sides of such leaves for mites. Heavier infestations cause leaves to have a bleached or bronze-col-
ored appearance and to turn brown eventually and drop from the plant. Spider mites are often
kept in check by natural predators, so one important way to manage spider mites is to avoid un-
necessary insecticide treatments. Many of the insecticides recommended for control of other rose
pests have a tendency to trigger spider mite outbreaks. Although it is sometimes necessary to use
these products, you do not want to do so unless you really have to. Hot, dry, dusty conditions also
favor mite outbreaks. Choose spider mite treatments carefully; spraying spider mite infestations
with ineffective products will cause the infestation to become worse!
leaf-cutter bees:
Leaf-cutter bees are solitary bees that sometimes cause minor, but rather obvious,
damage to rose leaves by cutting circular discs of leaf material to use in their nests. Nests are built
in hollow, tube-shaped holes that are approximately the diameter of a pencil. Occasionally an es-
pecially industrious bee, or group of bees, will create enough leaf damage on an individual plant
or group of plants to cause concern, but this defoliation is usually not severe enough to cause long-
term adverse effects.
Japanese beetle:
Fortunately, these non-native insects do
not occur statewide, but Japanese beetles are serious pests
of roses wherever they occur. Damage is caused by the
adults, which feed directly on the blooms, causing unsightly
damage. Light-colored blooms are more favored, but all
colors are subject to damage. They will also feed on leaves.
Currently, the heaviest Japanese beetle populations occur in
the northern part of the state, but infestations are spreading.
Japanese beetles are relatively easy to identify by their bronze
and green color and the small tufts of white hair that appear
along the edges of their wing covers.
rose Midge:
Although rose midge are
not common in the state, rose growers
need to be aware of this insect and the
damage it causes. Actually, you are not
likely to see the insects themselves be-
cause they are tiny, mosquito-like flies.
Heavy rose midge injury is hard to
overlook, but it is often not recognized as insect injury. Rose
midge larvae feed in developing shoot tips and flower buds,
causing them to be blackened and distorted, resulting in death
of the shoot tip and reduced bloom production. Heavily in-
fested plants will produce few blooms. This insect produces
many generations per growing season.
fire Ants:
Fire ants rarely damage roses directly, but their
mounds are unsightly in the rose garden and surrounding
landscape, and their stings are painful and annoying. It is dif-
ficult to fully enjoy a rose garden or landscape that is heavily
infested with fire ants. Fortunately, there are effective treat-
ments that can be used to control fire ants; long-term success
requires ongoing effort. See extension publication 2429, con-
trol fire Ants in your yard
, for information on how to best
control these vexing and persistent pests.
Diseases
Rose cultivars differ in their susceptibility to various diseases,
but because of our diverse climate and soils, most roses will
eventually have a problem with one disease or another.
Knowing something about common rose diseases and when
a preventive disease control program might be necessary can
help you be a successful rose grower.
A preventative disease program requires that you know
what a healthy plant looks like and that you check your
roses’ health once every two weeks. If the weather favors a
disease, start a preventative management program. If you
suspect a disease and can identify it, you should apply rec-
ommended products. If you do not know what the problem
is or cannot identify it, send a sample of the problem and re-
ceive a diagnosis and recommendation for only $6.00. Send
the sample to the
Extension Plant Pathology Lab
190 Bost North, Rm. 9
Mississippi State, MS 39762-9612
(662) 325-2146
(662) 325-8336 (Fax)
The most common diseases that infect roses are caused by
fungi or viruses. One is caused by a bacterium. Black spot,
powdery mildew, and stem cankers, the three most prevalent
Rose midge
enlarged 10 times
25
Leaf damage caused by leaf-cutter bees
Beetles (sizes vary)
and most costly diseases of roses in Mississippi, are caused
by fungi.
Always pay attention to weather conditions and take preven-
tative action before a disease gets out of control. Management
and fungicides work best when used preventively.
Black spot (Diplocarpon rosae)
Black spot is the most common disease of roses in the United
States. It affects the aboveground portion of the plant and de-
foliates leaves and causes cane blotches. While black spot can
be found on all roses, it is most severe on some types of hybrid
tea roses, especially those bred for their showy flowers. A reg-
ular preventive program should be employed for these roses.
The most prominent symptom of this disease is a black spot,
which may occur on either side of the leaf. A number of other
diseases cause dark spots in the leaves, but you can distin-
guish black spot by the darker color and the border of the
spots, which appear to be fringed, hairy, or feathery. Black
spot also infect the stems or canes. Cane lesions may not have
the fringed border.
The first symptom will be a small, pin-head sized lesion. De-
pending upon the genetics of both the rose and the fungus,
the plant may hold its leaves until the black spot grows to the
size of a dime or the leaves may fall soon after infection, be-
fore you notice the black spots. Premature defoliation is a
symptom of the disease.
Depending on the susceptibility of the plant to black spot, you
may see a little to a lot of chlorosis, or yellowing, around the
black spot lesion. In general, the more chlorosis, the more
quickly the leaf is likely to fall from the plant.
When unchecked, black spot can completely defoliate the rose
plant. Repeated defoliations weaken the plant and make it
more prone to dying from other causes. The fallen leaves har-
bor the fungus, and it moves from them to infect the new
leaves the following spring.
control.
The first step toward controlling black spot is
sanitation. If only a few leaves show symptoms of black spot
on a plant, remove and destroy them. Remove all diseased
leaves from around the plants, particularly at the end of the
growing season. The fungus overwinters in diseased leaves
and in the canes.
It is sometimes wise to prune canes drastically, without cut-
ting too close to the graft, before new growth begins in the
spring. Before new foliage occurs, remove and burn all old
mulch. Replace with new mulch.
An effective spray program
depends upon understanding
the lifecycle of the fungus that
causes black spot, Diplocarpon
rosae. The fungus depends
upon little “seed-like” struc-
tures, called spores, to move to
new infection sites. Most com-
monly, the spores move from
infested leaf residue at the base
of the plant or from fruiting
stem lesions. They may also be
borne by the wind to the rose.
Once on the plant, the spores
require water and high hu-
midity to germinate. The
longer free water (dew, rain, and irrigation water) is present,
the lower the humidity can be. The higher the humidity is,
the shorter time free water needs to be present. The fungus
can germinate and infect the plant from 32 F to almost 92 F,
but at the lower and higher temperatures, the infections will
not express until more favorable temperatures are reached.
The most favorable temperatures for disease development
range from 64 F to 75 F.
Because of this lifecycle, fungicide applications should start
as soon as any leaves appear on the canes, including during
January and February “warm periods”. A study has shown
that blackspot-susceptible rose plants that were sprayed ap-
propriately during the winter had better health in the spring
and summer. Fungicides used for these sprays should not in-
clude the class of fungicides called the triazoles (propicona-
zole, tebuconazole, metconazole and, myclobutanil), as they
can interfere with leaf emergence the following spring.
A spray program should require no more than 15 minutes
every two weeks if you have 10 or fewer rose plants, but it
should be done regularly. The protective sprays should con-
tinue throughout the spring, summer, and fall. Triazole-type
fungicides can be used once the plants are leafed out, in the
spring through fall. These fungicides are active enough that
sprays every two weeks should suffice.
For specific spray recommendations, please see Plant Doctor:
Black Spot and Powdery Mildew of Rose (IS 1667) and pub-
lication P2705 The Plant Doctor: How to Spray Fungicides to
Protect Your Home Landscape, Garden, and Turf
(http://msu-
cares.com/pubs/publications/p2705.pdf)
.
26
Black spot disease on rose leaflets -
the most common disease of roses.
As spots increase in size, they often
run together and cover the entire
leaflet. Severely infected leaves fall
from the plant. When unchecked,
black spot can completely defoliate
the rose plant.