(Oomycota: Class Oomycetes: Order Pythiales: Fam. Pythiaceae)
Pythium ultimum belongs to the Oomycetes, as does P. aphanidermatum, and is related to Phytophthora species. The fungus survives in the soil as sexual oospores. These are resting spores that are resistant to dehydration and both high and low temperatures. The oospores can either germinate directly by means of a germ tube or form sporangia. Sporangia can likewise either germinate directly or produce zoospores (swarm spores). Swarm pores spread through water towards a suitable host plant. They attach to the host and infect it. Germ tubes of oospores and sporangia can also infect host plants. This fungus is extremely prevalent and can survive in soil as a saprophyte. The fungus infects seeds, seedlings and roots. The infected tissue cells die and are used by the fungus as nutrients. New sporangia are formed, which can cause new infections. Zoospores are dispersed by water and oospores mechanically, by people, machines and other materials.
P. ultimum has a wide range of host plants, including flower bulbs, summer flowers and perennials.
The first symptoms are loss and stunting of plant growth. Leaves do not grow well, droop and turns yellow. Buds of infected plants dry out and fall off. Dark lesions on the stems and roots. The root epidermis comes away easily. In severe attacks, the entire root system will rot away.
P. ultimum thrives in soil or substrate with a high water content and at average temperatures.
- good drainage, do not give too much water;
- avoid strong variations in crop growth;
- avoid high N-fertilisation.