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.
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.
by Dr Dan Blanchon from Unitec's Diploma in Sustainable Horticulture and Bachelor
of Resource Management.
with permission from NZOOM Home and Garden content,
from the previous
The views expressed here are not necessarily those of the RNZIH