Biology, epidemiology

- Conservation, sources d'inoculum

The Pythium spp. are able to live in a state saprophytic at the expense of organic matter. Their low parasitic specificity allows them to attack a large number of hosts which also ensure their multiplication and conservation. Phytophthora cryptogea attacks many vegetable crops such as cucumber, tomato, cabbage, celery, asparagus, peppers Phytophthora porri has a host spectrum more limited to Allium , white cabbage and carrot. They are maintained in the soil by resistance structures , such as oospores , chlamydospores and, to a lesser extent, sporangia (Figures 1 to 4). In some species, oospores can be stored for two to twelve years (Figures 5 and 6). The preservative forms of these fungi are stimulated by the exudates emitted by the seeds or roots. They either produce a germ tube or zoospores .


- Penetration, invasion

These fungi enter the directly epidermal tissues or pass through wounds . They rapidly invade tissues, thanks to the combined action of various pectinolytic and cellulolytic enzymes, and progress between and into cells (Figure 7). The sporangium , of oospores are formed within tissues or on their surface.


- Dissemination

Oomycetes are perfectly adapted to life in the aqueous phase of soils and in the nutrient solution of soilless crops. The water largely ensures their dissemination through their many zoospores flagellated produced by sporangia. It is also favored by certain substrates, and by plants. In nurseries where the density of seedlings is high, Pythium spp. are transmitted from plant to plant during the progression of the mycelium through the soil. Aerial releases are sometimes possible as a result of splashing during sprinkler irrigation or heavy rains.


- Conditions favorable to their development

You should know that oomycetes do not all have the same pathogenicity. In addition, they need special conditions to infect plants:
- the presence of water is almost always essential. High soil humidity and reduced gas exchange constitute an ecological advantage for these fungi, to the detriment of other fungi and micro-organisms which are sometimes competitors for the organic matter of the soil;
- the temperature influences the behavior of these fungi differently. There are species that appreciate cold soils, at temperatures close to 15 ° C, such as Pythium ultimum (temperatures: optimum 15-20 ° C, minimum 2 ° C, maximum 42 ° C), others have more thermal optima. high. This is particularly the case with Pythium aphanidermatum (temperatures: optimum 26-30 ° C, min 5 ° C, max 41 ° C), which is found more in hot tropical areas and in soil-less crops;
- the host's receptivity is not constant throughout his life. Young seedlings, succulent tissue, are very sensitive. Subsequently, adult plants can become so, mainly when they are subjected to various climatic or agronomic stresses. It is especially the nourishing rootlets that are affected. This is sometimes the case in soilless crops.

Certain interactions unfavorable to salads between Pythium spp. and phytophagous nematodes ( Meloidogyne hapla , Pratylenchus crenatus ) have been observed.