Jun 07, 2024

Rx for Roses: Gardeners reporting cases of rosette virus, blackspot

Posted Jun 07, 2024 2:02 PM
Rose blackspot is a fungal disease that can cause dark, circular lesions with feathery edges on the top surfaces of leaves. <b>Image Courtesy KSRE</b>
Rose blackspot is a fungal disease that can cause dark, circular lesions with feathery edges on the top surfaces of leaves. Image Courtesy KSRE

K-State horticulture expert outlines treatment options

By K-State Research and Extension News Service

Rose rosette virus and rose blackspot have been reported in several Kansas counties, causing signs of stress on many rose plants this spring.

If you are concerned because your garden’s roses are looking a bit stressed right now, you might not be alone.

Kansas State University horticultural expert Cynthia Domenghini said gardeners in several Kansas counties are reporting problems with their roses due to the rosette virus or rose blackspot.

According to Domenghini, the classic symptoms that appear with rose rosette virus include:

⦁ Rapid elongation of a new shoot

⦁‘Witches broom,’ or a clustering of small branches

⦁Branches develop excessive thorns.

⦁Small, deformed, reddish-purple leaves

⦁Certain varieties may develop reddish-purple streaks or blotches on stems and petioles

⦁Plant death

“Infected plants, including the roots, should be removed and destroyed,” Domenghini said. “Garden tools that come in contact with the plant need to be sterilized to avoid spreading the virus.”

Rose blackspot is a fungal disease that can cause the following symptoms:

⦁ Dark, circular lesions with feathery edges on the top surface of leaves. Domenghini said usually the lower leaves are usually infected first

⦁ Raised, purple spots on young canes

⦁ Yellowing between spots on infected leaves

⦁ Leaf drop

“Prevention is the best approach for controlling blackspot,” said Domenghini, who recommends purchasing resistant rose cultivars.

Mindful gardeners should also consider watering roses with drip irrigation to avoid splashing water on the leaves, and ensure roses are planted in the sun with good air movement.

“Don’t crowd plants together in a planting,” Domenghini said. “Diseased leaves should be removed from the ground, and infected plant parts should be pruned out of the plant.”

For those gardeners who choose to apply fungicide, Domenghini suggests treating the plants on a 10-14 day schedule. Some recommended fungicides include tebuconazole, myclobutanil, triticonazole or chlorothalonil.

“As always, follow all label instructions carefully,” Domenghini said.

Domenghini and her colleagues in K-State's Department of Horticulture and Natural Resources produce a weekly Horticulture Newsletter with tips for maintaining home landscapes and gardens. The newsletter is available to view online or can be delivered by email each week.

Interested persons can subscribe to the weekly newsletter, or submit their garden and yard-related questions, by sending email to Domenghini at [email protected], More information also is available at your local K-State Research and Extension office.

Sidebar Question of the Week

How do I get rid of poison ivy growing around my roses?

Individual sensitivity to plant toxins varies; cover arms and legs when handling the plants. When poison ivy plants are growing among desired plants, herbicide treatment may not be ideal due to the risk of overspray and drift. When the soil is wet, use gloves and pull or dig young plants out of the soil.

Large vines can be cut at the base. Treat the remaining stumps with herbicide, including those with the following active ingredients: triclopyr, glyphosate, 2,4-D and dicamba. A second treatment may be necessary. Always follow label instructions.

Cynthia Domenghini, Kansas State University horticultural expert, [email protected]