Blackspot is a fungus that is exacerbated by hot, humid weather. It appears as – you guessed it – black spots on leaves.
It's most common on roses. (Though they are a bit high maintenance, are certainly worth the effort!)
The best way to control blackspot on roses is to practice good cultural controls
Remove leaves and dispose of them in trash, not compost. There is no guarantee that the compost bin will be hot enough to kill off the spores, in which case you are just spreading the fungus.
Clean your pruners with alcohol when you are pruning so you don’t transfer the fungus to another plant.
Water in the early morning, not the evening. Water the roots, not the leaves. During spring or fall when there are cool evenings, the added moisture can contribute to fungal growth.
Organic control of blackspot in a rose garden
An organic control for black spot is: 1 teaspoon baking soda + a few drops of dishwashing soap to a quart of water. Some people I know use diluted milk and water. Spray every 4 to 5 days. Try to get the plant completely saturated from top to bottom with the spray. You should also spray the ground around your roses. As with any spray, make sure the plant is well hydrated first.
As a New York City garden designer, I work in many places where there is a lack of moving air and humid conditions. This remedy is extremely effective on my own roof garden and on the many Brooklyn garden designs and New York City townhouse gardens I create for my clients.
