Primrose (Primula spp.)-Pythium Root Rot

See:

Cause Fungus-like microorganisms, Pythium spp., are a problem in cool, poorly drained soils or media. As a rule, these plants tend to be susceptible to overwatering. Although not formally reported from the Pacific Northwest, they have been found as a stem rot on primrose several times by the OSU Plant Clinic. May be on plants at any time in the production cycle causing damping-off to root rot.

Symptoms Older leaves may turn yellow, wilt and die. Entire plants may wilt suddenly. Stems, leaf bases, and/or roots may be brown, soft, and mushy.

Cultural control

  • Use a well drained soilless media with a pH of 5.5 to 5.8.
  • Do not overwater.
  • Remove and destroy diseased plants.
  • Improve drainage of soil or media before replanting.
  • Avoid reusing pots or trays from a previous crop for propagation. If pots must be reused then wash off all debris and soak in a sanitizing solution or treat with aerated steam for 30 min.

Chemical control Rotate fungicides that have a different mode of action for resistance management.

  • Alude at 5 to 10 fl oz/100 gal water applied as a soil drench at a rate of 25 gal solution/100 sq ft. Follow application with irrigation. Use only once per month. Can also be used as a foliar spray at 26 to 54 fl oz/100 gal water at 14- to 21-day intervals. Group P7 fungicide. 4-hr reentry.
  • Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
  • Mefenoxam 2 AQ at 0.12 to 0.24 fl oz/100 gal water as a soil drench at seeding, at 0.49 to 0.98 fl oz/100 gal water at transplanting; see label for media incorporation. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
  • MetaStar 2E at 0.25 to 0.5 fl oz/100 gal water as a soil drench at seeding, at 0.5 to 2 fl oz/100 gal water at transplanting, and see label for media incorporation. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
  • Organocide Plant Doctor at 1 teaspoon/8 gal water as a soil drench. Group P7 fungicide. H
  • Subdue MAXX at 0.13 to 0.25 fl oz/100 gal water as a soil drench at seeding; apply 1 pint of solution per sq ft. Or at 0.5 to 1 fl oz/100 gal water as a soil drench at transplanting. Group 4 fungicide. No restrictions on reentry when used as a soil drench.
  • Terrazole 35 WP at 3.5 to 10 oz/100 gal water. Group 14 fungicide. 12-hr reentry.

Biological control

  • Prestop (Gliocladium catenulatum strain J1446) at 0.03 to 0.33 oz/2.5 gal water as a drench. Do not use with other products in the tank. 4-hr reentry. O

Reference Ivimey Cook, W.R. and Collins, W.B. 1937. A Pythium wilt of primula caused by Pythium spinosum, sawada. Transactions of the British Mycological Society 21:29-33.