I have a special kind of sympathy for the folks who stop by the garden centre who are dealing with black spot. Finding it on your plants for the first time is devastating. One day everything’s all pretty flowers and shiny leaves, and then you take a vacation for a week and—HOLY MOLY! Your plant’s foliage has developed an alarming pox of death.
Spotting Black Spot
Black spot is an accurate description of this disease, as it causes black, dead-looking blotches that spread quickly on our beloved plants. While it is most common on roses, plenty of other plants are susceptible, too. If it has leaves and a stem, it can get black spot, and on fruit trees—especially crabapples—it can cause issues with harvest quality, so it’s worth keeping an eye out for if you have a rose or fruit tree on your property.
The cause of black spot is a fungus that thrives in the high heat and high humidity—and with the wet spring we’ve had, we can expect to see signs of it showing up fairly soon. It spreads through spores in the soil, so if you’re noticing it on one plant, if left unattended, it can quickly move onto the surrounding neighbours.
When the plant is first infected, the spots start out small and barely noticeable. Over time, they expand, rimmed with a grim yellow tinge. Eventually, you’ll start noticing larger black spots which, left untreated, will take over the plant leaf and slowly spread, especially if your garden sees a lot of action from wildlife, like birds and squirrels.
Treating Black Spot
As terrible as black spot looks, it’s not the Black Plague—it can be treated. Your plan of attack should depend on the severity of the condition.
If your plants are sporting just a couple of spotted leaves, simply pluck off the affected areas and get rid of them to control the spread.
If your plants have severely affected areas, like leaves that are yellowed or covered in black blotches, cut your losses and cut ‘em off with disinfected garden shears. Keep some alcohol wipes handy as you go so you can disinfect the shears every time you move on to a different plant. Otherwise, you may end up spreading the disease instead of controlling it.
If the disease persists, try a stronger natural fungus killer, like Green Earth’s dusting sulphur or Bordo, which we carry here at the garden centre.
If all else fails, look into a fungicide. This should be an emergency situation, like if the disease is spreading aggressively or keeps coming back every year. Always try to use the gentlest product you can.
No matter which treatment you’re working with, remember to retreat every 7-10 days until the plant is no longer showing any symptoms. Don’t forget to treat the soil, as well, to ensure you’re killing the fungus at the source.
Preventing Black Spot
Wet leaves are a haven for the fungus to grow and spread, so the best thing you can do is avoid getting your leaves wet! If you’ve never heard it before, there’s a great saying that goes “water the soil, not the plant”. The leaves aren’t the part of the plant that drinks the water, the roots are, so keeping the flow of water limited to the soil can deliver water to the right place.
Wait a moment, Tanya, you might be thinking, my garden is lush and beautiful, so I can’t help but get some foliage wet when I water. First of all, that’s awesome! Second of all, this is a valid concern. If you have a pretty impressive garden and you’re concerned about creating a black-tie affair for black spot, try installing a weeping hose under the mulch layer. They’re relatively easy to install, keep your plants watered at soil-level, and end up saving you a lot of time versus watering by hand.
Another important prevention tip is to create as much airflow as possible at soil level. Congestion around your plants creates ideal conditions for soil fungus and disease to incubate, so by opening it up a bit, we can reduce the humidity and give the leaves a better chance to dry out. Prune the lower stems of plants first and clear up dead plants and debris as soon as possible. Use a layer of mulch on the soil to counteract the drying effects of the improved airflow.
Once it looks like you’ve got your black spot under control, don’t make the mistake of thinking you have it beat! Keep monitoring your plants for symptoms for at least a year after exposure. Black spot spores are tough, but a well-informed, diligent gardener is tougher.