5 Tips for Healthy Roses: by Robert E. Thomas

Story by: Guest Contributor

First posted on 07-30-2007

Humid weather this growing season has taken its toll on roses. Black spot, a destructive rose disease, just doesn’t want to go away.

Black spot is a fungal disease that is most severe during wet and humid weather. It appears as black spots or rings on the leaf surface. Many times the black spots have a yellowish halo around them.

Affected leaves turn yellow and drop from the plant. If severe, this defoliation will weaken the plant, reduce flowering and make the plant more susceptible to winter damage, said Mary Kroening, University of Missouri Extension horticulturalist.

The fungus overwinters in dead leaves on the ground and infections in the stem. Rain and splashing water then spread the spores from old leaves or stems.

“Black spot is difficult to control as sprays are mostly preventative. The chemical must be on the leaves before the disease gets started. Once black spot spores germinate on the leaf, the infection cannot be reversed,” Kroening said.

Protection requires weekly spraying during wet periods. Hybrid tea roses are the most susceptible to the disease, though black-spot resistant varieties are available.

Kroening offers five tips for protecting roses from black spot:

Give roses at least six hours of sun daily.
Space plants to allow for good air circulation.
Avoid wetting leaves during watering.
Remove leaves at the first sign of black spot.
Clear away fallen leaves.