Pythium can be in commercially available soilless potting mixes. It is easily introduced into pasteurized soil or soilless mixes by using dirty tools, dirty pots or flats, walking on or allowing pets to walk on the mixes and by dumping the mixes on benches or potting shed floors that have not been thoroughly cleaned. Fungus gnat and shorefly activity may also be involved in moving Pythium from place to place in greenhouses. When introduced into a soil mix that has been heat-treated for too long or at too high a temperature, Pythium can cause severe root rot because it has few competitors to check its activity. P. aphanidermatum and P. irregulare pose a threat to crops grown in ebb and flow systems because they form a swimming spore stage that can move in water. This is likely to occur only if irrigation times are long (45 min. or longer) or if pots sit in puddles of water because the bench or floor does not drain completely. If Pythium infests a cutting bed or if contaminated water is used in propagation, large losses occur. Pythium ultimum is primarily associated with soil and sand. As growers switched to soilless mixes, this species became less important than when growers used field soil in the potting mix. P. ultimum does not form the swimming spore stage but can be a problem in ebb and flow systems if the reservoir becomes fouled with potting mix and plant debris particles harboring it. Almost all plants are susceptible to Pythium root rot. Root tips, very important in taking up nutrients and water, are attacked and killed first. Pythium also can rot the base of cuttings.
you can get more info at this link: http://extension.psu.edu/pests/plant-diseases/all-fact-sheets/pythium
