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This Information Was Obtained From The University Of Arizona
Cooperative Extension
Article by: LUCY BRADLEY, Extension Agent, Urban
Horticulture - cals.arizona.edu/pubs/garden/az1306.pdf
Troubleshooting Problems with
Roses in the Low Desert
Desert Southwest gardeners are
fortunate to have fewer insect and disease problems on roses
than most rose growers in other parts of the country. Some
advanced knowledge about potential problems will help you
recognize and manage problems before they get out of
control. Prevention and early intervention are the key to
healthy plants.
Insect Problems
Cane Borers
The first problem that might crop up in January is the cane
borer, an insect that tunnels into the cane soon after roses
are pruned in winter. Cane borers can be stopped by sealing
all newly cut canes with wood glue or other commercial
preparations. Their presence can be detected by a hole in
the end of the cane. Cut the cane back an inch or two at a
time until the hole is no longer visible. Seal the cane with
wood glue.
Aphids
When tender new growth appears on roses in February, aphids
are not far behind. The rose aphid is a soft-bodied pink or
light green insect that sucks the sap out of new growth and
buds. Aphids are best controlled with a forceful spray of
water from the hose. A second line of defense is to use a
soapy water spray (mix one tablespoon of dish detergent per
gallon of water). Spray when cool. Do not use a citrus-based
soap, and always spray one or two leaves and check their
reaction before you spray the entire plant. Spraying with
water or soapy water can be repeated daily, if necessary, to
control aphid populations. If you monitor the rose bushes
regularly and take action as soon as aphids start appearing,
it should not be necessary to use a chemical insecticide.
Beneficial insects such as lady beetles and green lacewings
will soon appear to feast on the juicy aphids.
Thrips
As the flower buds start to form on roses, look for thrips.
Thrips are slender, brownish-yellow winged insects, barely
visible to the naked eye. They hide inside newly opened buds
and munch on the flower petal edges, causing them to turn
brown. The damage is cosmetic and won’t harm the bush. Some
gardeners don’t experience problems from thrips and report
thrips are eaten by the same beneficial insects that consume
aphids. Gardeners trying to cultivate the perfect blooms for
competition may chose to use a chemical control for thrips,
spraying the buds before opening, (Use an insecticide
labeled for thrips on roses.) Spray only the unopened bud,
not the entire bush. Otherwise, you may destroy beneficial
insects that will consume these pests.
Spider Mites
When the weather becomes hot and dry (May through
September), be on the lookout for spider mites. The lower
leaves will become fuzzy yellow with red specks (mites),
with webbing on the underside of the foliage. Shake the
leaves over a white sheet of paper, look closely, and the
rascals can be seen scurrying around. Spider mites can be
controlled by washing off the plants with a strong stream of
plain or soapy water every two or three days. Soapy water
may be more effective. There are miticides available.
Leaf Cutter Bees
Circular or half-moon-shaped leaf cuts are caused by
leaf cutter bees. This is only aesthetic damage and is not
detrimental except for roses that will be entered in
competitions. Many gardeners see the hole left by a leaf
cutter bee as a badge of honor, a testament to the health of
their garden. Since the cutter bees use the leaves to make a
nest and do not ingest the plant tissue, it is impossible
and unnecessary to control them with chemicals. Try covering
show roses with floating row cover to keep cutter bees at
bay.
Disease Problems
Powdery Mildew
Powdery mildew will show itself about the same time as the
aphids appear. It is a seasonal problem that thrives with
cool, damp nights, warm daytime temperatures in the 70s or
above, moderate to high humidity, and poor air circulation.
Powdery mildew may first appear as small blisters on the
upper surface of leaves, followed by white or gray powdery
spots. The spots merge and eventually cover entire leaf
surface. Leaves may become twisted and distorted. Powdery
mildew can attack stems, leaves, petals and buds. The
growing tips and buds may die if the condition is severe,
but it seldom results in the plant’s death. The incidence of
powdery mildew can be lessened considerably by simply using
good gardening practices. When planting, provide sufficient
space among bushes to allow air circulation and sunlight.
Always clean up old leaves from the roses after pruning and
discard them. Periodically wash down the leaves of the roses
with water from the hose. Sulfur or fungicides can be
applied to the foliage as a preventative or to treat Powdery
Mildew. However use sulfur with caution when the
temperatures exceed 90 degrees Fahrenheit. Rosarians who
exhibit in shows usually begin their management program
before powdery mildew appears because it is difficult, if
not impossible, to eradicate once it becomes established.
They spray with a fungicide immediately after pruning in
January or February. Fungicides work better as a preventive
measure early in the season before the mildew takes hold. As
with any product, follow the instructions exactly. It is
also a good idea to test a few leaves first before spraying
the entire plant.
Crown Gall(Agrobacterium tumefaciens.)
This destructive disease causes a warty looking gall at the
base of the canes that can vary from the size of a pea to a
fist. Gall development causes gradual plant decline. The
bacteria may arrive on contaminated nursery stock and once
introduced, the bacteria persist in the soil for several
years. Infection takes place through wounds. To prevent the
disease examine plants for galls carefully prior to purchase
and prevent injuries during transplanting. There is no
treatment. Remove and destroy infected plants. Do not
replant roses, pyracantha or stone fruits in a place where
Crown Gall is present.
Other Problems
Salt burn
Edges of leaf turn brown. This common problem is caused by a
buildup of salts in the soil. Salts can accumulate from
ineffective watering practices or if an excessive amount of
fertilizer is used. Avoid this problem by periodically
watering slowly and deeply to leach salts past the root
zone. Also, carefully follow the instructions on fertilizer
containers and do not over apply. (“If a little bit is good,
then a lot must be better,” does not hold true for
fertilizer!) Water thoroughly before and after fertilizing
to help prevent burn.
Nutrient Deficiency
In the Southwest, the alkaline soils are typically high pH
(8.0 to 8.5). Ideal pH for growing roses is from 6.0 to 6.5.
If the pH of the soil is either too high or too low, some
nutrients may become unavailable for uptake by the plant
roots. A nutrient imbalance can cause symptoms of nutrient
deficiency. Nitrogen deficiency causes a general yellowing
of foliage, beginning with older leaves, then appearing on
younger leaves. Leaves turn light green and progressively
more yellow. Reduced growth and leaf size, weak and spindly
stems, and small flowers are other symptoms. Prevent a
nitrogen deficiency by fertilizing regularly. However, don’t
apply too much nitrogen, which will show up as abundant
foliar growth and very few blooms. Iron deficiency, often
called iron chlorosis, causes yellowing between the green
veins of young leaves. If this problem occurs, supply iron
in a chelated form, which is more readily available for
uptake by the plant roots. Magnesium deficiency is
manifested by older leaves which turn yellow at the edge
leaving a green arrowhead shape in the center of the leaf.
To prevent this condition, apply onequarter cup of magnesium
sulfate (epsom salts) to the rose bush two or three times
per year. Do not over-apply.
Nutrient Toxicity
Frequently, over-fertilizing is the problem. Follow the
directions on the bag, using no more than the prescribed
amount. If an excess is spilled, recover as much as you can
and water heavily to dilute and leach the rest of the
fertilizer away from the roots.
Acknowledgement
Special thanks to Rod McKusick, Master Gardner and
Consulting Rosarian, for his assistance on this publication. |
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