Plant Doctor Archive

Unhealthy rose

I have a rose with beautiful white flowers but as soon as they open out they seem to die back as far as the main stem. Sometimes a whole bunch goes a once. This particular rose bush is in a garden with seven others but are all different varieties and are very healthy.

The symptoms you describe could be one of many diseases that affect roses. Different cultivars have different levels of resistance, with old varieties often the most vulnerable to attack. This may explain why your other bushes are not affected.

From your description, with whole bunches of flowers dying at once, I suspect you have a case of either Botrytis Blight or Powdery Mildew. Both fungal diseases can cause damage and death of the buds and flowers. Botrytis is the likeliest culprit, usually occurring during periods of wet weather. Flower buds often fail to open and the flowers rot in whole bunches, being covered in a grey-brown fuzz. In severe cases the disease can kill entire branches. Powdery mildew usually attacks younger leaves and buds, distorting flowers and covering the plant in a cover of white fungus. Powdery mildew is more likely with high night time humidity and dry days.

Both diseases can be controlled by fungicides and some simple garden hygiene rules. You need to prune infected shoots back to healthy tissue and destroy the resulting diseased twigs. Rake and remove any fallen leaves as the fungi over-winter in dead twigs and leaves under the plant. A good dose of lime sulphur in early winter should kill most fungal spores. Follow this when the new leaves appear with a garden fungicide at regular intervals (look for one which mentions Botrytis). Remember to follow the instructions carefully.

Next time you are planting new rose bushes, choose disease resistant varieties if possible, plant in a sunny site and allow enough space around the bush to provide good air circulation and reduce humidity. This will hopefully mean fewer problems.

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

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