There are three common fungus problems of roses: Black Spot, Powdery Mildew and Cercospora Leaf Spot. Powdery mildew is addressed in a separate info-sheet.
Cercospora leaf spot look like most leaf spot fungus with a dark margin and a tan or grey center.
This leaf spot starts as a purplish spot then the center turns color as the leaf begins to die. Control is the same as black spot, both in timing and chemical.
Black Spot is often an irregular spot with feathery borders. Yellowing may appear around the spots but is starting in top as well.
Leaves are the most susceptible, but spots can be seen on other parts of the plant.
Infection usually starts on the lower leaves and works its way up the plant.
Infection usually starts on the lower leaves and works its way up the plant. As the yellow proceeds across the leaves, they will eventually drop, leaving the bottom of the plant bear with only a few leaves at the top. This defoliation decreases the plants energy reserves, reduces flowering and weakens the plant, making it more susceptible to other stress factors such as drought and winter injury. Black spots will be most severe when humidity is high, over 85%, warm temperatures over 75°, and six or more hours of leaf wetness.
The first line of defense is to try some cultural controls: Avoid planting susceptible varieties. Keep irrigation water off the foliage. Plant in full sun with good air movement.
Sanitation; remove diseased leaves that have fallen and prune out infected rose canes. Spray if needed.
NG Neem and ferti-lome Triple Action, are natural choices for the treatment of Black Spots. Both have the added advantage of insecticide protection, including a miticide for spider mites.
Apply at least three times 7 to 10 days apart.
Ferti-lome F-Stop is a very effective broad-use fungicide. As a preventative, apply every 2 weeks. As a control, apply every 7 to 10 days. If disease pressure is great, rotate with ferti-lome Liquid Systemic Fungicide.
Ferti-lome Liquid Systemic Fungicide is a systemic fungicide, apply at 10-day intervals. Another systemic is ferti-lome 2-N-1 Systemic, this has a fungicide and an insecticide, it can be used as a drench. No Spraying!
Due to the decline of plant vigor under the pressure of black spot, fertilize every 6 to 8 weeks to help with recovery.
Fertilize with ferti-lome Rose Food and ferti-lome Rose Food with Systemic. Both have micro-nutrients including Iron to improve color and vigor.
For an organic choice use NG Rose & Flower Food.
Contact or stop by to talk to the experts at The Big Tool Box and Highlands Garden center forideas for curing black spots on roses.