First, I do everything the experts tell you to do.
Plant roses in the sun. This may seem elementary, but roses need plenty of sunshine to produce more flowers and increase their resistance to all diseases.
Give roses enough food and agua. Roses are heavy feeders, and they get stressed without plenty of food and water. In the hot summer, when all of my plants, including the roses, are stressed, I spray them once a week with Sea Tea. I love that stuff. Like humans, when plants get adequate food and rest, they are less likely to get sick. You can also make your own compost tea, if you’re so inclined.
Provide roses with good air circulation, Roses like their leaves dry. This also helps with the occasional powdery mildew Oklahoma roses get in early spring.
Water roses from the bottom. Again, we’re going for dry leaves. I used soaker hoses for years, and now I have Netafin soakers as part of my irrigation system. If possible, water them in the morning.
Mulch plants. Our enemy, blackspot, multiplies in moist soil. During rain or irrigation, the spores will splash up onto rose leaves, starting the next round of disease. Mulch helps to block the spores.
Clear out leaf debris. Remove any leaf debris under the plants and throw it away. Do not compost diseased leaves unless you’re sure your compost pile is hot enough to destroy the spores.
Keep tools and implements clean. When pruning a diseased rosebush, wash pruners or any other tools in a mild, ten percent, bleach solution before using them on another rose. Also, switch out gloves and wash those used on infected shrubs in hot water. Blackspot spores can be transferred.
Leaf stripping. In the spring, if a particular plant is covered, and it is extremely hardy, strip the leaves, and let it start over. It will recover, and the new growth has a better chance of not being reinfected. However, I would not try this with a grafted rose. I strip the leaves of ‘Reine des Violettes’ because she is particularly prone to blackspot in the spring. Destroy the infected leaves. If a minor outbreak occurs, remove the infected leaves and destroy them too. I carry a small bag around with me in the garden for this purpose. Sometimes, I can stop blackspot before it takes hold.
Buy disease resistant varieties. Probably the most important tip of all. By listening to the buying public, hybridizers are continuing to create more disease resistant varieties. Health concerns and general busyness have changed the market for the better.
In spite of all these precautions, into every garden, a little blackspot must grow. So, what can be done to control it once it occurs?
Start with the least toxic prevention and control first. If I have time, in February, before leaf break, I spray a combination of lime and sulphur on my plants. (My, doesn’t that smell good?) This spray prevents some pest and disease problems and is a natural barrier. Spray in the early morning hours if you can.
In spring, before temperatures rise to 80 degrees daily, you can spray a solution of one tablespoon of baking soda and one tablespoon of horticultural oil mixed with one gallon of warm water Although I’ve never had much luck with this on blackspot, I have noticed it works well with powdery mildew. Any higher temperature, and the oil will burn the leaves.
I’ve used Neem oil with some success, but it is an oil, so that 80 degree rule still applies. Also, I think it smells like something rotten, so get ready. Neem oil is a fungicide and an insecticide, so it also helps with insects. Conventional wisdom cautions spraying it around bees or ladybugs. Neem oil is possibly toxic to them, although not to us, so spray it early in the morning before the bees get started. Green Light Organic Rose Defense – Pint #07516 is made from Neem oil if you’re interested.
For more organic tips, see The Organic Rose Garden, by Liz Druitt. I have to agree with her when she writes about tolerance:
“You don’t have to know the solution to every rose problem if you can learn to see beauty in good health overall, without demanding total perfection. It’s even possible to grow show-quality blooms on a bush that has blackspot on a few leaves.”
I do, occasionally (perhaps twice a season), spray only those bushes which are in full and total breakout. When I do, I use Ortho Garden Disease Control Concentrate – Pint #0297660, which has Daconil as its chemical ingredient. Of course, once you read the Pesticide Action Network’s page on Daconil, you may want to reconsider using it. Note, that there are even stronger chemicals used by large rose growers, but I won’t list them here.
If you do decide to engage in chemical warfare, suit up in full hazard gear: long pants, long sleeves, gloves, and a respirator mask because many chemicals are potential carcinogens.
There are a few roses I want to grow which have scent and beautiful flowers. Unfortunately, hybridizers are still trying to get the disease resistance and scent thing to occur consistently in the same plant. When they do, I will be dancing in the street. In the meantime, I do all of the above to combat blackspot, but I start with good garden practics.
Each year, I spray less and less, and frankly, a lot of my roses have blackspot in the middle of summer. I read somewhere that so do Queen Elizabeth’s roses, and if she doesn’t mind, then neither do I.