Ministry of Agriculture

1. Introduction

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 in older 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.

2. Disease cycle

Pythium can be introduced into a greenhouse in plug transplants, soil, debris, pond and stream water, growing media and roots or plant refuse of previous crops. 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, forming masses of 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.

3. Symptoms

‘Pre-emergence’ damping-off causes seeds and young seedlings to rot before they emerge from the growing medium, while ‘post-emergence’ damping off kills newly emerged seedlings. In ‘postemergence’ damping-off, the pathogen causes a water-soaked, soft brown lesion at the stem base, near the soil line, that pinches off the stem causing the seedling to topple over and die. In older plants, Pythium causes crown and root rot, where mature plants suddenly wilt when weather turns warm and sunny and when plants have their first heavy fruit load. Often, upper leaves of infected plants wilt in the day and recover overnight but plants eventually die. In the root system, initial symptoms appear as brown to dark-brown lesions on root tips and feeder roots and, as the disease progresses, symptoms of soft, brown stubby roots, lacking feeder roots, become visible (Figure 2). In larger roots, the outer root tissue or cortex peels away leaving the string-like vascular bundles underneath. Pythium rot also occurs in the crown tissue at the stem base. In cucumber, diseased crown turns orange-brown in color, often with a soft rot at the base; brownish lesions extending 10 cm up the stem base may be seen.

4. Monitoring & Identification

Routinely monitor your crop for slightly wilted plants and check wet areas in the greenhouse where Pythium is more likely to be present. Pythium occurs mostly in spring, at early fruit set and later in the season on mature plants. In cucumber, Pythium can also occur in the summer on young plants brought in for the fall crop. Monitor plants for wilting, and in cucumber, check the stem bases for discoloration. Always confirm Pythium diseases by promptly sending a representative plant sample with root, crown and foliage to the Ministry of Agriculture's Plant Diagnostic Lab.

5. Integrated Disease Management

Integrated disease management provides a combination of cultural, biological and chemical tools to control and/or manage crop diseases effectively. Cultural controls keep Pythium from reaching the roots while biological and chemical controls inhibit or suppress Pythium in the root zone.

5.1 Cultural Controls

5.1.1 Sanitation

Field soil and debris, pond and stream water, sand, sediments and roots of previous crops can contain Pythium. Follow a strict greenhouse sanitation program throughout the year and a thorough year-end clean up. Clean and disinfest all interior greenhouse surfaces and equipment including tools, hoses, walkways, carts, totes, troughs, tanks and water supply lines. Use sterile propagating media. Remove dying plants by placing them directly into plastic bags for disposal away from the greenhouse.

5.1.2 Irrigation water

Untreated water from rivers or streams poses a threat while treated, municipal water is considered safe from Pythium. Water storage and nutrient tanks need to be covered to prevent Pythium contamination and may require disinfesting.

5.1.3 Disinfesting nutrient solutions

Generally, greenhouse vegetables are raised on rockwool cubes and plastic sleeves or bags containing rooting medium (i.e. rockwool slabs, sawdust or coconut fibre) through which water and nutrient solution are circulated. Since Pythium and other pathogens can build up in nutrient solution, periodically disinfest recirculating nutrient solution using physical, biological or chemical treatments (Marchuk, 2006).

Physical treatments:
Filtration - slow sand filtration, ultrafiltration (membrane filters), micropore filtration (high pressure, rapid flow membrane or sediment filters).
Energy - heat pasteurization (95-97°C for 30 seconds or 85°C for 3 minutes), UV radiation (80-100 mj cm-2; e.g. Priva Vialux®), sonic energy, magnetism, aeration (i.e. oxygenation; bubbling air into water).

Biological treatments: biofiltration (slow sand or lava rock), water retention ponds

Chemical treatments: ozonation, chlorine, chlorine dioxide, copper (e.g. Aqua-Hort® from Aqua-Perl, Denmark), hydrogen peroxide, electrochemical (e.g. ECA®), soaps (wetting agents), iodine.

5.1.4 Resistant varieties

Although there are no resistant vegetable varieties, some vigorous varieties may have more tolerance to Pythium. Contact your local seed agent for further information on Pythium tolerant varieties.

5.1.5 Greenhouse Environment: Seedlings & Transplants

• Allow for good air circulation around seedlings by proper plant spacing and good aeration of irrigation water and recirculating nutrient solution. Water seedlings in the morning so that plants are not wet overnight.
• Use healthy transplants and handle them carefully to avoid wounding plants and roots and practice good sanitation when transplanting; do not let them dry when setting out. Transplant in the morning or later in the evening to avoid stress from high day time temperatures.
• Growers suggest that recirculation of the nutrient solution be suspended for the first few weeks after transplanting as a precautionary measure.
• Ensure that transplants have the proper root zone temperature and adequate moisture when moved into the greenhouse. The growing media must be well drained as saturated bags with low oxygen levels can predispose transplants to Pythium diseases.
• Use warm, aerated irrigation water (18-22°C). Avoid low light levels, low pH, high salts and warm growing conditions (above 28°C) which favour Pythium. In greenhouse cucumbers, the nutrient solution should be delivered at a pH 5.0 regime for approximately 5 weeks followed by adjusting the pH to a 5.8-6.2 regime for one week. (Tu, 2004)
• Target rockwool block wetness at 70-75% between watering.
• Use white/color-less drip lines instead of black or place driplines on the shaded side of the grow bags.
• Control fungus gnats (Bradysia impatiens) and shore flies (Scatella stagnalis) which spread Pythium.
• Avoid injuring roots with excess use of insecticide or fungicide drenches and follow an integrated pest management program, including cultural and biological controls.
• The temperature of growing media should not be less than 20-23°C. Minimum root temperature should be maintained at 20°C in the root zone.
• Remove and destroy severely infected plants and replant in new growing bags. Infected plant materials, including grow bags, must be safely disposed away from the greenhouse by deep-burying, incinerating or composting.

5.2 Biological Controls

Biological control strategies are gaining importance in Canada with the registrations of Mycostop (Streptomyces griseoviridis) and RootShield (Trichoderma harzianum), which directly protect seeds and young plants by suppressing Pythium in the root zone. Apply the biological control agents to the planting mix several days before seeding or transplanting (Table 1).

5.3 Chemical Control & Resistance

Previcur (propamocarb) and Ridomil (metalaxyl), the two fungicides registered in Canada for Pythium, have a high risk of Pythium developing resistance (Table 1).

• Prevent Pythium diseases by practicing integrated disease management strategies based on cultural and biological controls. Use fungicides as a last resort at the onset of disease.
• Rotate registered fungicides with different chemical groups and strictly follow label directions to avoid resistance development in Pythium.
• Routinely monitor plants and evaluate the level of disease control if fungicides are used. Stop fungicide treatment and get professional advice if fungicides fail.

Table 1. Registered biocontrol products and fungicides for Pythium diseases of greenhouse cucumber, tomato and pepper crops in Canada.

6. Further information

1. Cherif, M. et.al. 1994. Defense responses induced by soluble silicon in cucumber roots infected by Pythium spp. Phytopathology 84: 236-242.
2. Compendium of pepper diseases. 2003. K. Pernezny, et. al. editors. The American Phytopathological Society. http://www.apsnet.org/apsstore/shopapspress/Pages/43003.aspx
3. Diseases and pests of vegetable crops in Canada. 1994. R. Howard et. al. editors. The Canadian Phytopathological Society and the Entomological Society of Canada. http://phytopath.ca/dpvcc.html
4. Growing greenhouse peppers in British Columbia. A production guide for commercial growers. 2005. BC Greenhouse Growers’ Association and B.C. Ministry of Agriculture. www.bcgreenhouse.ca/publications.htm
5. Growing greenhouse vegetables. 2005. Ontario Ministry of Agriculture, Food and Rural Affairs. Publication order #371, Agdex #290. http://www.omafra.gov.on.ca/english/products/newpubs.html or products@omaf.gov.on.ca
6. Jarvis, W.R. 1992. Managing diseases in greenhouse crops. American Phytopathological Society. http://www.apsnet.org/apsstore/shopapspress/Pages/41221.aspx
7. Marchuk, R. 2006. Treatments for greenhouse recirculation water. Proceedings, 48th Annual Horticulture Growers Short Course, 2006. Lower Mainland Horticultural Improvement Association, Pages 3-8.
8. Pesticide label information for Canada: http://pr-rp.hc-sc.gc.ca/ls-re/index-eng.php
9. Tu, J. C. 2004. An integrated control measure for Pythium root rot of hydroponically grown greenhouse cucumbers. Acta Horticulturae 644: 571-574.
10. Zamir, P.K. and R. Yip. 2003. Biological control of damping off and root rot caused by Pythium aphanidermatum on greenhouse cucumbers. Can. J. Plant Pathol. 25: 411-417.

Contact:

BC Greenhouse Growers’ Association
Iris Bitterlich 604-591-5490

B.C. Ministry of Agriculture
Siva Sabaratnam 604-556-3029
Jennifer Curtis 604-556-3057

October, 2007