Black spot on roses is a fungus infection which causes black spots to appear on the rose plant's leaves. Tips on preventing and defeating this rose-marring disease. How to identify black spot on your roses. When to spray.
Pictures of infected leaves to help you confirm your diagnosis.
Black spot fungus Diplocarpon rosae is the bane of rose gardeners everywhere but is is much more common in humid climates than in dry places. This is the primary reason why roses grow so much more luxuriantly in California than they do in Florida.
Thankfully, climate is not the only thing that determines the probability of black spot disease infecting your roses. The plants themselves play a part.
The single best way to prevent black spot in roses is to plant black spot resistant rose varieties. But even a resistant variety can be infected if weather conditions favor it or if the rose shrub is stressed by poor cultural conditions.
Failing to feed the roses enough potash and warm, wet summer weather makes infection more likely.
Now that the rainy season has begun in earnest, my 'Don Juan' rose has become infected.
I'm waiting for a dry spell to spray it.
'Don Juan' is robust enough to shake off the infection.
If your rose develops a black spot infection, very noticeable black circles with fringed edges will appear on the foliage causing it to turn yellow and drop.
Severe infections will cause the flower stems to die back and denude the bush. Weak plants can be killed by the disease if it is not controlled.
There is a recipe for a homemade spray for black spot made with baking soda and water over on the Powdery Mildew on Roses page if you need something right now.
A better bet for defeating rose black spot organically, is Green Light Rose Defense. It is a neem based formulation that protects against black spot on roses,
Diplocarpon rosae fungus is specific to roses so there is no need to worry about the disease spreading to your other garden plants.
One of the most common ways black spot spores spread is in water
droplets. Rain and overhead irrigation systems are the culprits here.
There isn't much you can do about the rain but you can use drip
water your rose bushes.
Another thing you can do is observe proper spacing in the rose garden. Crowding the plants keeps air from circulating freely around them and causes moisture to remain on the foliage for too long.
Remove infected leaves from the garden. Bag and dispose of them. Do not attempt to compost them.
There is a systemic soil drench which can be watered into the soil in the spring which will prevent black spot and rust. It is a Godsend to gardeners who have a favorite rose that is disease prone as it eliminates the need for spraying.
This is not an organic product and it should not be used on or around edible plants.
Black spot spray controls black spot best when it is used before the infection takes hold. You can and should spray as soon as you notice black spots on your roses. This will protect uninfected leaves and plants. But it will not cure the infection that is already present.
Black spot control is best achieved in subsequent seasons.
What you do is you spray the plants which have been infected in the past. Spray when the leaf buds begin to open in the spring, again a week later and a third time if spots appear.
How many times the plants need to be sprayed will depend on how susceptible they are to the disease in the first place, how damp the growing environment is, how the plants are being watered and fed, etc.
Some types of roses can handle black spot fungus better than others. Some rose varieties will just shake it off and keep right on growing and blooming whether you treat the infection or not. Others will completely defoliate, weaken and go into a decline.
Ortho Rose Defense is the best chemical fungicide for use against black spot on roses. Like the other black spot control sprays I recommend, it also protects against the other fungal rose diseases.
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