Plant Doctor Archive

Bay tree blues

Question DUNEDIN sometimes is not the easiest of places to grow herbs so I have been most proud of my bay tree which has been battling the elements and coming through with flying colours. However, I have noticed that it has become ill. Of the three main branches, two have leaves on it which seem to have paled. There are indistinct yellow splotches (23 mm) covering the entire leaf except at the base which looks normal. Some, but not all, have tiny black spots on them. Apart from this, the plant overall is healthy and has a quantity of new shoots emerging. I surmised that this could be one of a few things:

  1. A mineral deficiency. In response to this I have been giving it feeds of worm liquid and minerals;
  2. An infestation of thrips or black spot?
  3. The cat has been using it for oneses.

Answer THEORETICALLY the Bay or Sweet Laurel (Laurus nobilis) should grow well in Dunedin, as long as it is protected from frosts and strong winds. It sounds like you have been doing all the right things if it has been growing so well for so long. However, the symptoms you describe suggest that all is not well with your bay tree. I think it is unlikely that a mineral deficiency is causing the problem, and your addition of worm liquid should deal with that anyway. It is more likely to be a disease or insect pest. I can think of several possibilities:

The first is an attack from scale insects. They usually cause yellow blotches on the leaves and stems and you see small brown, black or grey bumps on the undersides of the leaves and stems. If there are only a few, then gently wipe them off with a cloth or cotton bud dipped in soapy water. Destroy any that you remove. If it is a heavy infestation, then you could spray with white oil or an insecticide which specifically targets scale.

Another possibility is thrips, as you suggested. The symptoms don’t quite sound like thrips, which usually cause silvering of the leaves and leave their “poos” behind as deposits on the leaves. Treatment is usually by insecticide, such as carbaryl or diazinon.

Sometimes discoloured leaves are caused by a disease. One possibility is mildew, but this usually is visible as tiny grey threads. Treatment is by an appropriate fungicide. There is also an algal leaf spot, which is quite common in some native plants and will attack bay trees. This is usually found in light-supressed plants, so check that your bay tree is getting enough sun. It would also be a good idea to make sure that your tree has good drainage as bay laurels do not like wet feet.

Good luck with your tree, and remember, if using sprays read the instructions and follow them, and do not use any leaves from the tree for a period of time.