Curing and Preventing Rose Black Spot

Plant Diseases: Rose Black Spot

Black Spot disease of rose is the most important disease of roses all over the world. This disease is generally present, frequently epidemic, and a major problem on outdoor grown roses.

Identifying Black Spot: The characteristic black leaf spots are circular with feathery borders surrounding the spots. Leaf tissues surrounding the spots turn yellow and extend throughout the leaflet until the shedding of leaves or flowers occurs following the formation of the abscission zone.

What is Black Spot?: Black Spot disease is caused by a fungal pathogen. Microscopic fungus spores are easily transported through nature and are present in all gardens all the time. They take advantage of natural conditions or those inadvertently caused by gardeners.

Leaves are most susceptible while still expanding after bud break. Regardless of the relative humidity, the fungus spores must be immersed in water continuously for seven hours for germination and infection to occur. Fungal spores can be spread by splashed water. Fallen leaves blown by the wind may disperse the fungal pathogen.

The fungal pathogen is actually present only in the lesion itself but its damage extends throughout the leaf. The spot lesions are often small and rarely kill rose branches, but are extremely important in survival of the fungal pathogen over-wintering from season to season. The spot symptoms may even occur on rose canes. While the fungal pathogen can be spread to surrounding plants in the garden, only rose is susceptible to this disease.

Environmental Conditions & Disease Development: Environmental conditions that are most favorable for disease development include high humidity and 75F temperatures. Cultural conditions that favor disease development include dense plantings and watering late in the day when water remains on plant foliage for seven hours and longer since a dense leaf canopy does not allow good air circulation to dry the foliage.

Prevention is the Best Medicine: Once the disease has progressed to the point where black spots are visible it means that the plant tissue has been permanently damaged. There is no way to reverse or fully eliminate the spots, however, it is possible to kill the fungus, stop its spread and prevent additional infection. GreenCure® is a good preventative control by applying 1 tablespoon per gallon of water every 1 to 2 weeks when environmental and cultural conditions are ideal for the disease. Once black spot lesions are formed, GreenCure® will not eliminate the lesion since the leaf tissue beneath the spot is dead, however, GreenCure® will kill the spores to prevent the spread of the disease.

Since the fungal pathogen can over-winter in infected leaves, it is important to remove and destroy fallen leaves and debris surrounding the plants at the end of the growing season.