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.
Above ground, mature plant tissue is susceptible to infection to infection by P. aphanidermatum, particularly when it is in contact with the soil. It is most commonly seen on cucumbers and other cucurbits. The blight begins as water soaked regions, later enlarging and developing cottony mycelium on the rotting fruit.
Pythium can be introduced into a greenhouse in plug transplants, soil, growing media, plant refuse and irrigation water. Greenhouse insects such as fungus gnats (Bradysia impatiens) and shore flies (Scatella stagnalis) can also carry Pythium. Pythium spreads by forming sporangia, sack-like structures, each releasing hundreds of swimming zoospores (Figure 1). Zoospores that reach the plant root surface encyst, germinate and colonize the root tissue by producing fine thread-like structures of hyphae, collectively called mycelium. These hyphae release hydrolytic enzymes to destroy the root tissue and absorb nutrients as a food source. Pythium forms oospores and chlamydospores on decaying plant roots which can survive prolonged adverse conditions in soil, greenhouse growing media and water, leading to subsequent infections.