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What are those grey circles on my lavender leaves? Septoria Leaf Spot of Lavender

Lavender is typically thought of as a hearty plant with few pest problems when grown under the proper conditions. While this is generally true, lavender is not completely immune to insect and disease problems. Recently, we have observed greyish, circular lesions on the leaves of a number of varieties of lavender in southern Ontario. The culprit is a fungal disease called Lavender Leaf Spot.

What is it?

Also known as Septoria leaf spot, this disease is caused by the fungus Septoria lavandulae. It is only known to attack members of the lavender family including the common angustifolia varieties as well as the lavendin hybrids and has been reported from Africa, Europe and North America. The disease is spread by spores which are moved from infected bushes and plant debris by wind or by splashing during heavy rains. The fungus overwinters on lavender tissues and new spores are produced any time during the growing season. The disease is promoted by long and repeated periods of leaf wetness and high humidity, which are required for the spores to germinate and infect leaves. As a result, it is more commonly a problem in the late summer and fall. However, the disease is known to affect lavender at any time during the year in Europe, and it is likely that extended wet periods in the spring and summer can result in significant disease early in the season.

What does it look like?

Lavender leaf spot is primarily found on older leaves lower on the stem or in protected and shaded areas of the canopy. It first appears as small spots close to the leaf edges that eventually grow in size and may coalesce to form larger, irregular shaped lesions. The spots are 2-5 mm in diameter and appear grey with a purple/brown border (Figure 1). Eventually, affected leaves fall off the plant. In addition, weakening of the plant as a whole will occur and cause premature defoliation, even in less affected parts of the plant. As a result, diseased plants appear to have a thinner canopy and may be stunted compared to healthy plants. However, close examination of the plants is required to identify the disease, since it can be difficult to notice from a distance (Figure 2).

Figure 1. Lavender leaf spot appears as small circular lesions with a grey centre and purple/brown border mainly on older leaves.

Figure 2. Although this plant looks healthy, many of the older leaves are covered in lavender leaf spot.
Closely inspect all new plants to ensure they are free of disease before planting them in your field.

What does it do to lavender?

Depending on the time of year that the disease becomes established on the foliage, lavender leaf spot has the potential to cause considerable damage. Due to premature defoliation, the disease can retard the growth of the plant. One research study in Europe found that infected plants produced 24% fewer flowers and 13% less oil than healthy plants. Some research even suggests that the disease can also change the quality of the essential oils. We do not know how the disease will affect plant growth, flower yield or oil quality in Ontario.

What can be done?

There is not much that can be done to eliminate the disease once it is established in your field. Therefore, the key to managing lavender leaf spot is to avoid getting it in the first place. This can be achieved by appropriate site selection and by culling out diseased plants before planting them in the garden.

The best approach is to avoid moist conditions which promote infection and spread of the disease. Increasing air movement through lavender fields is very important. Consider orienting beds in the direction of prevailing winds and spacing plants far enough apart to allow room for air movement. Avoid planting lavender in protected locations with limited airflow, such as adjacent to woodlots or hedges, and try to choose sites with exposure to full sun. Any straw put onto beds for winter protection should be removed as early as possible in the spring to allow for rapid drying.

Research in Europe has found that, although no varieties have been identified as being resistant to the disease, the cultivar 'Munstead' may be the most tolerant. However in Ontario we have seen the disease on 'Munstead'. Growers evaluating lavender varieties under Ontario conditions may find it useful to record incidence and severity of this disease among the different varieties they are trying.

It is important to carefully inspect all new plants for lavender leaf spot, and other pests, before introducing them to your fields. If possible, keep new plants separate for a week or two and re-inspect before planting.

In a nutshell, while at present this disease does not seem to be having a serious impact, it does serve as a reminder that pests can impact lavender and should emphasize the importance of scouting for pests and employing preventative tactics.

If you see this disease or other pests on your lavender plants, please contact Melanie Filotas, OMAFRA's Integrated Pest Management Specialist, Specialty Crops at 519-426-4434 (email: melanie.filotas@ontario.ca) or Sean Westerveld, OMAFRA's Ginseng and Medicinal Herbs Specialist at 519-426-4323 (email: sean.westerveld@ontario.ca).