Hi,
Black spot is a fungal disease characterized by black spots on the upper side of leaves. Tending to occur in warm, wet weather, usually during wet summers, the leaves of infected roses turn yellow and fall off. This weakens the plant by making it more susceptible to other diseases or opening it up to injury for the next winter. The organism responsible for black spot spreads quickly and can move from plant to plant if proper care is not taken.
Black spot has the following indicators:
Circular black spots with fringed margins on the leaves.
The lower leaves tend to be infected first, after which it spreads quickly.
Upper leaves often turn yellow and fall off.
The plant becomes weaker and it either blooms less or not at all.
Remove infected leaves and branch areas as soon as you see signs of black spot. This helps to slow down any further progression of the fungus.
Remove black spot infected leaves that have fallen to the ground immediately. If left to lie below a rose, the spores of the fungi will start climbing the rose again as a soon as the temperature and moisture are right.
Spray roses every 7 to 14 days with a fungicide during growing months. Even without signs of black spot, this is an important part of good rose preventative maintenance. Fungicides include trifloxystrobin, Ziram, chlorothalonil, mancozeb, Thiophanate-methyl, and several others.
Good garden hygiene is important for treating black spot roses. During the growing season, overhead watering should be avoided. Removal of leaf litter and pruning of diseased canes (back to healthy wood) is also important.
Neem oil can also be used, which helps control many rose pests as well.
Another popular method includes spraying a solution of baking soda and water on foliage. Baking soda helps change the pH level on leaf surfaces, which makes it more difficult for black spot to infect plants. To make this organic solution, mix a couple tablespoons of baking soda with a gallon of water. To help keep the baking soda on the leaf, add a drop or two of bleach free dish soap. Spray both side of the foliage. Reapply weekly and repeat after any rain.
Avoid watering your plants on cloudy days. Bright sun and good air circulation are essential for getting rid of black leaf spot.
