Bacterial Disease

Like us, plants are subject to many bacterial diseases.

Bacteria are found everywhere on earth. Some bacteria live on the edges of volcanoes, and others thrive in jet fuel. (How’s that for resilience?) Luckily for us (and our plants), most bacteria are beneficial. Even as you read, millions of bacteria live on and in you, helping you to be healthier.

Plants have beneficial bacteria, too. Legumes, for example, have a relationship with Rhizome bacterium that helps them convert atmospheric nitrogen  into a form they use for food. Bacteria can also disfigure, dwarf, and even destroy many of your plants. 

Most plant diseases are bacterial, viral, or fungal infections. Fungi are responsible for 85% of all plant diseases. That said, approximately 100 species of bacteria can cause trouble for plant growers.

What are bacteria?

Bacteria are tiny one-celled beings without a clearly defined nucleus that can reproduce rapidly by simple cell division. They come in several shapes. They can be rods, spirals, spheres, or filamentous (“whiptails”).

The first bacterial plant disease ever identified was fireblight (Erwinia amylovora). This discovery occurred around 1877, just after anthrax, the first bacteria identified. Since then, we have learned more 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 infected with fireblight

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, they carry disease to every plant they visit. As they feed, bacteria invade the plant tissue. 

Bacteria can also enter plants through natural openings, such as stomas and injuries caused by limbs rubbing together, thorns, or wind damage. Onion maggots, cabbage maggots, caterpillars, crickets, slugs, 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!

How smart can one cell be?

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

How bacteria damage plants

Once inside, most bacteria grow between plant cells, in the apoplast, or within a plant’s vascular system. As vascular bacteria reproduce, they clog the xylem, phloem, or both, depending on the specific pathogen. Clogging the xylem blocks water rising from the ground while interfering with the phloem blocks access to sugars produced by photosynthesis, starving the plant.

Bacterial families

During medieval times, people believed bathing removed 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. 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, garlic, and many other crops
• Pseudomonas—bacterial blast, blight,, and canker in tomatoes, stone fruits, and many other tree crops
• Xanthomonas--black rot of crucifers and other crops

You will often see these words included in the names of the diseases they cause. Two other unique bacterial families are fastidious vascular bacteria and phytoplasmas.

Fastidious vascular bacteria

Until 1967, diseases caused by fastidious vascular bacteria were considered viral. Pierce’s disease, which attacks grapes, and almond leaf scorch are two diseases that fall into that category.

Phytoplasmas

This group of bacteria causes plants to produce more axillary buds, creating a bushier appearance and weakening the plant. Commercial greenhouses use phytoplasmas to generate 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 infections can be difficult to see at first. Water-soaked lesions or bacterial ooze are often the first signs. Other symptoms include leaf and blossom spots with yellow halos, cankers, galls, soft rots, the classic shepherd’s crook stem ends of fireblight, and wilting. Symptoms vary depending on the type of bacteria involved. 

Since bacteria are alive, they are constantly evolving to get the better of their hosts, pushing plants to develop better defenses, which motivates the bacteria to get better at what they do, and so on.

Conditions that promote bacterial disease

Most bacterial diseases occur in spring 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 diseases are difficult to control once they occur. Your garden plants will be far better off if you can prevent these diseases in the first place. The following good cultural practices can make a big difference in the number of diseases infecting your plants:

• Use certified pest- and disease-free seeds and seedlings.
• Quarantine and harden off new plants.
• Plant resistant varieties.
• Plant at appropriate times. 
• Use recommended plant densities.
• Monitor plants regularly for signs of infection.
• Provide adequate airflow through proper pruning.
• Irrigate plants properly, allowing the soil to dry out between waterings.
• Employ 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, 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 primary vectors of fireblight and southern bacterial wilt. As they move from flower to flower, they carry disease-causing bacteria. 

Marigolds provide some protection against nematodes. Research conducted at the University of Florida found that planting a cover crop of marigolds (Tagetes patula or T. erecta) can reduce nematode populations. Before you start, however, you must identify the specific nematodes in your soil to select the correct marigold variety to get the desired results. It makes a difference.

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