What To Do About Them
It’s often said that the conditions that favor the diseases of roses are also the conditions that are favorable to successful rose growth. Mild winters and warm sunny summers provide an excellent environment for the more common rose diseases but it is often poor rose bed management, or lack of any “disease control plan” that causes the most problem.
The most common diseases of roses are: Black Spot, Powdery Mildew, Rust, Canker, Botrytis, Crown Gall, Downey Mildew and Rose Mosaic Virus. Luckily, the last five are found infrequently so we really only have to deal with the more common, Black Spot, Powdery Mildew and Rust.
You find Black Spot on your leaves, especially the new young ones, starting in springtime. It’s usually a circular blackish spot with feathery edges which begins in the spring showers.
It is a fungus which produces the plant hormone, Ethylene, which causes the leaves to brown, dry and drop. This often results in bare stems with a few spotty, brown leaves. If a leaf surface stays wet for 24 hours or more, the spores germinate. Once established the cycle will continue every 10 to 18 days.
Once you have it, you can’t get rid of it!
As long as the temperature is between 50 and 80 degrees, and the leaves get wet, chances are you will get Black Spot.
Personally I don’t think there is a rose bush anywhere that is not susceptible, in some small way, to this leaf disease. Of all the diseases of roses, this tends to be the most common.
So what do you do?
First of all, plant rose bushes that are resistant to black spot. Check the labels and ask the nurserymen what they advise for your area.
Some roses are notorious….avoid them!
Secondly don’t let the spores overwinter in your rose bed. Clean up the beds, make sure there is good air circulation between and through your roses, and spray the bed and winter stems with a dormant oil.
Thirdly, spray for blackspot in the spring as the first new leaves begin to open. This will stop any over-wintered spores from gaining a hold. Spray (if you have to!) perhaps every ten days for the first month.
Finally, don’t water the leaves of the bush in summer unless the sun has time to dry them thoroughly……..in other words, water from below not overhead!
If you are unfortunate enough to get some black spot (and most of us do!) pick off the leaves and incinerate, don’t wait for them to fall into your rose bed.
As for what to spray?….. ask the garden centers in your area (preferably ones that sell good roses!) and try to use as near to organic as you can. Use goggles and gloves, don’t spray in a wind and read the label…. then read the label again and do what it says!!!!
During the warm summers you are more likely to find Powdery Mildew on your leaves and buds. This is the second most common of the diseases of roses.
Usually most prevalent when afternoons are in the 70 to 80 degrees and the nights are cool but high in humidity. You will see a white, powdery sort of web which is actually a clear fungus. Leaves become twisted and depleted of nutrients. If left to grow, the disease will eventually cover all the leaves of the bush.
Again the first infections come from over wintered spores….last years fungus returning! Unfortunately these spores can also move from bush to bush on air currents and it is the newer leaves that are most susceptible. Rainfall tends to destroy the spores.
So what to do now?
Same again…..provide good air circulation, clean up the rose beds, water early morning and not overhead. You could also “hose” the area down and wash the mildew away as long as it’s early enough in the day to let the leaves dry.
If you have to spray, talk to the experts at the nursery and read the labels. Try for the organic varieties first.
Rose Rust is another disease of roses that starts in spring and and peaks in early summer. You get rusty, orange blotches on top of the leaves and sometimes on the stems.
Underneath the leaf will be powdery, orange lesions. Like Black Spot, Rust prefers the warm sunny days with spring showers. Sometimes these leaf spores will turn to black in the fall, and then over winter if not dealt with. The best temperature for Rust growth is between 65 and 70 degrees with 2 to 4 hours of moisture.
You guessed it!!
Do all the things you did for the other diseases of roses. And if you have to spray, get the right one and follow the directions.
Diseases of roses invariably occur during the peak growing time, when new shoots, new leaves and new buds are freshly developing. For this reason you need to be ever vigilant, checking your roses carefully every week for signs of these attacks.
With good clean rose bed management however, the diseases will be minimized and your time better spend enjoying the blooms of your labors.