To begin with, that black spot disease you see on your roses (Diplocarpon rosae) is a fungus disease. You know you have it by the light spot it creates on the leaf – an initial yellowing of the leaf in a spot approximately one-half inch across.
This initial yellow spot can occur on either side of the leaf and distinguishes itself from other leaf spot fungi problems by having a slightly fringed edge to the spot. After establishing itself, the fungus will turn black (see picture below)
You’ll likely need a small lens or a magnifying glass to see the fringes so most gardeners assume black spot when they see the yellowing spots on the leaves. With the lens, you’ll also be able to see the black spore-producing bodies on the spot.
Why Don’t Chemicals Always Control This Problem?
The interesting thing about this problem is that the fungus “roots” (they’re really mycellium, but what the heck) penetrate down between the layers of the leaf so they are well protected and invulnerable to anything happening on the surface of the leaf.
You can’t destroy the fungus without destroying the leaf. That is one smart fungus. (no contact sprays – chemical or organic – will work between leaf layers)
Why do the leaves fall off
The fungus is reputed to produce ethylene which causes roses to drop their leaves. Some roses hang onto the leaves for a longer time than others and on these varieties, the black spot fungus can really develop.
It produces larger yellowing circles (sometimes edged with black dying tissue) and often several different circles on a leaf will meet, causing almost the entire leaf to go yellow.
Naturally, as the fungus works away, the leaf will become paler anyway as the nutrients aren’t getting to all parts of the leaf so the leaf just gets pale and yellow. Then it drops.
Traditional Chemical Controls
Gardeners have tried to control this black spot problem over the years by drenching their roses in all manner of fungicides.
Invariably, the fungicide works for a while and then after a few years, it loses its effectiveness as the fungus become immune to that particular compound. In this way, the chemical industry has convinced rose growers to use more and more powerful compounds to protect their roses from the effects of the blackspot fungus.
Let me suggest an alternative system of control for you.
The first step to healthy roses: cleanliness
To begin with, once the fungus is well established in your garden, it is necessary to clean up the garden.
Collect and remove all diseased leaves to the dump. Do not compost them.
By collecting all the leaves, and I start with those that have symptoms on the plant, and removing them from the garden area, you are denying the fungus the ability to breed and overwinter on plant debris.
Preventing successful overwintering is the first step in a control programme and a clean garden is the first step in this direction.
Second step: Dormant Oils
The second step is to understand that many fungus spores overwinter on the canes.
In my garden, I cut down the canes to the ground and remove them from the garden each year. I can do this because I plant my bud unions at least six inches deep (I describe this in my ebook Tender Roses for Tough Climates) and the roses survive quite nicely to resprout new and clean canes the following spring.
If you garden in a more traditional manner, I suggest you spray your roses with a dormant oil and sulfur spray in the fall before hilling as well as first thing in the spring after you remove the soil covering.
The dormant oil will suffocate the overwintering fungal spores. As a side benefit, it also suffocates the eggs and overwintering adults of several pests that can bother roses.
Third step: Preventative Sprays
The third thing a gardener has to do is develop a rotation spray in the garden as a preventative measure. Remember, once the fungus is established, the mycellium are protected between the leaves so spraying will not kill them.
Establishing a preventative spray programme is easy and several things work very well.
Organic Spray for Black Spot #1
The first is household baking soda.
Mix two tablespoons of baking soda into a gallon of water and add one squirt of detergent or soap to the mix. The soap acts as a spreader-sticker to help keep the baking soda on the leaf.
The mix should be sprayed onto the rose leaves – both top and bottom – to establish an alkaline leaf surface that will prevent the fungus from establishing itself.
This will have to be repeated after a rain as the rain will clean the leaves allowing the fungus a clean leaf to colonize. This mixture will last about a week in normal practice, the dew will wash it off and wind action will abrade it.
Organic Spray for Black Spot #2
Go to your favourite garden centre to purchase lime-sulfur (see below for resources) and use this the second week. Mix as per label directions.
Spray this mix onto both leaf surfaces as it creates a very acidic leaf surface that will prevent fungus from developing as well as killing off any immature fungal bodies.
Alternate organic controls
Each week you alternate sprays – one week the alkaline baking soda and the next, the acidic lime sulfur.
The poor black spot fungus will have to work very hard to establish itself if you are diligent.
There are other sprays coming on the market based on neem products and horticultural oil that hold good promise to be included as part of the spray rotation.
Do not rely on only one spray for rose protection but continue to use several in rotation.
Step Four: Resistant Roses
The last thing to consider – or possibly the first thing – is to only purchase roses that are “resistant” to the black spot fungus. This usually means the leaf surfaces are thicker so the fungus has a harder time penetrating and propagating.
It does not mean the rose won’t get the fungus but that it resists the fungus (resist and immune are two different concepts).
Using environmental controls, your roses are still likely going to get black spot, but they will get it later in the season and not as badly as under chemical spray programmes. This means you’ll get more blooms and your plants will be healthier. You’ve done two things – you’ve eliminated noxious chemicals from your garden and you’ve got better roses. How’s that for good gardening?
My Situation is Unusual
This article on black spot was written from a traditional point of view.
In current practice, I find I rarely have to spray my roses at all for several reasons.
The reason is my unusual planting method, the bud union goes six inches under the soil so the canes die off to the ground in the fall and are cut off and removed (no overwintering sites for the fungus).
And then, given the lack of an early start, the black spot doesn’t really get going until later in the summer and it’s not usually a problem. It doesn’t have the time to really make the plants look ugly.