Lavender is in full bloom across the province. Bloom was late to arrive this year, but may progress faster than normal due to the above-normal temperatures. Although growers are focused on many activities at this time of year and the lavender itself can be an afterthought, it is important to keep an eye on the crop to ensure it remains healthy and can recover quickly after bloom. Here are some things to watch out for:

Moisture Stress

While lavender can tolerate dry conditions, it does not necessarily thrive under these conditions. When plants are in full bloom, they have the highest water needs of any time of year. This is the stage when plants are at their largest and tender flower petals lose moisture faster than any other parts of the plant. The consequences of insufficient water at this time of year can include a shorter bloom period, premature wilting of the flower corollas (petals), and drooping during the afternoon. All of this can reduce the appearance of the lavender for agritourism and the duration of the season.

It is estimated that lavender plants require around 20 mm of water every week during the peak of summer for optimal growth, which is about half of what most crops need. Many areas of the province have received sufficient rainfall, but some areas are starting to become dry. Are you thinking of watering by hand rather than using an irrigation system? The estimated requirement of 20 mm per week translates to 200,000 L of water per hectare (80,000 L/ac). If you have a 500 L water tank, that is 160 tanks full per week to water 1 acre, if the lavender occupies the full acre.

Insect Pests

Some growers are noticing yellow sections on stems and spotting on leaves, without any insects present. This is the leftover damage from four-lined plant bugs. This pest only has one generation per year, and by now, the adults are finished feeding for the year. Management for this pest must occur in late May at the early nymphal stages. Note where the damage is problematic. You can expect worse damage in that area next year without some intervention, such as a spray of insecticidal soap.

Garden fleahoppers are another significant insect pest of lavender. They have several generations per year, and populations tend to increase through the season, peaking in August. They can lead to nearly complete defoliation of the plants. Monitor populations regularly by placing a white piece of paper under a plant and then shaking the plant vigorously. The insects will jump out onto the paper where the black adults and green nymphs can be easily counted. If you monitor the same way every week, populations can be tracked over time.

Spittlebugs are the nymphs of froghoppers, which are in the same order of insects as garden fleahoppers and four-lined plant bug. The nymphs suck sap from the stem and create a foam-like protective barrier around themselves to hide from predators and prevent desiccation (Figure 1). At this time of year, there is nothing that can be done to control spittlebugs on lavender. It may be possible to get some control when they first emerge, before they create their foam barrier. This is likely around the same time that four-lined plant bugs are emerging in May. An insecticidal soap application may suppress populations of all three sucking insect pests of lavender at that time. However, this has not been tested for managing spittlebugs.

Figure 1. A spittlebug on lavender.

For more information and photos of these pests, consult the Lavender Module of Ontario CropIPM.

Disease Samples Needed

Any root or stem disease issues that did not show up at green-up in the spring usually begin showing new symptoms during bloom. This is because they restrict water uptake, and the water needs of the plant are the highest during bloom. As I have mentioned in previous posts, I have been trying to diagnose an unknown disease issue that plagued my lavender research plot in Simcoe for its entire 6-year life. The disease is characterized by small branchlets scattered through the canopy progressing through the following stages: pale green, bright yellowish-green, wilting, rich brown dead growth, and grey (Figure 2). Often, each of these stages is present on a particular plant. The disease develops slowly, and often, the new growth can replace the dead growth. The plant becomes sparse and stunted over time. Several growers have reported similar symptoms over the past few years.

Figure 2. Mystery disease symptoms on lavender include a progression from pale green to yellow to wilting to death of small branches scattered through the canopy.

If you suspect you have this symptom and you live in Ontario, please send me an email with a photo at sean.westerveld@ontario.ca. We have funding through OMAFA to diagnose this disease and need new symptoms from which to isolate pathogens.