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Fusarium oxysporum

Fusarium Wilt is a fearsome disease of cyclamen.

Fusarium oxysporum f.sp. cyclaminis mostly attacks plants after re-potting and at the start of flowering.

The quality and nature of the growing medium has an effect on its development.

As in the case of Phialophora, the affected plant shows symptoms of withering and first partial and then total yellowing of the foliage.

On cutting the tuber across, brown or reddish regions of cell death can be seen along its length. Any form of stress (nutritional deficiency, wounds, other infections) can predispose to this disease.

No chemical treatment is effective.

There is a form of biological control using a variant of the same fungus, which gives very encouraging results.

> Prevent Fusarium

The very warm summer periods are favorable for the development of Fusarium. 

Which are the risk factors

How to recognize the symptoms, to understand its propagation for better prevention of the disease…

To obtain simple and concrete answers, download our new disease factsheet: « Fusarium ».

 
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> Fusarium oxysporum : fusarium wilt

Fusarium Wilt is a fearsome disease of plant vascular tissue.

There is a fungus, Fusarium oxysporum, which lives in the soil and in most cases feeds on dead organic matter (saprophyte). However, it has particular strains or special forms which cause disease in plants and are highly specialised.

Fusarium oxysporum f.sp. cyclaminis is the one which causes vascular Fusarium Wilt in cyclamen. This soil fungus is specific to cyclamen.

This cryptogamous disease has been known in Europe since the 1930s, and first appeared in France in 1973.

 

Though the fungus is capable of attacking all stages of the cyclamen plant, the first symptoms usually only show on plants that are at least 3 to 4 months old. They concern foliage, tuber and roots.

> Identification of the fungus and its life cycle

Fusarium oxysporum is a fungus of the class Adelomycetes (Deuteromycetes - ‘Imperfect Fungi’). These are fungi with mycelial septa, which are not known to reproduce sexually. Within this class, Fusarium belongs to the order Moniliales (Hyphomycetales) and the family Tuberculariaceae.

Asexual reproduction involves the production of spores known as conidia in a budding-off process from specialised hyphal tips (mycelium), known as conidiophores. These are short single strands grouped on a kind of cushion. They give off either (multicellular) macroconidia or (single-celled) microconidia.

The fungus also produces chlamydospores, resistant forms. These are spores capable of cell division but surrounded by nutritive substances and covered with a thick protective coat, ensuring the parasite’s survival in the ground.

 

These fungi are imperfect forms of ones in the class Ascomycetes.

> How the parasite spreads

The fungus can keep quite well in the soil, sometimes for more than a year; it can also survive on other things such as pots, benches, irrigation equipment and pipework, tanks of nutrient solution and plant debris. It can live at great depth (100 to 150 cm) and withstands both drying out and waterlogging.

 

Fusarium oxysporum is not transmitted in seed.

The spores which are drought-resistant (chlamydopores) can be spread in the neighbouring air and in dust; they can also survive for up to a year in water or nutrient solution.

They are produced in massive numbers and can also be spread during handling of the plants or by watering. A one-inch stalk with the pink fruiting bodies of Fusarium on it contains over 20 million spores: 50,000 are quite enough to kill a plant in a one-litre pot.

Insects and mites also can spread spores that are found on the plants above ground.

 

Since young plants may show no immediate symptoms even though infected, they may be brought into the glasshouse and act as a source of new infection.

> Optimal conditions for spread of the fungus

The ideal temperature for development of this fungus is of the order of 28ºC (82ºF), though it can attack at lower temperatures. It therefore finds glasshouse temperatures very suitable.

> The process of contamination

The spores land on the growing medium, sometimes after being carried from elsewhere. They infect the plant through the roots, by way of wounds or natural openings. The fungus develops first in the vascular system of the tubers, and then reaches the sap channels throughout the plant.

The incubation period is from 2 to 13 weeks.

> Symptoms observed on cyclamen

This pathogen causes disease of the vascular system - the sap- conducting elements. Symptoms are not therefore immediately to be seen on the outside. As a rule these will not show until the infected plant is 3-4 months old. On the other hand, plants can be a highly dangerous source of infection to other, healthy ones, right from the onset of the disease.

Symptoms are most readily seen at the end of the vegetative stage, when the plant is beginning to send up flowers, during hot spells. The more severely the plant has been infected as a young plant, the faster the symptoms will develop (particularly after re-potting during spells of strong sunshine).

 

But the growth medium has an important part to play: not all soils are equally hospitable to fusarium wilt disease.

Once attacked, the plant is irretrievably doomed.

 

Early on in the infection process the disease generally affects only one side of the plant.

The leaves turn yellow at the edges and their laminae curl up. Discoloration also appears where the stalk joins on; and these patches grow bigger and more yellow.

The worst-affected leaves wilt and then turn brown; the leaves flop onto the edges of the pots as their stalks are no longer able to hold them up.

These symptoms appear at first only on the older leaves. At this stage it will not be possible to detect any irregularity in the root system or the tuber’s surface, even on close inspection.

Later on the symptoms spread to the entire plant:

  • all leaves turn soft and yellow
  • flowers soften and then turn brown

 

As the disease progresses all the leaves wither and perish; this can be a very swift process, especially in periods of strong sunshine.

Finally, the perishing of the foliage is sometimes accompanied by a damp soft rot and a pinkish-white furry mass of spores at the base of the stalks (foot rot).

 

If the underground organs are cut longitudinally at the onset of symptoms, Fusarium oxysporum contamination can be seen in the conducting vessels. At first they show specks of red/brown or brown/black. Cutting into roots or tubers like this shows the precise way in which the disease affects specifically the vascular system, and gives us a means of following the fungus from the vessels of the tuber to those of the leaf stalks. As the fungus spreads, the blackening extends to all the vessels. The outward manifestations in the plant above ground (cell death in the spongy tissue of the leaves) and in the tuber proceed in parallel.

 

This invasion extends to the vessels which carry the ascending sap. The ‘blockages’ formed by bacterial lumps, and obstructions caused by the gum-like substances that are the breakdown products of cell walls or absorbent substances produced by the pathogen itself, deprive the upper part of the plant of some of its water and nutrients; indeed the plant itself tries to check the advance of the fungus by blocking its own vessels: it develops tyloses, cell outgrowths formed next to the vessels of the xylem.

 

The first symptoms in the plant above ground appear as a result of these deficits; the speed at which these aboveground symptoms appear is a function of temperature, relative humidity and the massiveness of the attack.

 

 

Horizontal cut of tuber contaminated by Fusarium

 

 

Longitudinally cut of tuber contaminated by Fusarium

 

 

Horizontal cut of tuber of young plant contaminated by Fusarium

 

 

 

White fruiting bodies of Fusarium in the heart of the plant

 

 

Half of the plant contaminated by Fusarium at the beginning of flowering period

 

 

> Fungal reproduction

At the end of the cycle, when the fungus has invaded the spongy tissue or parenchyma of the leaves, it forms its fruiting bodies and produce its spores (conidia).

> Possible misdiagnosis

Fusarium attack may be mistaken for Erwinia carotovora, Bacterial Soft Rot. However, while the bacterium invades the whole of the tuber, Fusarium remains confined to the conducting vessels. The disease caused by Erwinia is not vascular; moreover it causes wet rotting of the stalks which then are very easy to detach from the tuber, while in the case of fusarium wilt these stalks remain attached. Bacterial attack is also indicated by a characteristic oily, brown and star-shaped patch on the leaves at the junction of the stalk.

It is not rare, however, for the two pathogens to be present simultaneously.

 

Fusarium wilt may also be confused with another disease of vascular tissue, caused by Phialophora cyclaminis. Telling the two apart is tricky: this fungus causes yellowing and deformation of leaves, and their blistered or cross-hatched appearance may be put down to Fusarium. On inspecting the tubers, however, the vascular attack of fusarium will show up less thick, but darker; while the necrosis due to Phialophora cyclaminis is present in many parts of the vascular tissue, small and distributed far and wide. You will never find it outside this vascular tissue, however long-established the infection.

In many situations it is hard to make the distinction in the field, and the diagnosis must be made in a laboratory.

> Countermeasures

Taking action against these highly specific forms of Fusarium oxysporum is always difficult. There is at present no chemical treatment which is effective after the fact; the plants must be protected, so far as possible, by good management and precautionary chemical measures.

 

Efforts to control Fusarium can be successful provided an integrated approach is adopted, one which combines the best management practices with a well-considered use of chemicals that takes account of biological control measures.

> Management measures

The following practices are aimed at denying the fungus access to the glasshouse in the first place. For this, it is necessary:

  • to be careful to keep the glasshouse clean
  • to maintain a soil pH which does not encourage the fungus, ideally 5.8
  • to choose compost with a good aerated structure, avoiding black peat and too heavy mediums
  • to use new soil or compost, new seed-trays and new trays for pricking out seedlings
  • to prevent plants coming into direct contact with the soil in the glasshouse or from outside
  • to make regular checks and burn infected plants
  • at the end of a growing cycle, to disinfect all equipment (pots, shelves, irrigation and water recycling system; and when recycled water is used, the pump inlet tube must be at least 20 cm above the bottom of the recycling tank, since this considerably reduces the risk of sucking in spores)
  • to avoid extreme temperatures under glass in summer (ideally never over 23ºC - 73ºF)
  • to maintain nutritional balance between nitrogen, potassium and calcium. Infection risks are increased both by too little nitrogen and by too much
  • to choose watering systems with care: spraying (with rail, fixed jet or hosepipe jet) causes splashing which shifts the spores or contaminated particles of compost. Washing the spores down brings them to root level which is where infection starts.

> Biological control

Preventive biological control is a highly effective alternative.

It was noticed that on certain soils the disease was having little effect, in spite of the presence of the pathogen and suitable conditions for its development: such soils are known as “suppressive”. Upon analysis, these soils were found to contain strains of Fusarium oxysporum that are saprophytic, i.e. they feed on dead organic matter. These do not harm plants; and it is their properties that are used in a form of biological control.

This saprophytic strain has no harmful influence on the economic qualities of the plant (its growth and flowering), as was established in trials at the CNIH (National Horticultural Trade Committee) establishment at Angers in 1990.

 

The saprophytic strain sets up in opposition to the pathogen. It would appear that this strain follows the same plant colonisation pattern (hyphae of mycelium from chlamydospores penetrate the root system and colonise the xylem), but without bringing on cell death.

The most probable account is that there are three levels to this antagonistic activity:

  • the saprophyte colonises the area around the roots, and is in competition for nutrients, above all iron and organic matter
  • it colonises the sites on the roots which are the points of entry for Fusarium into the plant
  • it provokes or stimulates the activation of the plant’s self-defence and resistance mechanisms (challenges the immune system). The activating molecule would appear to be the fusaric acid produced by the plant.

 

This hostile strain FO 47, was discovered by the Dijon INRA (National Institute for Agricultural Research)

The fungus has to be introduced into the growing medium a number of times:

  • at sowing time
  • at pricking out
  • at potting-up

Strain FO 47 is sold under the name Fusaclean (though it is not on the Plant Medicine Register for 1997). Its mode of application is by inoculation of the growing medium: for the dosage, see the manufacturer's instructions.

This strain can only be used as a preventive treatment; for it to be effective, fungicide use obviously has to be rescheduled appropriately.

 

There is in the literature an Italian reference to another strain, FO 251/2 (Biofox) which is hostile to many specific forms of the Fusarium oxysporum pathogen. FO 251/2 is compatible with and activated by the application of Benlate.

Precautions for use

Before using FO 47, you must ensure that :

  • the growth medium is free of all contamination (soils must be fumigated so as to eliminate all soil micro-organisms, in order to help FO 47 to get established)
  • minimum soil humidity is over 10%

> Chemical control

Chemical treatment after infection is quite useless.

Though the benefits of chemical treatment against Fusarium oxysporum f.spcyclaminis are not proven, it is important to prevent cyclamen being damaged by other pathogens such as vectors of root disease. These leave the plant in a weakened condition which encourages the spread of fusarium.

 

The constant development of the regulations and homologations of phytosanitary treatment products, and the differences in regulations according to each country make it impossible for us to include updated information on homologations. Each producer will have to contact his local plant protection bureau to obtain the latest updates concerning the regulations and use of phytosanitary products. We strongly advise testing beforehand on a plant sample in order to measure the chemical’s activity (establishing the dose) and any effect on the plant (plant poisoning).



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.

 

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