Pythium species are fungal-like organisms (Oomycetes), commonly referred to as water molds, which naturally exist in soil and water as saprophytes, feeding on organic matter. Some Pythium species can cause serious diseases on greenhouse vegetable crops resulting in significant crop losses. Pythium infection leads to damping off in seedlings and crown and root rot of mature plants. In Canada, several Pythium species, including P. aphanidermatum, P. irregulare and P. ultimum, are known to cause damping-off and crown and root rot in greenhouse cucumber, pepper and tomato crops. There are no Pythium resistant varieties available although some varieties may have disease tolerance. Over watering, poor root aeration, root injury and improper root zone temperatures can weaken the crop and, thus, trigger Pythium outbreaks. Saturated growing media that are either too cold or too warm can be conducive to Pythium build up and spread in water and recirculating nutrient solution. Plants grown under optimal environmental conditions are less susceptible to Pythium than plants grown under poor conditions.
Pythium sp infects seeds, juvenile tissue, lower stems, fruit rot and roots. The symptoms and extent of damage caused depend on the region infected.
Plants are most vulnerable to infection by Pythium during the germination and juvenile stages. The initial symptoms may be poor or uneven germination (pre-emergence damping off). Seedlings that do germinate are susceptible to post-emergence damping-off. An infected seedling will appear water-soaked and the plant will collapse. Entire plantings can be completely destroyed at this stage.
Stem rot infects the lower stem of many annual and bedding plants, especially during the juvenile stage. Symptoms begin as a water soaked region near the soil line. The plant tissue becomes slimy. If the lesion expands to encircle the stem, the plant will collapse. If the pathogen is limited fungicide the plant, the lesion will eventually dry out and the plant begins to recover. The lesion eventually becomes sunken and is brown in color.
Pythium root rot causes wilting, loss of vigor, stunting, chlorosis and leaf drop. Root growth is inhibited and roots are blackened, mushy and rotten. Symptoms begin at the root tips. The rotten part of the roots may slough off exposing the inner root core. Underground storage structures are also at risk of infection by P. aphanidermatum. Even mature tissue can be infected and destroyed. Beets and other fleshy plant organs are susceptible to rot in the field and during storage.
P. cubensis is an obligate parasite or biotroph, meaning that it requires live host tissue in order to survive and reproduce. Because of this characteristic, the pathogen must overwinter in an area that does not experience a hard frost, such as southern Florida, and where wild or cultivated cucurbits are present. The spores are dispersed via wind to neighboring plants and fields and often over long distances. Symptoms appear 4 to 12 days after infection. The pathogen thrives under cool and moist conditions, but can do well under a wide range of conditions. Optimum conditions for sporulation are 15 °C with 6 to 12 hours of moisture present, often in the form of morning dew. Even when high daytime temperatures are not favorable for the pathogen (>35 °C), nighttime temperatures may be very suitable. Oospores (thick-walled, resting spores) of P. cubensis are rare and their role in nature is unknown.