See:
Greenhouse Plants, Ornamental - Pythium Seed Rot, Damping-off, and Root Rot
Cause Pythium ultimum, a fungus-like organism that survives long periods in the soil with a resting structure called an oospore. Excess water and flooding conditions favor the disease as well as cooler soil-media temperatures. As a rule, these plants tend to be susceptible to overwatering.
Symptoms Stems near ground level of established plants become darkened and water-soaked and collapse. Seedlings may simply die.
Cultural control
- Remove and destroy infected plants, increase ventilation, and avoid moistening leaves by watering.
- Sterilize soil or media before replanting. Also disinfect any tools and equipment that might be used and contaminate the media.
- Do not overwater, and keep water-hose ends off the ground.
- Be careful not to bury begonia seed too deeply when planting.
- Do not overfertilize. Use a low, constant feeding with ammonium fertilizer. Check the electrolyte concentration of the medium regularly.
- 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 For bedding plants. Rotate fungicides from different groups that have a different mode of action for resistant management.
- Aliette at 1.25 to 4 lb/100 gal water. Do not use with adjuvants. Group P7 fungicide. 24-hr reentry.
- Areca at 1.25 to 4 lb/100 gal water. Group P7 fungicide. 12-hr reentry.
- Banrot 40 WP at 6 to 12 oz/100 gal water. Group 1 + 14 fungicide. 12-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, and 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.
- Monterey Garden Phos at 1 teaspoons/8 gal water as a soil drench. Group P7 fungicide. H
- Subdue MAXX at 0.5 to 1 fl oz/100 gal water as a soil drench. Use 1 pint of solution/sq ft at time of 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 as a soil drench. For use only in commercial greenhouses. Group 14 fungicide. 12-hr reentry.
- Truban 30 WP at 4 to 6 oz/100 gal water for 800 sq ft of bed area. Group 14 fungicide. 12-hr reentry.
Reference Bolton, A.T. 1981. Specificity among isolates of Pythium splendens from geranium, chrysanthemum and rieger begonia. Can. J. Plant Pathol 3:177-179.