Lavandula angustifolia cultivars are in full bloom in most of Ontario, with lavandins starting to bloom further south. Some cultivars are already past peak, progressing through bloom faster due to the hot conditions. It is becoming very dry in most of southern Ontario, except for localized areas that have received thunderstorms. During full bloom, the moisture needs of lavender are the highest and restricted moisture can cause petals to not fully open or wilt in the middle of the day, reducing the appearance of the lavender for agritourism. Irrigation will help reduce this stress and is important for ensuring a quick transition to vegetative growth after bloom.
Some growers have noted either yellowing of sections of stems or leaves, or brown patches on the leaves and down the stem with curving or malformation of some flowers. Bright yellow patches are probably due to Alfalfa Mosaic Virus (Figure 1). The virus appears to be transmitted by leafhoppers from nearby crops. Although the damage is unsightly, the virus does not appear to cause much stunting of flowers or plants. The symptoms appear under certain weather conditions and then may not appear at all in some years. Control of leafhoppers in nearby crops and frequently sterilization of pruning/harvesting equipment are the only effective means of preventing the spread of the disease. There are no control options once the plant is infected.
Brown patches on stems and leaves are usually leftover damage from four-lined plant bug (FLPB) (Figure 2). The adults are long gone for the year, so there are no control options until next spring. When pruning plants this summer, remove the leftover debris from the field, since the eggs of FLPB are inserted into the side of green/soft stems. Removal of the debris may remove considerable numbers of egg masses from the field. Keep an eye on plants beginning in mid-May next year to catch the hatch of nymphs, which is the best time to try insecticidal soaps as a management option.
Many growers are also noting a range of strange and sometimes severe symptoms this year that have not been seen before:
Collapsed Plants or Branches
Growers with lavender on black ground cloth, including within my own lavender plot, have noticed a sudden collapse of parts or all of the plant in the middle of bloom. Affected plant parts turned brown and died rapidly over a matter of a day or two (Figure 3). The reports of this damage so far have all been on two-year old plants. The most pronounced symptoms are on plots with solid plastic between rows. The cause of these symptoms is unknown, but similar symptoms have not been reported for growers with mostly grass between rows and minimal ground cloth showing. Certain cultivars appear to be more affected than others.
Since we have not seen this damage before in other hot summers on the same age of plants on black plastic, it is unlikely to be directly due to the recent hot temperatures and the additional heat of the black ground cloth. It is possible that the November or May cold periods caused damage to the crown of the plant either because the black plastic may be colder at night, or due to fluctuations in temperatures caused by heating during the day in the sun followed by cold at night. If this is the case then the damage could have blocked movement of sugars from the leaves down to the roots, which would have killed the roots at a time when moisture demands were high.
The symptoms are very similar to those caused by Phytophthora root rot, and that disease cannot be ruled out, although the lack of rainfall this summer would not be ideal conditions for the disease. Plants will be submitted for Phytophthora testing. In the current cultivar verification trial, at least 40% of plants are affected so far, with about 20% that are unlikely to recover.
Multiple other growers are reporting plants of all ages with weak and/or variable bloom. Although plants looked healthy coming out of the winter, blooms are both sparse and short compared to normal. Sometimes this damage is variable with several plants in a row healthy and then many more in a row that are stunted. More stunting may be occurring on the north side of plants in some cases, suggesting an effect of the cold. Since these symptoms only appeared this year, soil diseases or other pathogens are less likely to be the issue, since they would normally develop more gradually over time in most cases and these symptoms are not consistent with any of the known diseases of lavender.
It is possible there was damage during the May freezes when the flower buds were initiating within the shoots. If so, the plant may have initiated secondary buds within the shoots, which would have been weaker when they emerged. However, it is unclear how a whole plant could be affected next to a whole plant that is unaffected if it was cold directly acting on the buds. Soil samples will be collected to determine if this could be disease related.
If you are experiencing either of these symptoms this year please email me (firstname.lastname@example.org) along with a description of your ground cover, the cultivar(s) affected, the age of the plants and when symptoms were first noted. Your observations could help us determine the cause of the problem and how to prevent it from happening in the future.