See:
Greenhouse Plants, Ornamental - Pythium Seed Rot, Damping-off, and Root Rot
Cause Pythium ultimum, a fungus-like organism that is a common soilborne pathogen. The disease can be severe in overwatered soils or media with poor drainage. It survives unfavorable periods in soil and infected plant debris. Under favorable conditions, such as cooler soil-media temperatures, spores germinate and infect roots. Crowns and leaves or petioles in contact with the media can also be invaded. Leaf cuttings are also susceptible. After infection, the fungus-like microorganism spreads mainly in the inner tissues of the root and stems. A thick-walled survival structure, oospore, are produced that can persist for several years. Movement of infected plants and/or soil can spread this fungus-like microorganism. African violets are more susceptible when grown in high intensity light for prolonged periods or when infested with root knot nematodes. Most cultivars are susceptible.
Symptoms Plants suddenly wilt and die. Sometimes only the lower leaves and petioles become necrotic and dry. Plants are easily separated from the pot and media when the crown has decayed. Roots and root crowns become brown, soft, and mushy.
Cultural control
- Sterilize rooting media and potting soil before planting. Also disinfect any tools and equipment that might be used and contaminate the media.
- Avoid overwatering but keep soil evenly moist by sub-irrigating. Let plant get dry between waterings when not flowering.
- Destroy diseased plants.
- Fertilize only when plant is beginning to flower.
- 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 Soil drenches are useful before disease develops. Best used in conjunction with cultural control methods. Rotate fungicides from different groups that have a different mode of action for resistance management.
- Banol at 2 to 3 fl oz/10 gal water. Group 28 fungicide. 12-hr reentry.
- Banrot 40 WP at 6 to 12 oz/100 gal water. Group 1 + 14 fungicide. 12-hr reentry.
- Mefenoxam 2 AQ at 0.49 to 0.98 fl oz/100 gal water as a soil drench. See label for media incorporation. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
- •MetaStar 2E at 0.5 to 2 fl oz/100 gal water as a soil drench. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
- Proplant at 2 to 3 fl oz/10 gal water. Group 28 fungicide. 12-hr reentry.
- Subdue MAXX at 0.5 to 1 oz/100 gal water as a drench. 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.
- Truban 30 WP at 3 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 Thompson, H.S. 1958. Pythium rot of Sainpaulia, the African violet. Canadian Journal of Botany 36:843-863.