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Bacteria

Bacterial rot : Genus erwinia

Erwinia carotovora is the main bacterial disease of cyclamen.

The symptoms of its onset are a sudden rotting of the tuber, which gives off a fetidsmell, and a drooping of the leaves.

The bacterium is present in most soils, but it is fostered by too much warmth, too much fertiliser, and the use of too heavy a growing medium which makes for too much humidity.

Prevention is the only countermeasure.

> Bacteria of the genus erwinia : bacterial rot in cyclamen tubers

Bacteria are single-celled living organisms about a micron in size (1µm). They reproduce asexually, by cell division giving two daughter cells genetically identical with the mother cell.

Erwinia carotovora and E. chrysanthemi are bacteria that cause disease in plants, and both attack cyclamen.

They belong to the family Enterobacteriaceae in the order Eubacteriales, one of two in the group of True Bacteria; and take the form of small straight rods capable of moving around by means of flagellae which they have on every surface. They are able to live in both aerobic (oxygen-containing) and anaerobic conditions, provided there is free water, and they multiply rapidly. They are opportunistic parasites, developing as a disease in any situation of plant weakness, however caused; and conditions which stress the plant are favourable to the proliferation of the bacteria.

> Symptoms in cyclamen of disease due to E.carotovora

E.carotovora is an agent of Soft Bacterial Rot. This is a wet rot and is found on fleshy organs such as cyclamen tubers. This bacterium belongs to the group of Erwinia known as pectin-dissolvers: it produces enzymes which have a high potency for lysis of pectin and of cell walls: they can dissolve the middle membrane separating cell from cell, which is what maintains the cells’ coherence and rigidity. The pectins composing this membrane are hydrolysed by these enzymes into simple compounds which the bacteria can assimilate. Cells are broken down in this way, and plant tissue disintegrates, resulting in soft rot (tissue disorganisation) and a nauseous smell from the affected matter.

The bacterium is particularly apt to develop in the parenchyma.

 

The first visible symptoms are in the upper parts of the plant; while the interior of the tuber is going brown (and it is this which will eventually rot, giving off an unpleasant, fetid smell), the upper parts wither and droop abruptly onto the pot. Roots also are attacked. The leafstalk becomes soft; and oily-looking patches can be seen where this stalk joins the leaf. Sometimes the leaves yellow before wilting. The onset is very rapid, sometimes occurring overnight; and one can only diagnose affected plants at the highly advanced stage when the soft and malodorous tuber has become squashy to the touch. Occasionally a whitish mucus may be seen on the tuber’s surface. Eventually the whole plant is infected as the disease travels upwards, and the transport of water and nutrients is blocked.

> How the bacterium gets into the plant

The bacterium can only get into the plant by means of natural orifices (stomata) or wounds on the plant’s surface: it has no special organ for mechanically forcing an entry, nor any enzyme which can break down the waxy epidermis.

In the case of cyclamen, access is essentially through wounds and cracks in the tuber and at the sites where leaves or buds have been removed. Such sites are particularly numerous after re-potting, and infection often occurs a few days after this operation. Bacteria can survive in the soil and in plant debris. Once inside the plant it invades its food reserves as well as spreading throughout the plant through the vascular system.

> Sources of infection

Bacteria that cause disease in plants do not have resistant forms (spores) for endurance and dispersion: their development requires a medium of vegetable matter (a plant, or plant debris). Most often infection moves from plant to plant: the bacterium spreads like a stain, or by an infected plant dripping on another.

The bacteria can also survive in water and in growing mediums. The germs can only come from infected plants introducing the disease into the establishment.

As the tuber disintegrates, partly or wholly, billions of bacteria are released into the soil and washed along by watering.

The bacterium multiplies more readily at high temperatures (between 25ºC (77ºF) and 30ºC (86ºF)) and high humidity.

> Another bacterium which infects cyclamen: Erwinia chrysanthemi pv. chrysanthemi

This bacterium also causes wet rot of tubers and on occasion even the wilting of the plant. It also is a pectin-dissolving bacterium.

The bacterium attacks mainly the vascular system, and it is this which causes the wilting which accompanies the soft rotting of the tubers.

Moreover, if the tuber disintegrates completely the bacterium is capable of spreading a considerable distance around the infected pot, above all when the type of surface favours it (e.g. moist mat surfaces).

> Possible mistaken diagnosis as another disease

Erwinia attack can be mistaken for fungal disease:

  • of the Pythiaceae family (Pythium, Phytophtora)
  • Fusarium oxysporum f.sp. cyclaminis. In this instance the tuber stays hard, and in a horizontal cross-section, the plant’s vessels are black because they are blocked by the fungus, and the leaves are sulphur-yellow.
  • Botrytis, especially in dry atmospheric conditions where the damaged parts lose their watery look.

> Countermeasures

Erwinia disease is frequently the result of defects in management such as temperature variations, planting too deep in the pot, or too much nitrogenous fertiliser.

There is currently no chemical means of combating the breakdown of tubers once this is under way.

There are no antibiotics authorised for agricultural use. Prevention is the only means of defence.

The main defensive techniques consist of the creation and maintenance of optimal growth conditions for the cyclamen plants. It is often stress on the plants which encourages bacterial attack. It is necessary therefore:

 

  • to ensure strong vigorous plants by arranging for plenty of light and ventilation
  • to avoid excessive humidity, and temperatures either too high (over 25ºC - 77ºF) or too low (below 10ºC - 50ºF)
  • to avoid irregular watering (over-watering, drought)
  • to avoid too peaty a growing medium (ensure a well aerated soil) as well as excessive nitrogen content or large doses of fertiliser all at once
  • not to mist-spray as a means of lowering temperature
  • to eliminate infected or suspect plants
  • to avoid over-acid soil (pH < 5.5)
  • to avoid sudden variations in pH
  • not to cover the tuber with compost at re-potting time
  • to watch out for infected drainage water carrying infection from pot to pot (take care with subirrigation and gutter irrigation)
  • not to reuse either the earthenware pots or the compost used by infected plants; disinfection is not easy to achieve.
  • to disinfect materials and tools used, with alcohol or dilute bleach
  • to install foot-baths at strategic positions

 

Bacteria can on occasion be transported by insect vectors (a small coprophagous fly in the heart of the plant, for instance), by nematodes, glasshouse pests and humans. Pest eradication can make an important contribution to control of this disease.



Caution

This advice sheet is based on the methods used at the SCEA at Montourey (Fréjus, France). These procedures may need some modification to adapt them to other climatic situations. Before starting to grow cyclamen there needs to be a review of precautions against pests and diseases.   We must point out that our advice and suggestions are offered for information purposes and therefore cannot include any guarantee of specific results; it is a good idea to carry out trials beforehand.

 

Bacteria :

S.A.S Morel Diffusion

2565, rue de Montourey
83600 Fréjus - France

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Switchboard : + 33 (0)4 94 19 73 00
Fax : +33 (0)4 94 19 73 19

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