At this time of year when lavender is being harvested across Ontario, spots on lavender leaves and stems can significantly affect the quality of products or the appearance of lavender for farm festivals and tours. Not all spots are created equal though, and recognizing the symptoms is important for management in the future. There are two main causes of spots on lavender in Ontario: four-lined plant bug (FLPB) and Septoria leaf spot.
FLPB is a sucking insect that feeds on new leaves and developing stems from late May to early July. Nymphs are red and black and colour and move rapidly on the plant, often hiding when the plant is approached (Figure 1). Adults are 7 to 10 mm in length and yellow/green with four black stripes running down their back (Figure 2). There is only one generation per year and adults die off quickly after maturity. As a result, there is no additional damage beyond mid-July.
Figure 1. Four-lined plant bug nymph.
Figure 2. Four-lined plant bug adult.
FLPB adults and nymphs feed by sucking juices out of the new leaves and developing stems. A small circle of the leaf dies where they feed. Eventually the leaves and stems are covered in uniform spots that are white or light brown/grey in appearance (Figure 3). Feeding damage has minimal effect on lavender flower yield, but can have a significant effect on quality of flower bundles.
Figure 3. Damage from four-lined plant bug on leaves and stems. Lesions on stems are often elongated.
Septoria leaf spot is caused by the fungus Septoria lavandulae. The disease usually appears on weakened plant material in the late summer and fall as dew periods become extended and leaves remain wet for long periods. With the recent humid and wet conditions, the disease has been found in lavender much earlier than normal. The disease also causes round spots on leaves similar in size to those caused by FLPB (Figure 4). The disease spreads by wind-blown spores, infecting new lavender leaves if there is a sufficient wet period. Little is known about the impact of Septoria leaf spot on lavender, but it is likely that plant growth and subsequent flower yield will be impacted if it occurs early in the season due to diminished photosynthetic capacity.
Figure 4. Septoria leaf spot lesions.
Now that the two pests are occurring at the same time of year, they can be difficult to distinguish. However, there are several differences between them (Figure 5). Septoria leaf spot does not often appear on stems. Since the Septoria lives within the leaves, lesions enlarge over time and lesion size is irregular, whereas FLPB lesions are generally uniform in size. Septoria lesions are often surrounded by a yellow or pale green halo, and affected leaves begin to turn yellow and eventually die. By contrast, leaves damaged by FLPB usually remain green. The two lesions can also be distinguished by colour. Septoria leaf spot lesions are dark brown/grey sometimes with an inner grey circle. FLPB damage is uniform white or light grey, sometimes falling out partially or entirely, leaving a hole in the leaf.
Figure 5. Comparison of lesions. Top: four-lined plant bug, Bottom: Septoria leaf spot. Unlike FLPB, Septoria lesions are dark in colour, variable in size and often surrounded by a yellow halo.
There are no products registered for the control of either FLPB or Septoria leaf spot in lavender. However, Septoria leaf spot severity can be reduced through cultural management practices. Since the disease mostly affects weakened plants, keeping plants healthy is important for minimizing disease. Plants at this time of year are naturally weakened by devoting energy to flower production. Avoiding additional stresses on the plant includes ensuring adequate fertilization and preventing ponding of water in the field or saturated soil conditions which can restrict the root system. The disease is also promoted by high humidity and leaf wetness. Lavender plantings should be located in open areas where there is good air flow. Plants should be adequately spaced to ensure good airflow through the canopy. Planting rows in an east/west orientation will also improve airflow down the rows. However, this should be balanced with the need for good wind protection over the winter and good drainage out of the field. For both pest issues, damage can be higher on certain cultivars. Growers should monitor different cultivars for damage from both pests and use less susceptible cultivars for new plantings. Ratings will be conducted in the provincial lavender cultivar trials to identify less susceptible cultivars.