Plant Doctor Archive

Scale and brown tips

I HAVE two problems. My lemon tree has soot-like build up on the leaves and seems to be lacking the usual number of flowers for this time of year. Also the new leaves on my hydrangea are curved and deformed and some have brown marks on the ends. Any ideas?

THE sample you sent shows that your lemon tree is infested with scale, the excretions of which encourage the black sooty mould growth on the leaves. The easiest way to solve the scale problem is to spray the whole tree thoroughly with an insecticidal oil such as Conqueror, Clear White Oil or the canola-based Eco-oil. Make sure to get the spray on as much of the foliage as practicable, particularly the undersides of the leaves where many of the scales are sheltering. Avoid spraying in hot, sunny weather or you run risk of burning the foliage - early morning on a cloudy day is ideal. Once you get rid of most the scale the sooty mould will shrivel up and drop off.

Don't worry if you don't get rid of all the scale, as most lemon trees produce quite good crops even with a moderate population of scale on them. And give the tree a good feed with citrus fertiliser to encourage new growth and better flowering.

The hydrangea sample was not in very good condition, but I suspect the distorted growth and brown tips may have been caused by some adverse environmental condition such as a particularly cold biting wind, a drying wind or perhaps a light frost which has damaged the new growth. If that's the case then the plant should quickly recover and subsequent new growth should be fine.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.