Hot-season pythium

Three of the most commonly encountered Pythium species are Pythium irregulare, Pythium aphanidermatum and Pythium ultimum, said Gary Moorman, professor of plant pathology at Penn State University.

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 minutes 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. Root tips, very important in taking up nutrients and water, are attacked and killed first. Pythium also can rot the base of cuttings.
Watch for stunted plants, brown or dead root tips, plants that wilt at mid-day and may recover at night, plants that yellow and die, and brown tissue on the outer portion of the root that easily pulls off leaving a strand of vascular tissue exposed.

P. aphanidermatum is active at high temperatures (higher than 77°F). And it is a problem for mum production. Paul Lopes of the University of Connecticut advises growers to select sample plants, remove them from their pots and examine the roots carefully. Symptoms of poor root health are blackened or rotted roots or lack of roots.
To keep P. aphanidermatum at bay, avoid allowing water to sit around containers, deep planting mum cuttings or overwatering. Provide even moisture and avoid high electrical conductivity values. Any root stress may open the plant up to pythium attack.

Pythium is favored by high fertility and high moisture. It’s a natural inhabitant of the soil and can survive there indefinitely as well as in dirt and debris in greenhouses. Keep irrigation hose ends off the floor and avoid contaminating growing media with soiled hands, tools or flats.

To see how BioWorks can help your fight against P. aphanidermatum, click here:

Photo by Paul Lopes, University of Massachusetts