When Pythium spp. shows up late in the crop cycle, there are few options for salvaging the crop. Once the fungus is managed, it takes time for roots to re-grow, and there may not be enough time for plants to recover prior to sale. Signs of infected plants are wilting and stunting. Roots are soft and decayed, sometimes extending up into the stem where it causes a canker. Looking closely, the rotted outer covering of the root slips from the central core. There are different species of of Pythium that can cause problems on poinsettias. Late season Pythium is often due to Pythium ultimum which is favored by cooler temperatures. In studies the disease was worst at 62F and caused no serious damage at 80F.
Pythium is a natural inhabitant of the soil and can survive there indefinitely as well as in debris in the greenhouse. Growers having re-occurring problems with Pythium, should review their overall production practices including fertilizing, watering and media handling. Over-watering and excessive fertilizer levels promote Pythium. Good sanitation is crucial for prevention. Keep hose ends off the floor and avoid contaminating growing medium by cleaning areas where plants are held, media is handled and plants are potted.
Suspicious plants can be diagnosed through your University diagnostic lab or through a private diagnostic lab. If Pythium is diagnosed on poinsettias now, run a soil test and use clear water if high soluble salts are indicated. There are several materials registered for Pythium including potassium salts of phosphorous acid (Alude, Fosphite, FRAC Code 33), which works by stimulating the plant's natural defenses, and the more traditional fungicides fosetyl aluminum (Aliette, FRAC Code 33), 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. After application, irrigate with additional water to move the fungicide into the root system. Note that many Pythium isolates are insensitive or resistant to Subdue.
Fact Sheet: Chase A. R. Pythium Root Rot on Poinsettias Chase Agricultural Consulting.
Photos: Poinsettia roots with Pythium,
Wilted and stunted plant - signs of root rot
Poinsettia roots with Pythium and recovering roots
Tina Smith, UMass Extension with input from Angie Madeiras, UMass Extension Plant Diagnostic Lab
