, use drip irrigation or water the base of plants, as watering from above can allow moisture to collect on the foliage.
Is leaf spot contagious?
Yes, leaf spot is extremely contagious to other plants in your garden. That’s why it’s imperative to prevent bacterial leaf spot and treat for it as soon as symptoms appear. Although leaf spot itself isn’t contagious to humans, it provides a handy access point for pathogens humans can catch to infect the crops, so you also must discard any infected fruit or vegetables by burning, burying, or including in hot compost. Do not eat produce that shows signs of bacterial leaf spot.
What causes bacterial leaf spot?
The culprit behind bacterial leaf spot is a bacterium called xanthomonas campestris pv. Vesicatoria. It lives on tomato seeds as well as certain weeds. Bacterial leaf spot can be spread through water via rainfall, irrigation, or moisture that collects on plants, and the bacteria enter plants and their fruit or vegetables through openings such as pores and nicks, cuts, or other injuries.
What is bacterial leaf spot?
Bacterial leaf spot is a plant disease that impacts tomatoes, peppers, trees, and certain weeds. It is caused by the bacterium xanthomonas campestris pv. Vesicatoria and spread by rainfall, irrigation, moisture on plant foliage. The bacteria enter plants and their fruit or vegetables through pores and cuts or other injuries. Crops infected with bacterial leaf spot must be discarded by burying, burning, or including in hot compost.
How do you get rid of leaf spots?
Once leaf spot has infected your plants, spray with copper fungicide for seven to 10 days. After that, continue to treat every 10 days when weather is dry or every five to seven days when weather is wet. Copper fungicide can also be used preventively after sowing seeds but before moving plants into the garden.
Preventive treatments are recommended, as the loss resulting from a bacterial leaf spot infection can be devastating. In addition to preventive copper fungicide treatments, gardeners should ensure their seeds are certified disease-free and soil is sterile, whether you sterilize your own soil or purchase commercial soils. If seeds aren’t sterile, soak them in 1.3% sodium hypochlorite for one minute to sterilize them on your own. Crop rotation and avoiding too-wet conditions are other strategies to prevent leaf spot. Opt for drip irrigation, or water plants at their base instead of from overhead, and do your watering in the morning instead of later in the day.
How does leaf spot spread?
The bacteria behind leaf spot spread through water, whether via irrigation, rainfall, or water droplets on plants. Then the bacteria enter plant cells or crops through their pores or openings that result from injury (like nicks or cuts).
What’s the difference between bacterial spot versus bacterial speck versus bacterial canker on tomatoes?
All three of these bacterial diseases cause spots on the fruit of the tomato plant. It is very hard to differentiate between bacterial spot and bacterial speck based on the spots that occur on the leaves of the plant, and both can cause spots to occur on all above ground plant parts. Fortunately, figuring out which one is affecting your plants is not too important because both bacterial diseases require the same control methods.
Bacterial canker causes vascular discoloration and wilting and its effect on the plant is quite different than spot or speck diseases. The main difference between the three are the pathogens that infect the tomato plant. Bacterial spot is caused by Xanthamonas campestris pv. vesicatoria. Bacterial speck is caused by Pseudomonas syringae pv. tomato. Bacterial canker is caused by Clavibacter michiganensis subsp. Michagensis.
Visually, you can distinguish the three diseases with some careful observation. Bacterial speck causes dark spots similar to bacterial spot, but around half the size, specks are around 1/16 of an inch, while spots are ⅛ of an inch. Bacterial canker spots are the same size as bacterial spot (⅛”) but lesions usually appear on the tomato as bird’s eye spots, white spots with raised dark centers. For more detailed information on how to differentiate between these three diseases, check out the chart on this link.
We are looking for photos of bacterial spot on tomato plants? If you have bacterial spot on leaves or fruit and have pictures, we would appreciate you sending them our way.
Want to learn more about Bacterial Leaf Spot?
NC State Extension covers Bacterial Spot of Pepper and Tomato
PennState Extension covers Bacterial Spot of Tomato: Biology and Management
University of Minnesota Extension covers Bacterial Spot of Tomato and Pepper
Gardening Know How covers Bacterial Speck Identification and Tips For Control For Bacterial Speck On Tomato Plants
University of Wisconsin-Madison covers Bacterial Spot of Tomato
University of Illinois Extension covers Spots on Tomatoes