Professional area | Diseases factsheet | 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.
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 temperature for growth is between 20ºC (68ºF) and 25ºC (77ºF), but it can develop in soil as cool as 13ºC (55ºF) to 15ºC (59ºF).
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
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.
And, in addition:
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.
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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