The Pythium spp. , and Phytophthora spp. are able to live in a state saprophytic at the expense of organic matter present in the soil or substrates. In the latter case, the root exudates, but especially the debris of dead roots, constitute important substrates for the development of saprophytics and the maintenance of these chromists on and in the soil.

The oospores (Figure 1) and chlamydospores perfectly ensure their conservation in the ground, whether in wet or dry conditions.

Their low parasitic specificity allows them to attack a large number of hosts , cultured or not, which also ensure their multiplication and conservation.  

In soil as well as above ground, the sources of inoculum can be diverse : the substrate, a few plants, water of various origins: pond, dam, more or less important watercourse, etc., plant debris , sludge. For example, in the region of Almeria in Spain, several species of Pythium (P. aphanidermatum, P. spinosum ) have been isolated from dust settling on shelters, with sometimes high concentrations of propagules. These dusts thus constitute sources of inoculum in certain cases.

These chromists directly penetrate the epidermal tissues of young roots and fruits, but also through wounds . They rapidly invade tissues, thanks to the combined action of various pectinolytic and cellulolytic enzymes, and progress between and into cells. Depending on the parasitic potential of the species, their presence in the roots can go unnoticed or quickly lead to browning of the tissues, and therefore to significant damage. When they colonize a fruit, it does not take long to rot. Subsequently, sporangia, oospores (which will ensure the conservation of Oomycetes) form inside the tissues or on their surface. The temperature at which their production takes place varies from one species to another: 15-35 ° C for Pythium aphanidermatum and P. myriotylum , 10-20 ° C for P. ultimum .
 

The Pythium spp. and Phytophthora spp. are perfectly adapted to life in the aqueous phase of soils and in the nutrient solution of soilless crops. As we pointed out previously, they sporulate more or less abundantly in and on the tissues which they have invaded (roots, crown, fruits ... ). In particular, they form sporangia there which can germinate directly or produce zoospores flagellated and motile . The latter are easily disseminated in the aqueous phase of soil and in the nutrient solution of soilless crops, and are attracted by root exudates. In nurseries and in soil-less crops, where the densities of seedlings but especially the quantities of roots are important, they are transmitted from plant to plant during the progression of the mycelium in the soil or in the substrate. Aerial releases are possible as a result of splashing during sprinkler irrigation or heavy rains.

Some insects , including flies, could cause contamination through the transport of oospores . Thus, transmissions of aerial Pythium aphanidermatum by the Scatella stagnalis fly have been reported on cucumber crops.

The development of these Chromists, which do not all have the same pathogenicity, can be influenced by various parameters:

- the high density of seedlings in nurseries and roots in the breads of soilless crops;
- excess nitrogen , which would worsen the root symptoms associated with certain Phytophthora . On the contrary, the addition of potash would reduce its severity. In addition, the high salt concentrations would increase or reduce, depending on the species, their effects, in particular on young seedlings;
- the presence of water which is almost always inevitable. High soil humidity and reduced gas exchange constitute an ecological advantage for these chromists, to the detriment of other fungi and microorganisms which sometimes compete for soil organic matter. Heavy and / or compacted soils are very conducive to their attacks because they penalize the vigor of the host and generate an environment conducive to the diffusion of the exudates necessary for the germination and growth of these oomycetes. In addition, soil moisture contributes to the production and subsequent dissemination of zoospores;
- temperature influences their behavior differently. There are species that appreciate cold soils, at temperatures close to 15 ° C, such as Pythium ultimum (optimum temperatures 15-20 ° C, min 2 ° C, max 42 ° C), others have higher thermal optima . This is particularly the case with Pythium aphanidermatum (optimum temperatures 26-30 ° C, min 5 ° C, max 41 ° C). Let us point out, for example, the optimal temperatures in some species of Phytophthora : 22-25 ° C for P. cryptogea , 28-31 ° C for P. drechsleri , 24-28 ° C for P. citrophthora and 28 ° C for P. capsici ;
- the host's receptivity is not constant throughout his life. Thus, succulent or etiolated seedlings are very sensitive while adult plants are less so, but can become so, in particular when they are subjected to various climatic and agronomic stresses (low availability of oxygen, water stress, etc.);
- the intervention of other pests and diseases leading to much more destructive interactions for Cucurbits.