See:
Greenhouse Plants, Ornamental - Pythium Seed Rot, Damping-off, and Root Rot
Cause Several species of Pythium can rot Delphinium species as well as many other bedding and garden plants. It has been found by the OSU Plant Clinic. All Pythium species are strongly favored by abundant moisture and can be especially problematic in poorly drained potting media and planting sites. Pythium usually attacks seeds or seedlings, however, older plants may also be affected, especially if the soil is too wet. Pythium spp. can survive in dust, planting media, or soil particles on greenhouse floors, flats, and pots. Spread can also occur via movement of infected plants. Damaged plants are more easily infected. The disease is usually favored by cool, moist conditions in the field and wet waterlogged soils.
Symptoms Often the first symptoms are yellow and brown leaves. The plant may be stunted and, at more advanced stages, may wilt. Inspection of the collar area of the stem, at and just below the soil line, reveals brown, water-soaked lesions. The lesions may be limited or girdle the whole stem. Diseased roots are brown, often starting at the root tips. The cortical layer of rotted cells can easily be pulled away from the white vascular tissue. If roots are pulled when the rot is extensive, the cortical tissue may have already sloughed off, leaving the white vascular tissue.
Cultural control
- Plant in a well-drained site.
- Plant in raised beds if the site is poorly drained. A small grade of composted bark can be incorporated in the soil of the raised bed to improve aeration. Also, put in drainage tile to facilitate soil drainage and direct water away from the site.
- 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 from different groups that have a different mode of action for resistant management.
- 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.
- MetaStar 2E at 0.5 to 2 fl oz/100 gal water. 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
- OxiPhos at 1.3 to 4 quarts/100 gal water as a foliar spray. Group P7 fungicide. 4-hr reentry.
- Subdue MAXX at 0.5 to 1 fl oz/100 gal water and apply 1 pint/sq ft of solution. 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.
Reference Bloch, E.D., Raabe, R.D., and Hurlimann, J.H. 1976. Control of Pythium root rot of larkspur. Plant Disease Reporter 60:600-601.