Pythium is considered a ‘weak pathogen’, meaning that it can usually only affect plants that are in a sub-optimal condition or suffer from stress, caused for example by too much water, too high or low temperatures or sudden fluctuations in temperature. Pythium ultimum mostly occurs at low soil or water temperatures and causes among others cereal diseases.
Life cycle and appearance of Black leg of seedlings
Pythium belongs to the Oomycetes, a separate group of microorganisms and is therefore not a real fungus. It survives as oospores, resting spores which are resistant to unfavourable conditions, especially dehydration. Germination of the oospores is stimulated by exudates from roots of a susceptible host nearby. The oospores germinate and form so-called sporangia from which subsequently zoospores are produced. Zoospores are spores with flagellae, which enable the spores to move through water. The zoospores are chemically drawn to the growing roots of young plants and move towards them through the soil water. They infect the roots through small wounds, where the side roots are formed or through other similarly damaged spots.
On the infected plant, new sporangia with zoospores are formed, that allow the disease to spread to neighbouring plants. The zoospores disperse via water. Therefore, this disease spreads most rapidly in wet conditions with a high (>70%) water content of the soil or substrate. Oospores are dispersed when soil particles are moved around by humans or machinery. Pythium can survive very well in the soil or substrate in the absence of plants. Many species are entirely saprophytic, so these do not harm plants.
Pythium affects roots, in particular those of seedlings. Infected plants lag in growth. Seedlings fall over (‘damping-off’). In slightly older plants, the leaves lose their turgor or the leaf tips turn yellow, for example in flower bulbs. At first, the plants recover during the night, but later the plants wilt irreversibly. In potted plants the flower buds fall off. The side roots rot and very little of the root system survives. On the border between air and soil or substrate the plant turns brown. A typical symptom of a Pythium infection is the easy stripping of the skin from the root cortex with a finger nail.