| Missouri Environment and Garden |
Volume 13, No. 7 |
| News for Missouri's Gardens, Yards and Resources |
July 2007 |
Black Spot on Roses
The humid weather this spring has taken a toll on roses in the garden. Many
gardeners have roses that contain hardly any leaves or the blooms are
greatly reduced, and wonder what is wrong with the roses.
Black spot, the most destructive disease on roses, is a fungal disease that
is most severe during wet and humid weather, as a wet leaf surface is
essential for spread of the disease. Black spot appears as black spots or
rings on the leaf surface and many times the black spots have a yellowish
halo around them. The affected leaves then turn completely yellow and drop
from the plant. If severe, this defoliation will weaken the plant, reduce
flowering and make the plant more susceptible to winter damage.
Black spot is difficult to control as sprays are mostly preventative. The
chemical must be on the leaves before the disease gets started. Once black
spot spores germinate on the leaf, the infection cannot be reversed. To
achieve protection takes diligence with weekly spraying during wet periods,
and can be quite a chore that has led many rose gardeners to give up.
Hybrid tea roses are the most susceptible to black spot. In recent years,
rose breeders have directed their efforts toward black spot resistant roses
and there are now many available. Modern shrub roses can tolerate black
spot and still bloom well despite it. There are now hybrid tea roses that
have improved black spot resistance. Keeping roses healthy will help them
resist black spot. It is important to provide roses with at least 6 hours
of sun daily and space them far enough apart to provide good air
circulation to enable leaves to dry and not have moisture sitting on the
leaf surface. If possible, it is good to avoid wetting the leaves when
watering. It is also important to remove and destroy any diseased and
fallen leaves, and pick off infected leaves as soon as you see black spots
developing. The fungus is carried over-winter by dead leaves on the ground
and infections on the stem. Rain and splashing water then spread the spores
from old leaves or stems.
For more information, refer to MU Guide G6601 – Roses: Care After Planting
(
http://muextension.missouri.edu/explore/agguides/hort/g06601.htm).
Mary Kroening
KroeningM@missouri.edu
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