Bacterial Disease

Like us, plants are subject to many bacterial diseases.


Bacteria are found everywhere on earth. There are bacteria that live on the edges of volcanoes and bacteria that live in jet fuel. (How’s that for resilience?) Luckily for us (and our plants), most bacteria are beneficial. Even as you read, there are millions of bacteria living on and in you, helping you to be healthier. Plants have beneficial bacteria, too. Legumes, for example, have a relationship with Rhizome bacterium that help them to convert atmospheric nitrogen into a form they can use for food. Bacteria can also disfigure, dwarf, and even destroy many of your plants.


Pests and diseases are estimated to reduce crop and garden yield by 10 to 25%, depending on who you ask. That’s a big loss, considering how labor intensive gardening and farming are. Plant diseases are caused by bacteria, viruses, and fungi. You may be surprised to learn that fungi are responsible for 85% of all plant diseases. That being said, there are approximately 100 species of bacteria that cause trouble for plant growers. Most of these diseases are more commonly seen in the Tropics, but the bacterial diseases we do see are certainly worth learning about. But, first, what exactly are bacteria?


What are bacteria?

Bacteria are tiny one-celled beings, without a clearly defined nucleus, that can reproduce rapidly by simple cell division. There are two categories of bacteria: Gram negative and Gram positive. This is from a lab test called the Gram stain. When tested, the cell walls of one family of bacteria will stain purple (Gram positive), while the other family stains red or pink (Gram negative). Out of these two categories, countless types of bacteria emerge. They come in several shapes. They can be rods, spirals, spheres, or filamentous (“whiptails”). All bacteria go through the same basic life cycle phases:

1. lag phase - also known as epiphytic or resident phase; populations become established on top of healthy tissue
2. log phase - also known as pathogenic or endophytic phase; populations explode within tissue, causing costly spots, cankers, and lesions
3. stationary phase - populations stabilize as resources equalize death and division rates, or as external limiting factors kick in
4. death phase - resources are used up, or temperature extremes occur, and the bacteria begin to die

The study of bacteria as pathogens

The first bacterial plant disease ever identified was fireblight (Erwinia amylovora). This happened sometime around 1877, just after anthrax, the first bacteria to be identified. Since that time, much has been learned about how these one-celled creatures help and harm our food crops and ornamental plants. But, to cause harm, they must first gain entry.

Apple with fireblight (Kate Russell)

How bacteria enter plants

Unlike viruses, which inject genetic material to reprogram cells to make more viruses, nearly all bacteria grow in the spaces between plant cells, working to destroy the cell wall and consuming its contents. [One species, Agrobacterium, comes close to crossing the line between viruses and bacteria by genetically modifying their hosts to cause cancer-like growths called crown galls.]


Most bacterial diseases are transmitted by sap-sucking insects. Aphids, leafhoppers, nematodes, and psyllids are the most common disease vectors. Once these pests are infected with a bacteria, they carry disease to every plant they visit! As these pests feed, bacteria are inserted into the plant tissue.


Bacteria can also enter plants through natural openings, such as stoma, and through injuries caused by rubbing, thorns, or wind damage. Onion maggot, cabbage maggot, caterpillar, cricket, slug, and other pest feeding damage can also open the way for bacteria. Other physiological conditions that provide entry for bacteria include citrus fruit split, mummies, and your very own pruning shears!


One bacteria, Pseudomonas syringae pv. tomato, the bacteria responsible for bacterial speck, causes massive losses each year. Researchers at Virginia Tech discovered that this pathogen has figured out how to drop the bits of its own genetic information that were used by tomatoes to recognize it as a threat!


How bacteria damage plants

Bacteria have five basic methods of breaking through cell walls and consuming host plants:

• enzymes - chemicals that dissolve the cell wall and its contents
• toxins - these chemicals can be host specific or more generalized, and they act similarly to enzymes
• effector proteins - secretions that suppress the host’s defense processes by interfering with internal signaling mechanisms or reducing phytochemical production (Wikipedia)
• phytohormones - some bacteria alter a plant’s auxin levels to generate tumors within the plant
• exopolysaccharides - material produced by some bacteria, that blocks the xylem, killing the plant

Once inside, most bacteria grow between plant cells, in an area called the apoplast, or within a plant’s vascular system. As vascular bacteria reproduce, they clog either the xylem or the phloem, depending on the specific pathogen. If you’ve been following The Daily Garden, you will realize that clogging the xylem blocks the flow of water up from the ground, while interfering with the phloem starves the plant by limiting access to sugars produced by photosynthesis. Do you think the symptoms would be different? How so? Tell us your thoughts in the Comments!


​Bacterial families

Back in medieval times, it was believed that bathing would remove a protective coating that saved us from the Evil Spirits that caused death and disease. [Thank goodness those days are over!] In the same way, scientific research is changing the way we look at many bacteria. Some have been reclassified due to more accurate information. At this point, the most destructive plant bacteria fall into one of these families:

• Agrobacterium - crown gall in peach and nectarine, walnut, flowers and many perennial, woody plants
• Clavibacter- bacterial canker of tomato
• Erwinia - fireblight in apple and pear
• Pectobacterium - bacterial soft rot in potato, onion and garlic, and many other crops
• Pseudomonas syringae - bacterial blast, blight, and canker in tomatoes, stone fruits and many other tree crops
• Xanthomonas - black rot of crucifers and other crops

Acidovorax, Burkholderia, Pantoea, Ralstonia, Streptomyces, and Xyella are bacterial families that cause problems in other regions. To learn which bacterial diseases are likely in your area, contact your local Cooperative Extension Office. Two other unique bacterial families are fastidious vascular bacteria and phytoplasmas.


Fastidious vascular bacteria

Fastidious vascular bacteria were not properly identified until 1967. Up to that point, it was believed that many of the diseases they cause were actually the result of viruses. Two of those diseases are found in California. They are Pierce’s disease, which attacks grapes, and almond leaf scorch. These bacteria clog the vascular systems of plants.


​Phytoplasmas

This group of bacteria cause plants to produce more axillary buds, creating a bushier appearance, and weakening the plant in the process. Phytoplasmas are used by many commercial greenhouses to create plants with lush, thick growth. These bacteria also cause vivipary in strawberries. The same group of bacteria, carried by leafhoppers, cause corn stunt, cherry X-disease, and aster yellows in lettuce, celery, and other plants.

Black rot on cabbage leaf (David B. Langston, USDA Forest Service) CC BY 3.0

Symptoms of bacterial infection

Bacterial infection can be difficult to see, at first. Water-soaked lesions or bacterial ooze are often the first signs. Traditional symptoms of bacterial disease include leaf and blossom spots with yellow halos, cankers, galls, soft rots, or the classic shepherd’s crook stem ends of fireblight, and wilting. Symptoms vary, depending on the type of bacteria involved. Also, since bacteria are alive, they are constantly evolving to get the better of their hosts, pushing plants to evolve, which then pushes the bacteria, and so on.


Conditions that promote bacterial disease

Most bacterial diseases are seen in late April and early May in the Bay Area. This is because temperatures are rising, insects are active, there is enough moisture, and new, vulnerable buds and leaves are emerging. Most bacterial diseases prefer temperatures between 55°F and 85°F, with a high relative humidity.


Preventing and controlling bacterial disease

Bacterial disease is very difficult to control once it gets a foothold. Your garden plants will be far better off if you can prevent it in the first place. The following good cultural practices can make a big difference in the number of bacterial diseases affecting your plants:

• use certified clean seeds and seedlings
• quarantine and harden off new plants
• plant resistant varieties
• plant at appropriate times
• use recommended plant density
• monitor plants regularly for signs of infection
• provide adequate air flow through proper pruning
• irrigate properly, allowing soil to dry out between waterings
• crop rotation, to reduce overwintering
• remove weeds
• avoid working crops when they are wet
• isolate and destroy infected plants
• clean and disinfect tools regularly
• apply least damaging treatments, such as fixed copper or Bordeaux mixture, and only when necessary

Of bees, disease, and marigolds

Many garden resources point to honey bees and marigolds as protection from bacterial disease. There is even some measure of truth to these claims. Initial research, published in the Journal of Food Engineering suggests that the pollen bees carry with them may have antibacterial properties. Other research, from the University of Florida, shows that bees are the major vector of fireblight and southern bacterial wilt. As they move from flower to flower, they carry disease-causing bacteria with them. Marigolds, however, have been shown to provide protection against nematodes. Research conducted at the University of Florida found that planting a cover crop of marigold (Tagetes patula or T. erecta) can reduce nematode populations. Before you start planting, however, it is important to identify the specific nematodes in your soil so that you can select the correct marigold variety to get the desired results.


If you suspect bacterial disease in your garden, take a closer look and monitor regularly. Very often, you can break the disease triangle once you know what is growing on and in your plants.