Black Spot on Roses - HYG-3072-96
- Jim Chatfield &
Black spot is the most important infectious disease of roses. It
occurs only on roses (Rosa spp.), and is widespread among rose species
and cultivars, although some of the shrub roses and rugosa roses show
more resistance. Many hybrid tea roses are very susceptible. Lists of
black spot-resistance hybrid tea roses often are variable due to
localized races of the pathogen.
Round to irregular black splotches with fringed margins are quite
obvious, mostly on upper leaf surfaces. Leaf yellowing develops around
these black spots, with defoliation of these infected leaves common.
Repeated defoliation weakens plants, leading to poorer blooming and
greater sensitivity to other stresses. Occasionally symptoms are noted
on petals (red dots, distortions), and on petioles, fruit and canes.
Using a hand lens, the fungus can often be seen fruiting in the black
(click pictures to enlarge)
The fungus overwinters on fallen leaves and diseased canes.
Microscopic spores are then splashed to newly emerged leaves and stem
tissue in the spring. Under ideal conditions of leaf wetness, humidity
and temperature the spores can germinate and infect in 1 day, cause
symptoms in 4 to 5 days, and produce new spores that can infect
additional leaf, flower and cane tissue within 10 to 11 days. Spores are
easily spread to new locations by air currents.
- Keep foliage dry. Plant roses in sunny locations to encourage
drying after rains. Avoid sites with dense surrounding vegetation,
so that good air movement will dry leaves. Avoid overhead
irrigation, especially late in the day. Black spot is most severe in
summers with sustained rainy periods.
- Sanitation. Remove all black spotted leaves from and around
plants. This should be done throughout the season. Before winter,
remove and clean up all diseased leaves and remove diseased canes
- Disease resistance. Listing susceptibility and resistance of all
roses would take volumes. In addition, the occurrence of local races
of the pathogen often result in a particular cultivar being listed
as susceptible in one area and resistant in another. However, some
lists have general usefulness; see Table 1. Also, consult local
Extension publications and books, consult local rosarians and garden
center horticulturists, and make observations of relative disease
incidence in local rose collections and your own gardens.
- Preventive fungicide applications. Fungicide controls are not
successful if cultural and sanitation practices listed above are not
followed. For fungicides to work, applications must be made
preventively, providing a protective fungicide barrier which kills
germinating fungal spores that have landed on plant tissue.
If conditions for infection are present and a high level of
control is desired, preventive spray programs often start as soon as
rose foliage emerges in the spring and continue throughout the
summer at frequent intervals (as frequently as every 7 to 10 days in
wet weather). Frequently used fungicides for black spot control
include triforine (Funginex), and phaltan.
|Table 1. Rose varieties reported to have
resistance to black spot.
|Resistant hybrid teas:
Miss All-American Beauty
|Resistant shrub roses:
All that Jazz
Baby Betsy McCall
Resistant Rugosa hybrid:
F. J. Grookendorst
Ohio State University Extension Fact Sheet
Plant Pathology - 2021 Coffey Road, Columbus, OH