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Pythium & Phytophthora Root Rots

by Coralie Farinas, Nancy J. Taylor & Francesca Peduto Hand


Pythium and Phytophthora, also known as water molds, are fungus-like organisms that cause damping-off of pre- or post-emerged seedlings, as well as root and crown rot diseases on mature plants. The pathogens are ubiquitous in soils and they are present worldwide, infecting a wide range of greenhouse floral crops, including begonia, cockscomb, geranium, gerbera daisy, madagascar periwinkle, new guinea impatiens, petunia, poinsettia, snapdragon, and vinca.

Pythium is the most common pathogen associated with root rot of potted plants. Several species have been isolated from infected plants with Pythium irregulare, P. sylvaticum, P. ultimum, and P. aphanidermatum being the most prevalent. Concerning Phytophthora, more than 100 species have been identified to date, with P. criticola, P. citrophtora, P. nicotianae, P. cactorum, P. dreschleri, P. cryptogea, and P. cinnamomi being the predominant species in the Northeastern US.


Pythium and Phytophthora cause root and crown rots resulting in yellowing, stunting or wilting of the aboveground tissues of the plant. Foliar symptoms resemble nutrient deficiency because the compromised root system cannot take up the nutrients needed. Indeed, roots are soft and mushy due to the tissue decay and may appear water soaked, glassy, and dark brown. The root cortex separates easily at the touch leaving the central part of the root -the vascular cylinder exposed. The rot may extend to the crown resulting in stem or crown rot or canker.

Symptoms caused by Pythium and Phytophthora are not unique and laboratory culturing is necessary to distinguish among pathogens. Samples should be submitted to a diagnostic laboratory for confirmatory diagnosis so that the right management recommendations are provided.

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For assistance in identification, contact the C. Wayne Ellett Plant and Pest Diagnostic Clinic.

Pathogen Biology

Pythium and Phytophthora are fungus-like organisms (oomycetes) that colonize host plants through tubular cells, called hyphae, which grow together in a thread, called mycelium. Through hyphae, the pathogens extract nutrients from the host cells. When hyphae from opposite mating type meet, they produce thick-celled spores called oospores. Oospores are the overwintering sexual structure of the pathogens and can serve as source of inoculum for future seasons or crops. Once the oospore germinates, it can produce more hyphae or develop a spore-bearing structure called sporangium (plural sporangia). The sporangium contains asexual spores, called zoospores, which possess motile appendages (flagella) that allow them to swim for several hours in a film of water. Zoospores are spread by irrigation and surface water and they are attracted by root exudates. Once they reach the plant cells they lose their flagella, encyst, and then infect and invade the host tissues causing the disease to develop.

Phytophthora and Pythium have similar disease life cycles. Mycelium can infect directly new plant material when growing through the foliar canopy. The pathogens survive in the soil as oospores for several years or as mycelium in plant debris. They are polycyclic and zoospores normally act as secondary inoculum. However in greenhouses, new hosts are constantly introduced and the pathogens do not need to overwinter so zoospores are the primary means of reproduction. Sporangia could also directly germinate and infect plant tissues.

Multiple production management practices pose the risk of spreading Phytophthora and Pythium in the greenhouse: infected tools, hose ends that came in contact with contaminated soil, workers’ footwear or soiled hands. Contaminated propagation  material including cuttings and seeds are a frequent way of disease introduction. High density of plants in the greenhouse also facilitates pathogen spread. Free water is essential to cause infection and mere moisture is not sufficient. Droplets can splash onto new foliage, and overhead watering help zoospores propagate and spread. Adult shore flies can serve as vectors and introduce the pathogens in the greenhouse. Fungus gnat larvae that have ingested oospores and encysted zoospores can spread the fungus in feces once the fungus gnat is an adult.

Favorable Environmental Conditions

The virulence of Phytophthora and Pythium is dependent on the pathogens’ species, the host that is being attacked, and the prevailing environmental conditions. For example, geranium and poinsettia are more susceptible to Pythium with high fertilization rates; whereas low fertilization rates to Peperomia increase its susceptibility to the pathogen.

The temperature range for pathogens’ development is very wide, ranging between 59°F and 95°F. Optimums are specific to the species of Phytophthora or Pythium with a majority preferring temperatures between 68°F and 86°F. Pythium ultimum and Pythium aphanidermatum, the most commonly encountered Pythium species in the greenhouse, have an optimal growth temperature range of 63-68°F and 80-93°F, respectively. The optimum growth temperature range for Phythophthora cryptogea is 71-77°F.

All Phytophthora and Pythium species favor high soil moisture conditions, low oxygen and high soil soluble salt levels in the potting medium.

Management Guidelines

Integrated management practices are key to control Phytophthora and Pythium diseases in the greenhouse. The use of proper cultural practices combined with exclusion methods to avoid introduction of the pathogens in the greenhouse is essential to limit the diseases’ impact.


  • Clean and disinfect benches, pots, trays and any tool and equipment coming in contact with the plant or soil. If you have confirmed cases of infection, it is preferable not to recycle used pots and trays.
  • Quarantine brought-in plants, and separate plants according to their origins.
  • Scout thoroughly for disease to ensure early detection and remove and dispose of diseased plants immediately as well as any plant and soil debris.
  • Keep hose ends off the ground to avoid contamination with soil particles that may be infested.
  • Limit visitors’ access to production areas; disinfecting foot pads could be placed at the entrance of greenhouses
  • Do not leave pots on bare soil, but use tarp to prevent contamination from the ground.

Cultural Practices:

  • Do not overwater – select potting media with good draining capacity, preferably soilless.
  • If field soil is used as a component of the potting medium, it must be steamed prior to utilization (180°F for at least 30 min).
  • Do not place pots or flats in standing water.
  • Fertilizers should be applied in strict accordance to the plant’s needs. High accumulation of salts increases root damage, which consequently favors pathogen infections.
  • Plants should be spaced to avoid touching canopy and promote air-drying.
  • Management of the irrigation system is key to control Pythium and Phytophthora. Particular attention should be paid to recycling irrigation systems. Many filtration systems options are available and will help in preventing pathogen dissemination (slow sand filtration, membrane filters, chlorine, ultraviolet light, etc.).
  • Drip irrigation is preferred over overhead irrigation because minimizes water and soil splash and consequently pathogen spread.

Chemical control:

  • Several fungicides are available to control Pythium and Phytophthora in the greenhouse and best results are obtained when they are applied preventatively.
  • Misuse of fungicides has led to development of resistance in several pathogen populations. A high level of resistance has been observed to mefenoxam and metalaxyl (phenylamides) for both pathogens. Check with your State specialist or County Extension educator for the most updated information on product efficacy.
  • To minimize the risk of fungicide resistance developing in your greenhouse, it is best to alternate fungicides with different mode of action (different FRAC codes).

Biological control:

  • Some biocontrol agents have shown efficacy in controlling Pythium spp. in conditions of low disease pressure. Check with your State specialist or County Extension educator for the most updated information on products efficacy.