Cause The OSU Plant Clinic has found Fusarium, Phytophthora, Pythium and Rhizoctonia associated with rotting roots of lavender. These pathogens, including Fusarium avenaceum, P. megasperma from field grown plants and P. nicotianae from nursery stock, have also been found in Washington. In the eastern USA, many different Phytophthora species have been found with P. nicotianae and P. palmivora as the most common. They are typically found in cold wet soils where lavender does not do well. Overwatering is another problem that encourages these diseases.
Symptoms Roots develop a discolored vascular system and rot. Aboveground plant parts wilt and dieback. Foliage may be off color at first before it collapses. In general, root rots due to Pythium do not move up into the stem area like Phytophthora does.
- Plant lavender in sunny locations with well-drained soils.
- Shading may be necessary to manage Phytophthora when growing plants in black plastic pots.
- Do not incorporate high levels of organic matter that retain moisture around plants.
- Do not overwater.
- Does best at a pH of 5.8 to 7.0.
- Keep plants off the ground for nursery production.
Chemical control The following materials may be helpful before symptoms develop especially if Pythium or Phytophthora are involved. Focus on cultural controls first.
- Adorn at 1 to 4 fl oz/100 gal water plus another fungicide. Group 43 fungicide. 12-hr reentry.
- Alude at 26 to 54 fl oz /100 gal water at 14- to 21-day intervals. Can also be used as a soil drench at a rate of 5 to 10 fl oz/100 gal water. Use 25 gal solution/100 sq ft. and follow application with irrigation. Use only once per month. Group P7 fungicide. 4-hr reentry.
- Fosphite at 1 to 3 quarts/A. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
- Organocide Plant Doctor at 1 teaspoon/8 gal water as a soil drench. Group P7 fungicide. H
- Reliant at 2 qt/A as a foliar spray. Group P7 fungicide.
- Segway SC at 1.5 to 3 fl oz/100 gal water. Drench rate is based on pot diameter. Group 21 fungicide. 12-hr reentry.
- Subdue MAXX at 1 oz/100 gal water as a drench. Group 4 fungicide. No restrictions on reentry when used as a soil drench.
Reference Álvarez, L.A., Pérez-Sierra, A., Armengol, J., and García-Jiménez, J. 2007. Characterization of Phytophthora nicotianae isolates causing collar and root rot of lavender and rosemary in Spain. Journal of Plant Pathology 89:261-264.
Guarnaccia, V., Hand, F. P., Garibaldi, A. and Gullino, M. L. 2021. Bedding plant production and the challenge of fungal diseases. Plant Disease, 105:1241-1258.