Plant Doctor Archive

Bugs on rhodies

  My rhodies are growing back from being trimmed down last year, but the leaves are being chewed up and have brown spots on them. What kind of spray is the best to keep rhodies bug free? Is there something natural you can make?

The brown spots sound very much like an attack of thrips, which are very common on rhododendrons in New Zealand.  If the leaves are also silvery-looking, and the brown spots mainly on the underside, then it does sound like thrips are the culprit.

I don't know of any effective "natural" remedies, but any insecticidal spray which mentions thrips should do the trick.  Liquid diazinon or orthene both work well.  Make sure you spray the underside of the leaves and into buds and any other nooks and crannies.  Read the instructions carefully to maintain safety and to protect bees.  You may also need to spray any neighbouring rhododendrons or camellias, which also get thrips.

If the problem is not thrips, then it could be a fungal problem.  In that case a fungicide or formulation such as Shield could be effective.

Make sure that your rhododendrons are as happy as possible — are they in the right spot? Is the soil a nice acidic well-drained but moist humus? If not, you may need to think about changing the site or soil to make sure your plants are as strong and disease-resistant as possible.

Advice by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor of Resource Management.

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