Pythium Root Rot
Wednesday, 26 October 2011 12:22
Monitor poinsettia crops for Pythium root rot.
Pythium is a water mold that has number of species with different temperature requirements. Growers can have Pythium problems throughout poinsettia production however it is a usually a problem late in the growing season. Favorable conditions include cooler temperatures, overcast weather, poorly drained and excessively wet media. Fungus gnats can disseminate Pythium and other soilborne pathogens, and should be also be closely monitored and managed. Pythium is a natural inhabitant of soil and can survive there indefinitely as well as in debris in the greenhouse.
Symptoms of Pythium infection late in production include wilting; chlorosis of the mid-vein region and lower foliage; stunting due to lack of roots; black or brown cankers at the soil line; blackened, mushy roots, and eventual plant death. Plants with root rot tend to be heavy and waterlogged. It is important to periodically check the roots of your plants even if none of the above symptoms are observed. Healthy roots are white and firm; decayed roots may be dark colored and the rotted outer covering of the root slips from the central core exposing a thread-like root. Since symptoms can be confused with other causes such as high soluble salts or other diseases, suspicious plants should be diagnosed through your University diagnostic lab or through a private diagnostic lab.
Growers having re-occurring problems with Pythium, should review their overall production practices including fertilizing, watering and media handling. Over-watering, poor media drainage and excessive fertilizer levels promote Pythium. Good sanitation is also crucial for prevention. Keep hose ends off the floor, wash hands before handling plants and avoid contaminating growing medium.
Some fungicides registered for Pythium include trichoderma harzianum (RootShield), (biological fungicide when disease pressure is low), potassium salts of phosphorous acid (Alude, Fosphite, FRAC Code 33), (works by stimulating the plant's natural defenses), fosethyl aluminum (Aliette, FRAC Code 33), fenamidone (FenStop, FRAC Code 11), propamocarb (Banol, FRAC Code 28), thiphanate methyl + etidiazole (Banrot 40 WP, FRAC Codes 1+14), mefenoxam (Subdue Maxx, FRAC Code 4), etridiazole (Truban, Terrazole 35 WP, (FRAC Code 14) and cyazofamid (Segway, FRAC Code 21).
Apply treatments as a drench, following label directions. FRAC Codes are provided for disease resistance management. After application, irrigate with additional water to move the fungicide into the root system. Note that about half of most isolates received at the UMass diagnostic lab over the past few years have been resistant to Subdue.
For more information on Pythium see the New England Recommendation Guide .
Fact Sheets: Disinfecting the Greenhouse , Root Diseases of Greenhouse Crops
Article: Pythium Root Rot by Gary Moorman and Margery Daughtrey
Tina Smith, UMass Extension
