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Professional area | Diseases factsheet | Fungi | Phytophthora

Fungi

Phytophthora

Fungi of the genus Phytophthora are soil fungi which after infection develop in the plant’s tissue.

As with Pythium, this is a fungus which causes seedlings to deliquesce (damping off disease) and roots to rot. On occasion, Phytophthora works alongside other agents of disease.

Chemical controls are applied as a preventive.

> Phytophthora : cause of soft rot and damping off of seedlings

Fungi of the genus Phytophthora are Phycomycetes, characterised by a hyphal thallus without transverse septa.

There are two stages to the life cycle of this fungus, and it is environmental conditions that determine the emergence of each. In unfavourable conditions, or at the end of the cycle, sexual reproduction takes place and produces the survival form of the disease organism (a coated oospore): the other stage is one of spreading and growth with asexual reproduction (conidia spores) and growth of mycelium. When humidity is low, these conidia produce hyphae straight away; when it is high they develop into sporangia and form zoospores, a primitive kind of spore which can only move around in water (as in the case of Pythium).

Since its life cycle involves a stage of sexual reproduction, the fungus belongs to the class of Oomycetes. It is in the Pythiaceae family, of the order Perenosporales.

It is a soil fungus which after infection develops in the plant’s tissues. The mycelium grows between cells and puts out suckers to fed from them.

 

There are a great many species, many of which are polyphagous and capable of infesting a number of plant species. Along with Pythium, Fusarium and Rhizoctonia solani it is one of the main causes of damping-off disease in seedlings and root rot.

> Optimal conditions for growth

Optimal temperature for growth is between 20ºC (68ºF) and 25ºC (77ºF), but it can develop in soil as cool as 13ºC (55ºF) to 15ºC (59ºF).

> Symptoms

Infection starts with the roots (rootlets).

The plants affected start to rot, or show necrotic lesions, in the root system, in the section (hypocotyl) between shoot and radicle (in seedlings and young plants), or at the collar and the base of the stalks. The attacks occur most often after the seedlings have shown above ground, and they kill the plant.

When the seeds themselves are attacked this results in a failure to show; the impression is that the seeds have rotted.

In adult plants, the roots are blackened. The plant above ground turns yellow and then withers, but this is of course not a distinguishing sign of this disease. Phytophthora spp. are vascular fungi, which block the sap-transporting vessels and the plant dies of ‘apoplexy’. Dry soil makes these symptoms more likely, since the dead roots can no longer assimilate free water from the soil, and the plant suffers from drought.

 

Phytophthora spp. may produce these symptoms on its own or in concert with other fungi: the specific cause(s) can only be determined by exact analysis.

 

Glazed tuber of a plant contaminated by Phytophthora capsici

 

 

Foliage and stems of a plant contaminated by Phytophthora capsici

 

 

Half mature plant affected by Phytophthora during the summer

 

 

Completely withered plant affected by Phytophthora capsici

 

> How the fungus spreads and survives

Phytophthora spp. is capable of living as a saprophyte in the soil, making use of decomposing organic matter. Even without plants or plant debris, it can survive: to do so under hostile conditions it forms resistant organs known as chlamydospores.

The zoospores can move around in water and infect new host plants. They are chemotropic, moving towards the roots by smell. Water that splashes or drips is an important vehicle of spreading this fungus.

> Management measures

  • destroy infected plants as soon as the disease is apparent.
  • avoid too wet a growing medium; choose well drained mediums rather than fine ones which  retain water and warmth and in so doing encourage the disease’s development.
  • avoid letting puddles or streams of water form beneath the plant pots
  • avoid recycling water wherever possible
  • use new growing medium and store it away from possible contamination.
  • be careful with organic manures which encourage the fungus as saprophyte
  • do not sow too thickly since the resulting seedlings will be more susceptible
  • do not sow too deep
  • nitrogen fertiliser given as the shoots are appearing will encourage the growth and spread of the fungus.

 

And, in addition:

  • disinfect maintenance equipment and storage and service areas of the glasshouse at least once a year.

 

> Chemical control

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).

 

It is possible to treat the growing medium, by mixing the chemical in, to sprinkle the ground, or to spray the above-ground parts of the plants.



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.

 

Fungi :

S.A.S Morel Diffusion

2565, rue de Montourey
83600 Fréjus - France

International telephone : +33 (0)4 94 19 73 04
Switchboard : + 33 (0)4 94 19 73 00
Fax : +33 (0)4 94 19 73 19

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