| Black spot is caused by the fungus, Diplocarpon
rosae. It is one of the most common diseases found
everywhere roses are planted. The disease does not
kill the plant outright, but over time, the loss of
leaves can weaken the plant making it more
susceptible to other stresses and winter damage. The
disease is active during cool, moist weather.
Conversely, extreme summer heat during July and
August will limit development of the disease.
Symptoms and Diagnosis
Black spots, one-tenth to one-half inch in diameter,
develop first on upper leaf surfaces. Areas adjacent
to the black spots turn yellow and leaves drop
prematurely, usually beginning at the bottom of the
plant and progressing upward. Less commonly,
raised purple-red blotches develop on immature
wood of first year canes. These spots may later
become blackened and blistered.
Life Cycle
The disease overwinters in diseased canes and
infested fallen leaves. The fungal spores germinate in
the spring and are disseminated by splashing water.
Fungal spores on the leaf surface must be
continuously wet for at least seven hours for
infection to occur. Once infection is established, the
fungus will develop fruiting bodies called acervuli in
black lesions which, in turn, produce spores that
splash to new tissue, spreading the disease.
Integrated Pest Management Strategies
1. Remove diseased leaves. As soon as diseased
leaves are noticed, remove and dispose of them.
Dead leaves on the ground should also be
collected and destroyed. Compost only if this
material will not be used back in rose beds. To
minimize overwintering of the fungus, collect and
remove all leaves from the ground in the fall,
mulch with 2–3 inches of leaf mold or fine bark,
and prune diseased canes before growth begins
in spring.
2. Keep the foliage dry. The fungal spores need a
wet surface to germinate and cause infection.
Therefore, keep the foliage as dry as possible. Do
not use overhead watering, or if you do, water
early in the day so the foliage is dry by nighttime.
Prune plants to allow more air circulation and
facilitate the drying of foliage.
3. Use fungicidal sprays. Fungicides applied as a
protectant provide good control. Fungicides will
not cure infected leaves, but if applied regularly,
will prevent infection of new leaves. Pesticides
registered for use include captan, chlorothalonil
(Daconil), copper, ferbam, mancozeb, maneb,
sulphur, thiophanate methyl (Cleary 3336), and
ziram. Many other products listed as rose dusts
are also effective. Check the label for control of
black spot. Most fungicides need to be applied
every 7–14 days. Fungicides need to be reapplied
following rain or overhead watering in excess
of 1/4 inch.
4. Move the plants. You can limit the disease by
growing roses in locations where they receive
morning or, preferably, all day sun. This facilitates
drying of the foliage.
5. Replant with more disease-tolerant varieties.
Avoid highly susceptible cultivars, including most
yellow and copper-colored roses. Roses reported
to be highly resistant are: ‘Fortyniner,’ ‘Coronado,’
‘Carefree Beauty,’ ‘Simplicity,’ ‘Bonica,’ and
‘Grand Opera.’ Ask about others at your nursery
or check mail order sources. |
