See:
Greenhouse Plants, Ornamental - Pythium Seed Rot, Damping-off, and Root Rot
Cause The OSU Plant Clinic has found several oomycetes associated with root rots of hydrangea including Phytophthora spp. and Pythium spp. These diseases are generally found where plants are in cold wet soils and/or overwatered. Poorly draining potting media can also favor these diseases.
Symptoms Samples with oomycetes have displayed root rots, root crown and stem rots, and shoot diebacks. Leaves may turn chlorotic before wilting, becoming necrotic and may not fall from dead plants. A reddish-brown discoloration can be found in the roots or root crown of plants. There may be a line between discolored and healthy tissue.
Without good root development iron chlorosis may be an early symptom.
Cultural control
- Do not overwater plants.
- Blue cultivars should be grown at a pH of 4.5 to 5.0 and pink cultivars at 5.5 to 6.0. Aluminum sulfate is needed for the blue color development but additions to the soil may tie up needed phosphorus.
- Provide good drainage for plants in beds, fields, or containers. Place containers on gravel beds (4 inches or more deep) to allow drainage. Do not place containers on poly sheets; they can prevent containers from draining into soil and allow contaminated drainage water to spread from the base of one container to another.
- Avoid reusing pots 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 Use when new root growth is expected. The Group 4 and Group P7 fungicides have been useful for these diseases on other crops but none specifically list hydrangea on the label. They can be legal used but test them on a few plants before widespread use.
- Empress at 1 to 3 fl oz/100 gal water can be used for cuttings or seedlings. Group 11 fungicide. 12-hr reentry.
- Segovis at 1 to 3.2 fl oz/gal water for drench applications to container or 1.2 to 9.15 fl oz/25 gal water for field use. Group 49 fungicide. 4-hr reentry.
Reference Baysal-Gurel, F., and Kabir, M.N. 2019. Evaluation of fungicides and biocontrol products for the control of Phytophthora root rot of hydrangeas. Archives of Phytopathology and Plant Protection 52:481-496.