Most lavender growers know that lavender does not like wet feet, which means that roots cannot sit in saturated soils for long periods of time. In most of Ontario, the last few weeks have been very wet. Prolonged periods with wet soils can cause problems for lavender, even if planted in well-drained soils. Last year, the first spreading root rot of lavender was identified in a low area of a lavender field (Figure 1). A subsequent study of this disease suggested the water mould (oomycete) Phytophthora cactorum is likely involved. This pathogen has swimming spores that can move through saturated soils very rapidly. Results of this study will be reported in a later article. Lavender growers have no chemical options for controlling soil-borne diseases, and therefore should be extra vigilant to reduce the chances of the disease developing in the field and spreading through the field.
Figure 1. A root disease of lavender spreading down the row. The plant at the bottom died first and the next two plants down the row were partially killed shortly after.
Preventing root diseases of lavender begins with proper site selection. Growers planning to expand their acreage should take the opportunity after a heavy rainfall to examine their fields and identify drainage issues. Lavender should not be planted in sites in which water sits after a heavy rain for longer than a few minutes. These sites are likely to be saturated for extended periods over the winter, when drainage is impeded by frost and snow cover. If planting in those sites cannot be avoided, consider modifying the site to remove the water rapidly such as the installation of a drain, drainage tiles, and or trenches. Planting lavender on raised beds in these areas can keep a portion of the root system out of the saturated zone.
Growers should constantly scout lavender plantings for signs of disease. If a plant or a portion of a plant suddenly collapses after sprouting normally in the spring, there is a good chance that a root disease is affecting the plant. Affected plants should be removed immediately and not replaced with a new lavender plant. The spot should then be flagged and neighbouring plants should be monitored closely for symptoms of the disease. If the disease spreads to neighbouring plants, it is a good idea to remove at least two plants in each direction from the initial plant, and avoid planting lavender back into that location. While this might be unsightly in the middle of a lavender field, it is a small price to pay for avoiding a disease epidemic.
It is important to avoid taking cuttings from areas containing any disease issues. Even if the tops are not affected, spores of the pathogen may have splashed onto the leaves and can be introduced into the potting soil. This could result in the introduction of the pathogen into different areas of the farm when transplanted back into the field. Likewise, even if the plant is purchased, do not plant into the field if there are any symptoms of disease.