Biology and Epidemiology

- Survival and sources of inoculum

Pythium  spp. may be present and survive as saprophytes in organic matter. Due to the large range of hosts they can attack their survival and multiplication from year to year is very common.  They survive in the soil through structures such as oospores (figure 1), chlamydospores and to a lesser extent sporangia. In some species, oospores can survive from two to twelve years. Germination of oospores and sporangia are stimulated by exudates from germinating seeds and growing roots. They produce either germ tubes or zoospores.


- Penetration into the plant and invasion

These fungi penetrate the epidermal tissues directly or through wounds. They quickly invade the tissues with the combined action of various cellulolytic and pectinolytic enzymes and progress in between and within the cells. Sporangia and oospores are formed within or on the surface of infected tissues.


- Dissemination of the pathogen (figures 2 et 3)

Pythium spp. are perfectly adapted to live in the soil and in the nutrient solution of float water production systems. The water allows zoospore release which is further favoured by some substrates and root exudates of the seedlings themselves.  In the nurseries Pythium spp. are transmitted from plant to plant by growth of mycelium in the soil and thus plant density is important for disease spread. Spore release through the air may occur when water splashes during sprinkler irrigation or heavy rainfall.


- Favourable conditions for the disease development

All oomycetes do not have the same pathogenic potential. They need special conditions to infect plants;

- The presence of water is almost always essential. High soil moisture and reduced gas exchange gives an ecological advantage to these fungi at the expense of other fungi and micro-organisms with which they are sometimes competing for soil organic matter;

- The behavior of these fungi is influenced differently by temperature. Some species prefer cold soils, with temperatures around 15 ° C, such as Pythium ultimum (opt. temp. = 15-20 ° C, mini 2 ° C, max 42 ° C).  Other species have higher optimum temperatures, for instance Pythium aphanidermatum (opt. temp. = 26-30 ° C, mini 5 ° C, max 41 ° C), which occurs rather in warm tropical areas;

- The reaction of the host is not consistent throughout its lifecycle. Young seedlings and succulent tissues are very susceptible. Mature plants are more resistant than seedlings but may become susceptible, if they undergo various climatic or agronomic stresses. Usually the feeder roots are affected;

- Attacks from other pests such as root-knot nematodes make plants susceptible to Pythium spp.