Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
7/21/03
Contacts:
Anne Dombroski, Plant Pathology, (515) 294-1741, annedom@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Yard and Garden Column for the Week Beginning July 25
Black Spot of Rose
By Anne Dombroski
Plant Pathology Graduate Student
Iowa State University Extension
While the heat and humidity of the Iowa summer chase us indoors to the sanctuary of air conditioning, many of our landscape roses are falling prey to a fungus that thrives in Iowa's summer weather. This fungus, known as black spot, causes leaves to develop black spots, turn yellow,and fall off. A severe case of this disease can leave your roses looking like bare twigs with only a few unsightly flowers.
The fungus Diplocarpon rosae causes black spot and it is considered the most serious disease affecting roses in the Midwest. It thrives in our moist summer climate and receives an added boost during rainy periods. The fungus attacks new leaves as they emerge, beginning at spring bud break, and continues throughout the growing season. Black spot affects the leaves most severely, but it can infect canes and flower parts as well. It is transferred by splashing water from infected to healthy plant parts, and the fungal spores can also be carried by wind.
The disease produces rounded black spots on leaves. These spots often have feathered edges, can be up to half an inch in diameter and frequently are surrounded by a yellow halo.. If the infection becomes severe, spots may coalesce, forming large irregular patches on the leaves. As the infection progresses, spotted leaves will turn yellow and fall off. This leaf fall can occur with as few as one or two spots per leaf. Eventually you are left with an unsightly plant, which may then flush its new leaves and drop them repeatedly after each new cycle of infection.
Black spot symptoms usually appear first in the lower leaves and spread upward throughout the plant. In addition to leaf symptoms, flower number and quality is reduced and raised purple bumps may appear on younger canes. These bumps eventually turn into blackened, blistered areas. Leaf damage has an immediate impact on plant health, and cane and leaf infections also enable the fungus to survive the winter and infect new leaves as they emerge the following spring.
If left untreated, black spot can wreak havoc on susceptible rose varieties. Losing leaves early makes a plant short on stored food as it enters the harsh Iowa winter. As a result, infected plants are much more susceptible to winter injury and death.
Because the fungus is a threat throughout the summer, chemical control of black spot can be quite demanding. Weekly sprays are sometimes recommended to protect highly susceptible varieties. Once black spot gains a strong hold on your rose plant, it can be tough to subdue. Unless you are committed to using achemical sprayer all summer, the best way to control black spot in your yard is to purchase resistant varieties of roses. Lists of rose varieties with strong resistance to black spot can be obtained on the World Wide Web or through your local extension office.
Good cultural practices help to reduce the severity of black spot. First, try to keep foliage as dry as possible. Adequate plant spacing promotes good air circulation and rapid drying, as does planting in sunny spots. When watering, it's helpful to apply water only to the base of the plant. Avoid using overhead sprinklers that can leave the foliage drenched. If you must use a sprinkler, it's better to water early in the day rather than in late afternoon or evening, when water can linger on the foliage all night.
Proper sanitation can also help control the spread of black spot. Because the fungus can survive the winter on both fallen leaves and canes, it is important to clean up all diseased leaves and prune out any infected canes. Cleanup should be done regularly during the growing season and again at the season's end.
Can't live without that highly susceptible rose in your yard? For the rose enthusiast willing to mount a full-scale attack on the black spot fungus, several fungicides are available to control the disease. Spraying should begin as soon as foliage emerges in the spring and continue throughout the summer. Plants need to be sprayed every 7-10 days in order to protect the new leaves as they emerge. As with any pesticide, check the label for application instructions and confirm that it is labeled to control black spot of rose.
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Editors: There is no photo this week.
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