When we talk about "phytophthora ", We are not talking about one and only disease. The term actually groups together numerous phytopathologies with more or less significant consequences (crown rot, mildew, ink disease, etc.). It is therefore more correct to speak of "phytophthoras".
Fortunately, the symptoms, the means of prevention and of fight are quite similar between the different diseases. Follow our guide to learn more about phytophthoras.
- General information on phytophthoras
- Plants concerned
- Conditions of appearance
- Root and foot
- Fight and treatments
General information on phytophthoras
Biology and propagation:
Phytophthoras are mushrooms of the Oomycetes family. They are therefore the cause of so-called fungal diseases. Etymology the word comes from the Greek "phyto Which means "plant, vegetable " and of "phtora Which can be translated as "I destroy ».
Contamination and spread of disease occurs via the "seeds" of the mushrooms called spores where the zoospores (motile spores). Once in the ground, they can be stored for many years while waiting for a new host.
Some species like P. citricola primarily attack the root system. However, they can still affect the leaves if spores or contaminated soil settle there.
What plants are concerned? :
There is more than one hundred species different from phytophthoras. Some only affect one host in particular :
- Phytophthora phaseoli which is bean late blight;
- Phytophthora infestans which corresponds to late blight of Solanaceae (such as tomatoes);
- Phytophthora fragariae also called "red stele" and which attacks strawberries;
Other species, on the other hand, have a much broader scope: they are said to be polyphagous. Those which generally affect ornamental plants are :
- Phytophthora cryptogea;
- Phytophthora cinnamomi, responsible for the ink of oak or chestnut;
- Phytophthora citricola;
- Phytophthora cactorum, responsible for bitter rot or leather rot on fruits such as apples or strawberries;
- Phytophthora cambivora.
Phytophthoras can attack all types of plants: vegetables, perennials, shrubs, trees, etc.
Important note : the Phytophthora ramorum is a particular species virulent. Long confined to the United States and Great Britain, it first appeared in France in the early 2000s. The first subjects affected were viburnum and rhododendrons. However, since 2017 larches have been contaminated and Japanese larch is not recommended for planting.
Learn more about the P. ramorum :
- Article from the Ministry of Agriculture
Conditions of appearance:
There are many situations that make it easier the appearance phytopathologies in general and phytophthoras in particular:
- Plants are also subject to stress (lack of water or Conversely overabundance, excess nitrogen, etc.). This stress weakens the plants and thus makes them much more vulnerable to attack.
- Carry out cultures too close from each other not only promotes the appearance of fungi (increased humidity, poor ventilation), but also the spread of diseases by simple contact.
- The clay soils and compact, often referred to as "heavy earth", hold water easily. In some situations this is a good thing, but when it comes to phytophthoras, standing water is a very favorable development ground.
Symptoms related to phytophthoras
Phytophthoras can attack all parts of a plant; of roots to the leaves through the rods.
Roots and bases of stems (foot):
As a rule, it is the roots and the base of the plant (the crown) that are most often affected by phytophthoras. This then results in the rot of the root system, its discoloration and the absence of rootlets (small nourishing roots).
When the collar is reached and the plant is dying, it is possible to see the attack zone which is located just above ground level. It is quickly recognized, because it is located at the border between the healthy part on top and the affected part below which presents a brown or reddish brown.
When symptoms are visible on the leaves (wilting, dieback), it means that the root part is already well affected or that the foot is invaded. The plant is then unable to synthesize chlorophyll due to the lack of water and nutrients.
Some species of phytophthora can infect directly the leaves without going through the roots. The main symptom is the appearance of leaf spots dark brown to black in color and moist in texture. The disease then spreads through the leaf blade, reaching the petiole and then the stem.
Like leaves, other phytophthora species can target directly the stems. They then cause brownish to black lesions. The direct consequence is a wilt and one wasting away foliage above the lesions. On the other hand, the part of the plant located under the lesion remains healthy (roots and crown included).
Prevent the appearance of a phytophthora
For limit the risks of developing a phytophthora, just follow a few rules:
- Since zoospores have the particularity of moving in soil water, it is necessary limit cultures in heavy earth (clayey) or poorly drained. Stagnant water is then conducive to the spread of the disease. If necessary, lighten your soil by incorporating sand.
- Well space plants and crops. This will prevent the creation of favorable conditions for the disease and limit the risk of contagion should it occur.
- Perform the watering to the feet and not by sprinkling on the foliage.
- Disinfect your tools after every job. Hygiene is important, even in the horticultural environment. You can also treat pots or trays that you reuse when repotting with a disinfectant.
- After a purchase, inspect plants and do not hesitate to put them in quarantine before planting them in your garden or vegetable patch.
- When you multiply your plants (by division, cuttings, layering, etc.), be sure to use healthy subjects. In case of cuttings, take cuttings far from the ground.
- In the vegetable garden, diversify your cultures and keep susceptible species away from each other. Likewise, rotate your crops every year.
Control and treatment
Despite all the precautions taken, your plants may still be attacked by a phytophthora. In this case, it is imperative to clean the area by collecting all the infected elements (leaves, stems, roots) and burn.
The last step is to treat contaminated subjects. For this, there are some means of fight which will not be the same according to the affected parts.
This is perhaps the easiest situation to deal with. This is because phytophthoras that attack the leaves are, most of the time, responsible for downy mildew. To treat it, you just need to get Bordeaux mixture and spray the affected subjects with it following the instructions for use provided by the manufacturer.
Telluric phytophthoras, evolving in the soil and attacking the roots, are not not easy to deal with. The best way to prevent and to fight in case of attack is to rotate your crops, choosing species with little or no susceptibility to the fungus.
A few fungicides can nevertheless be used as the fosetyl-Al (aluminum fosetyl), marketed asAliette. The treatment is both preventive and curative and can be used on many crops (vegetables and fruit trees). This is a product called "systemic ", That is, it enters the body of the plant in order to fight the disease. It is both systemic descending (action on the roots) and ascending (action on the leaves).
Other products based on mancozeb or from dimethomorphic can also be used.
For all defenders of organic farming, know that, unfortunately, at the present time, there is no no natural curative solutions to phytophthoras. Nevertheless, studies are being carried out to determine the effectiveness biological control agents (BCA) in the fight against Oomycetes such as phytophthora (1).
Security rules :
When you use phytosanitary products to fight against the attack of a phytophthora, it is essential good you protect and of respect the recommendations from the manufacturer. If necessary, and in case of doubts, do not hesitate to refer to the Safety Data Sheets (F.D.S.) that manufacturers must make available free of charge on their website.
Sources : (1) Thesis by M. Manasfi Youssef on “Control of telluric pathogens in a horticultural context: case of the pathosystem Choisya ternata / Phytophthora spp. »P. 66