Plant Bacteriosis: How to Treat and Prevent It?

Plant Bacteria: The Good and the Bad

Bacteria are among the simplest living organisms. They are present in nature in almost incalculable numbers. Unicellular, measuring just a few thousandths of a millimetre, these bacteria are only visible under a microscope. For example, it is estimated that more than a billion bacteria colonize a single gram of soil. An astronomical figure explained by the dizzying growth of bacteria. Indeed, they multiply by simple cell division. Among these bacteria present in soil, air, or water, some play a fundamental role for plants, while others are particularly harmful, depending on the relationship they establish with them.

Legume roots with nodules where Rhizobium bacteria attach

Indeed, some bacteria are completely neutral to the surface or tissues of plants. There are saprophytic bacteria (feeding on dead organic matter), which include epiphytic or endophytic bacteria. Other bacteria establish a true symbiosis with plants, in a mutually beneficial relationship. This is the case of Rhizobium group bacteria, settled on legume roots, which fix atmospheric nitrogen to return it to plants and soil.

Some bacteria are very useful for maintaining soil fertility. Others are multiplied and intentionally introduced in biological control against parasites. This is the case of the bacterium Bacillus thuringiensis (Bt), naturally present in the soil, which has been synthesized for use as a biological insecticide against larvae of certain lepidopterans.

Finally, there are phytopathogenic bacteria that parasitize plants, causing qualitative and quantitative damage. There are about a hundred of these bacteria, harmful to plants, a number significantly lower than that of phytopathogenic fungi. They are responsible for bacterial diseases, called bacterioses, but their importance in plants is less than fungal diseases.

To go further:

• Bacillus thuringiensis: a natural insecticide
• How to improve your vegetable garden soil with legumes?

How do bacteria develop and spread in the garden?

These bacteria, whether good or bad, which can take the form of shells, rods (bacilli), or spirals (spirilla), live and persist in the soil, in plant debris from cultivation or pruning, on seeds and seedlings, on buds and foliage, on gardening tools, or even in the water from water collectors. In case of extreme heat or cold (late frosts affecting plants), or in high humidity, resistant forms can appear and multiply, in the form of spores.

Then, comes the dissemination which occurs in various ways. Either it is carried by the wind over a fairly long distance, but weather conditions and insects such as aphids or scale insects also allow bacteria to travel. Just like the use of infected tools or the human transportation of plants, also infected.

Once they have reached their destination, bacteria penetrate the plant tissues through the stomata (pores) of the leaves or the lenticels of the roots. More likely, it is through wounds, pruning cuts, or the bites of sucking insects that bacteria infest plants. A plant weakened by multiple factors is logically more vulnerable than a healthy and vigorous plant.

Leaves affected by bacterial leaf scorch[/caption>

As soon as the bacteria have invaded the plant tissues, various symptoms appear: wet spots (grease), necroses, cankers, galls or tumors, wilting, browning and/or scorching of foliage, rot, gum oozing, mosaics… These symptoms may be similar to those of fungal diseases, which sometimes makes diagnosis difficult.

What are the most common bacterioses in young plants?

Depending on the bacteria involved, there are different types of bacterioses, more or less widespread depending on the regions and climatic conditions… As phytopathogenic bacteria are subdivided into different strains, pathovars, they can attack multiple species.

• Fire blight (Erwinia amylovora) : this is the most dangerous bacteriosis for fruit trees, bushes, and plants of the Rosaceae family, with pear trees being particularly sensitive. It can kill a tree in 8 to 15 months. This bacterial disease is subject to very strict legislation which, among other things, requires reporting any suspicious symptoms to the town hall. I invite you to read Eva’s article to learn all about this disease: Fire blight: identifying and combating this disease
• Bacterial canker (Pseudomonas syringae) : this condition has spread widely in recent years, mainly affecting fruit trees (cherry, peach, plum, apricot, kiwi), but also chestnut trees. The bacterium, naturally present on the leaves, develops in autumn and winter on the foliage, buds, and May clusters, through leaf scars, cracks, or bark wounds. In spring, symptoms appear: light green then brown spots on the leaves, black spots on the fruits, depressed areas (necroses) on the bark with gum oozing. By early summer, entire branches or the tree eventually die.
• Soft rot or bacterial rot (Pectobacterium carativora or Erwinia chrysanthemi): the first bacterium attacks the roots, stems, and fruits of cucurbits, solanaceae, lettuces, cabbages, celery… The second affects vegetable plants such as onions or potatoes, but also ornamental plants like carnations, chrysanthemums, or dahlias. In practice, the foliage wilts, browns, then liquefies and decomposes. It can emit an unpleasant odor.
• Bacterial blights (Pseudomonas syringae or Xanthomonas spp.) which mainly affect beans and peas, but also cabbages, artichokes, tomatoes, cucurbits. These bacteria are transmitted through seeds and develop when a humid and windy period is followed by warm and dry weather during flowering. Angular, oily, transparent spots with yellow edges appear. The leaves brown and die, then the plant perishes within a few days. On peas and beans, dark green spots bordered in red appear on the pods.
• Bacteriosis caused by Xylella fastidiosa: this is a worrying bacterium, recently arrived in Europe, which attacks a large number of species. I invite you to discover Olivier’s article: Xylella fastidiosa: a deadly bacterium for many plants!

Please note that this list is not exhaustive!

Olivier affected by the Xylella fastidiosa bacterium

How to fight against these bacterioses?

Unfortunately, it is very difficult to fight against these bacterial diseases, some of them such as fire blight being incurable. Few curative means are sufficiently effective. However, for certain infections like bacterial canker or bacterial rot, if detected early enough, it is possible to treat them 2 or 3 times, three weeks apart, with colourless Bordeaux mixture. But success is not guaranteed, and it should be noted that the copper in Bordeaux mixture persists in the soil and pollutes the water.

Similarly, it is essential to remove and destroy the contaminated parts, often by digging quite deeply into the affected bark areas, before applying a healing agent. Burning them is the best way to eliminate them, but it is prohibited in gardens. The only remaining option is the waste disposal site. Bacteria should normally be destroyed there thanks to the heat in the compost.

The best way to fight against these bacteria is to implement very strict prophylactic measures:

• Carefully remove all crop or pruning waste that remains on the ground
• Fertilize appropriately, and avoid fertilizers too rich in nitrogen
• Regularly maintain your plants (appropriate watering, mulching) to avoid stress
• Prune moderately and at the right time to thin out the branches, and apply a healing sealant on the cutting wounds
• Try not to wet the foliage when watering, as excessive moisture is a vector of proliferation
• Adhere to a strict crop rotation
• Combat transmission vector organisms, such as scale insects and aphids
• Carefully clean and disinfect gardening tools after each use
• Boost plant immune defenses with nettle liquid fertilizer.
  

  Fighting bacterioses involves prophylactic measures

Lastly, if your vegetable garden or orchard is frequently affected by bacterioses, make sure to select certified seeds or resistant varieties.