Diseases of Roses – Black Spot

Black spot is the most common rose disease throughout the world, and it’s not pretty. The good news is that black spot is preventable in most cases and if your bushes already have this disease you can fight it.

What is it? The name is pretty self explanatory. Black spot is a fungus that shows up as black lesions on both the upper and lower surfaces of leaves. If your roses aren’t treated they’ll weaken and weaken as leaves turn yellow and drop off, which causes new leaves to surface, which in turn drains your entire bush of important food reserves. It takes a lot to grow leaves. Finally you’ll end up with a weak plant with poor looking wood. Additionally, a rose bush that’s infected will see fewer and fewer blooms as time goes on.

Prevention first. Choose rose bushes that are resistant to black spot. Black spot, being a fungus needs moisture to thrive so water roses from below, not from above (especially in the evenings), and keep your bushes well mulched, changing to fresh mulch with the seasons. You should also grow plants in a nice sunny spot to avoid lingering wet foliage.

Stop black spot: Even if a bush is infected you can often stop the disease cold. If you notice the telltale black spores and spots, remove all leaves from the plant quickly and carefully. There is some debate about leaves on the ground. I’ve read that the spores can’t survive in the soil for very long and recommend raking up leaves in the spring, but here in Oregon, my local extension recommends getting all infected leaves off the ground during the whole year anyhow to halt the spread to other plants.

If you need to move to a chemical treatment, fine ground sulfur dusted on the leaves is considered one of the least toxic, but note you can only apply this if the temperature is 85 degrees or less to avoid burning your leaves.

The University of Maine Cooperative Extension has a good section on black spot, including other fungicide treatments, but not all are low toxins, so try physical removal methods first (such as removing all infected leaves).