By: Bonnie L. Grant, Certified Urban Agriculturist
Many ornamental and edible plants display dark, necrotic looking spots on their leaves. Bacterial leaf spot on plants will discolor, and in extreme cases, kill leaves. Tiny, microscopic single celled organisms are what causes bacterial leaf spot. There are several methods on how to treat bacterial leaf spot and save your plant’s glorious leaves. Early identification is crucial to effective management of bacterial leaf spot disease.
Symptoms of Bacterial Leaf Spot
Bacterial leaf spot on plants may manifest in several different ways. Symptoms of bacterial leaf spot may include black edged lesions, brown spots with yellow halos, or just light and dark areas on the foliage. Spots are irregular and measure between 3/16 and ½ inch (0.5 to 1.5 cm.) wide. They can occur on the top or bottom of a leaf and kill sections of the tissue when they cluster together.
Symptoms of bacterial leaf spot may also appear on the edges of a leaf, where it appears brownish yellow and the tissue dries and breaks off. The leaves become quite papery and delicate when the bacterial disease attacks the leaf edges. The disease is most prevalent on older leaves, but will quickly establish on newer tissue.
What Causes Bacterial Leaf Spot?
Organisms that cannot be seen with the naked eye are the cause of this visibly damaging plant disease. Wet, cool conditions promote the formation of these bacteria, which can spread on plants quickly. The bacteria splash onto leaves or overwinter on plant debris in soil.
Bacteria divide to reproduce and one bacterium can multiply quickly in just a matter of hours. Bacteria reproduce the fastest when temperatures are 77-86 F. (25-30 C.). High rates of infection will cause leaf loss and can seriously compromise a plants health. This makes the disease extremely contagious and bacterial leaf spot disease treatment extremely important.
The pathogen is also carried in infected seed; however, there are some disease resistant seed strains for food crops. Additionally, choose disease free transplants, rotate crops, and avoid overhead watering to prevent spreading the bacteria.
How to Treat Bacterial Leaf Spot
In addition to the previous tips on preventing the spread of the disease, you can use a copper fungicide on crops. This has limited management use unless it is applied early in the disease cycle.
On ornamental plants, remove the affected leaves at the first sign to prevent the bacteria from jumping onto adjacent leaves. Some of the most common hosts are lettuce, beets, eggplant, peppers, and large leaved ornamental plants, such as philodendrons.
Remove old vegetable debris in the garden and do not plant new crops where host plants were once growing. There are no recognized chemical treatments for bacterial leaf spot disease. Your best bet is prevention and mechanical control at the first sign of symptoms of bacterial leaf spot.
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Read more about Plant Diseases
Bacterial Spot Control: How to Identify and Get Rid of Bacterial Spots
Bacterial spot is also known by other names, such as bacterial shot hole, bacterial leaf spot, or bacteriosis. The cause of the disease is Xanthomonas arboricola pv. Pruni. The bacterium was discovered in North America in as early as 1903. When you look under the microscope, you will see that the bacterium has a rod shape and is motile.
This disease can spread in a number of ways, but one of the most common is through infestation in its seed, as well as transplants that are already suffering from bacteria. It can affect the different parts of the plant, including twigs, leaves, and fruits. Aside from planting a seed with contamination, there are other ways by which it can multiply and spread to other hosts. Some of these ways include dew and wind. Overhead irrigation can make healthy plants susceptible to the disease if there are plants with bacterial spot nearby. Even blowing of the sand as a result of wind will spread infection. Wounds and natural openings in the plant will be ideal entry points for the bacteria. In terms of the external environment, an infection will be common in places with high moisture.
Bacterial Spots Can Affect the Different Parts of the Plant
Tomato Diseases: How To Fight Bacterial Spot
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?
University of Minnesota Extension covers Bacterial Spot of Tomato and Pepper
University of Wisconsin-Madison covers Bacterial Spot of Tomato
University of Illinois Extension covers Spots on Tomatoes
Zinnia-Bacterial Leaf Spot
Spots turn brown and angular and may have a yellow halo.
Spots begin as dull gray water-soaked areas on the leaf then turn brown and angular.
Cause Xanthomonas campestris pv. zinniae , a bacteria that can survive in dried zinnia leaves for as long as a year. They can reside on foliage for several months before initiating disease. The bacteria can also be seedborne contaminating the surface of the seed. Infected plant material is probably the most important source of contamination. Bacteria can be spread from plant to plant by splashing water from overhead or sprinkler irrigation. The disease spreads rapidly during warm, humid and rainy weather. Hot water soaks or antibiotics meant to clean the seed also reduced germination or were phytotoxic. There may be other experimental hosts but they have not been naturally infected.
Most common zinnia cultivars and Z. haageana are susceptible. The Crystal and Star series are highly resistant while the Profusion series are moderately resistant.
Symptoms Spots begin on lower leaves as dull gray water-soaked areas on the leaf. Numerous yellow or tan spots can develop across the leaf forming irregular dead areas. Often there is a yellow halo around individual lesions. Spots turn brown and angular which could then be confused with Alternaria leaf spot. Small brown spots may form on the ray flowers and if severe flower heads can be disfigured and completely decay.
Cultural control Practice strict sanitation and rigorous scouting to identify and remove infected plants in the early stages before the pathogen spreads.
- Start with pathogen-free seed. Seed can be treated with a dilute bleach solution (10%) for 15 to 30 minutes. Be sure to test a small batch of seed for sensitivity to these treatments.
- Plant resistant cultivars or species.
- Plants with leaf spots should be discarded. Diseased plant debris should be removed from the growing area.
- Workers should wash their hands after handling diseased plants or soil.
- Handling of wet foliage should be avoided.
- Minimize splashing when watering.
- Increase spacing between plants.
- Rotate plantings to different beds each year and choose open sunny sites.
Chemical control Bactericides are only marginally effective so focus on cultural control tactics.
- Badge X2 at 1.5 to 2 lb/A. Group M1 fungicide. 24-hr greenhouse reentry. O
- CuPRO 5000 at 1.5 to 5 lb/A. Group M1 fungicide. 48-hr reentry.
- EcoSwing at 1.5 to 2 pt/100 gal water. Group BM01 fungicide. 4-hr reentry. O
- Grotto at 0.5 to 2 gal/30 to 100 gal water/A. Group M1 fungicide. 4-hr reentry.
- Phyton 27 at 1.3 to 2.5 oz/10 gal water. Group M1 fungicide. 48-hr reentry.
- Bayer Advanced Natria Disease Control RTS ( Bacillus subtilis strain QST 7z13) is registered for the home garden. Active ingredient is a small protein. H O
- Cease or Rhapsody ( Bacillus subtilis strain QST 713) at 2 to 8 quarts/100 gal water. Active ingredient is a small protein. Efficacy in the Pacific Northwest is unknown although it was effective in one trial in Maryland. 4-hr reentry. O
- Stargus ( Bacillus amyloliquefacien s strain F727) at 2 to 4 qt/A plus a nonionic surfactant. Group BM02 fungicide. 4-hr reentry. O
- Triathlon BA ( Bacillus amyloliquefaciens strain D747) at 0.5 to 6 quarts/100 gal water. Group BM02 fungicide. 4-hr reentry. O
Features - Equipment & Technology
Check out our latest round-up of covering products in the marketplace.
SOLEXX TWIN-WALL HIGH-DENSITY POLYETHYLENE
The pitch: Developed to help reduce heating costs, it provides a durable, long-lasting cover and reduces plant loss due to leaf burn.
- Twin-wall, high-density polyethylene, two thicknesses: 3.5mm (2.1R factor), 5mm (2.3R factor)
- Available in 4- x 8-foot and 4- x 12-foot panels, and continuous rolls of 2-, 4- and 6-foot wide up to 900 feet long
- Year-round growers can see a return on investment in two to four years in fuel savings
- Durable Solexx prevents damage from high winds, hail, snow and driving rain
- Backed by a 10-year UV warranty
- Flexible to bend around corners and over peaks for a watertight seal
- Easily installs horizontally or vertically by overlapping and screwing into place
- Diffuses light to prevent leaf burn and create ideal growing conditions
For more: Farm Wholesale Ag, farmwholesaleag.com
The pitch: New to the market, it is an affordable sliding wall ventilation system that can be added and retrofitted to most greenhouse walls with polycarbonate cladding.
- Vertically sliding vent
- Can be operated with manual or motorized drive shaft
- Opening sidewall a few inches at the top in the early season creates a fast cooldown without affecting young plants
- Low or no maintenance required
- Can be operated in stages by most existing control systems for motorized units
- Offered with a low-cost 3 rpm drive motor and five-stage operating controlling for two sides
For more: DeCloet Greenhouse Manufacturing, decloetgreenhouse.com
The pitch: Reduces daytime heat buildup, lowering the greenhouse temperature by up to 10 degrees when used as the inside layer of a double-layer installation.
- High PAR light transmission for fall and winter production
- Two-directional drip control
- Nighttime infrared heat retention can help save up to 20 percent on heat costs
- Can be part of Two In One rolls in 20-foot and 24-foot sheeting combinations with K50 Clear
For more: Klerks Hyplast Inc., klerksusa.com
SUN MASTER IV – SULFUR RESISTANT (SR)
The pitch: Introduced last year, this co-extruded film product is UV stabilized for up to four years of film life.
- Film is 6 mil thick and made of three layers
- Provides strength, high-light transmission and better fold, tear and puncture strength
- PAR light transmission (400-700 nm) is 86 percent with a slight yellow tint
- Resistant to sulfur
For more: Lumite Inc., lumiteinc.com
The pitch: Offered in a variety of widths, lengths, thicknesses, profile structures, colors and glazings, the Policarb line consists of structural multi-wall polycarbonate sheets.
- Thickness options range from 4-25mm
- Anti-fog coating
- Resistance to UV rays and hail
- Applicable for vertical windows, roofing, curved roofing and ceilings
- Offers light transmission
- Saves energy
- Economical and versatile
For more: Gallina USA, gallinausa.com
The pitch: This shade coating diffuses light and reduces heat on greenhouse surfaces, including glass and hard and soft plastic glazing materials.
- Coating can be used on all greenhouse roofing materials, including glass, acrylic and poly
- Formulated to provide better uniform coverage of greenhouse surfaces
- Packaged in a concentrated form to be easily diluted
For more: Becker Underwood, beckerunderwood.com
The pitch: Designed as a glazing panel for commercial greenhouses where an extremely high light transmission is required.
- 16 mm double-skinned sheet
- Patented anti-condensation coatings
- Light transmission of up to 91 percent
- Provides an energy savings of up to 50 percent over conventional single-glazed products
- 30-year, non-yellowing guarantee
- 10-year hail warranty
- Available in clear UV Transmitting and clear UV Blocking
For more: Evonik Industries, acrylitebuildingproducts.com
The pitch: This reinforced transparent greenhouse cover blocks 90 percent of the sun’s UV radiation and saves energy when used inside the greenhouse.
- Withstands hail, downpour and wind
- Can be used for roll-up side walls both inside and outside, and doesn’t stretch
- Made of 100 percent polyolefin
- 0.3 mm thickness
For more: Svensson, svenssonglobal.com