black spot on roses

Asked May 29, 2016, 12:02 PM EDT

Black spot on my rose plants has really gotten away from me this year. many of my rose plants are almost leaf-less now. I'm considering cutting them down to about 10" or so, spraying them thoroughly and hoping they send out new leaves which I will then continue spraying in hopes of preventing the spread of the disease. Advise please??

Douglas County Oregon

1 Response

From your description it sounds as if you have a serious infection of .Diplocarpon rosae. Black spot on roses are common in the spring and the fall and can also be found during a wet summer or on plants that receive overhead watering. It survives from year to year on living or dead plant tissue and spreads when water activates the spores. Once infected leaves will fall off. Start by removing any dead plant material from around your roses as this will be the source of continuing infection. You did not mention where your roses are located. Roses do not like to be in a dense area or in the shade. Either of these conditions can contribute to the problem. And as mentioned before they do not do well with overhead watering. If you are watering your grass and the roses get the spray that will be just as harmful as rain or intentional watering. As leaves fall remove them from the area and at the end of the season rake up and burn all plant residue. Burn the residue and do not put it in your compost pile. Once the wet season is past the infection should stop spreading and your leaves will probably grow back. If your canes are infected prune them back to two buds. Remove and destroy the diseased canes.

Chemical treatments are available and should be applied prior to bud brake or during the dormant season. When using a chemical treatment alternate the chemicals used so as to prevent creating a resistant fungus in your garden.

The Pacific Northwest Guide to Plant Disease lists the following products as available for home use.

  • Bonide Captan 50 WP at 2 Tbsp/gal water can be used in home gardens.
  • Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. Group M5 fungicide.
  • Bonide Liquid Copper Fungicide at 1.44 fl oz/gal. May result in purple spots on some cultivars in cool conditions.
  • Ferti lome Liquid Systemic Fungicide II at 1 fl oz/gal water plus another fungicide. Group 3 fungicide.
  • Hi-Yield Captan 50 W Fungicide may be used in Oregon home gardens at 2 Tbsp/gal water.
  • Infuse Systemic Disease Control at 2 Tbsp/gal water plus another fungicide. Group 3 fungicides.
  • Monterey Liqui-Cop at 3 Tbsp/gal water. May discolor blooms.
  • Ortho MAX Garden Disease Control at 1.5 Tbsp/4 gal water.
  • Ortho RosePride Disease Control at 1 Tbsp/gal water. Group 3 fungicide.
  • Spectracide Immunox at 1 oz/gal water. Group 3 fungicide.
  • As with any poison, the user is responsibility for its use. Carefully read the label prior to making your purchase to insure it is for your plant and the disease. Before you mix and apply the chemical read the label again. Adding extra chemical to a solution does not improve the efficiency and is some cases can cause harm to the plants. Store the unused portion of the chemical and or dispose of the empty container in accordance with the instructions on the label.

    For a positive identification of your disease bring in a sample of the infected leaves to the Master Gardener Plant Clinic. The Plant Clinic is located in the OSU extension office at 1134 SE Douglas Ave. Roseburg OR 97470 (next door to the courthouse). If you have further questions you can call the Master Gardeners at 541-672-4461. Summer hours are 1 - 4 p.m. Monday through Friday.