Black spot is basically what it sounds like: roundish black spots on leaf surfaces. The spots grow over time, and the foliage around the spot yellows. Eventually, the entire leaf falls off.
Damage to Plants
The issue with black spot is that the defoliation that occurs when the fungus is allowed to get out of control weakens the plant. And not just for the current growing season, but for the next season as well. The main function of plant foliage is to absorb energy from the sun and perform photosynthesis. If your plant loses all or most of its leaves to black spot, it will be unable to collect and store energy for the winter, as well as for the burst of growth that occurs in spring. You'll see weaker growth and reduced bloom in the season following severe black spot damage.
Disease Life Cycle
Black spot spores overwinter on infected foliage and canes, including infected foliage that has fallen and been left on the ground. In spring, spores are splashed up onto newly emerging foliage during rains or irrigation. Once the weather begins to stay consistently warm and humid, the spores germinate and infect the plant within one day. Visible symptoms (black spot and some yellowing) will be evident within five days, and it will produce and spread new spores within ten days. The new spores will infect other parts of the plant, or be carried on the wind to any other nearby rose bushes.
Treatment and Prevention
There are several organic ways to control black spot. Most of them rely on regular monitoring and upkeep. None of them are difficult and can be accomplished while you're admiring your roses.
- Plant roses in full sun. If you give your roses a spot where they receive a full six to eight hours of sun per day, you'll not only have plants that grow more robustly, but also plants that are more able to resist black spot. Black spot loves moisture, and, in shade, water evaporates much more slowly. In full sun, evaporation happens more quickly, which not only helps prevent black spot, but other fungal diseases as well.
- Plant roses in an area with good air circulation. This accomplishes the same thing as planting in an area with direct sunlight: moisture evaporates more quickly. In addition, with plenty of air circulation, hopefully the breeze will blow any newly-germinated black spot spores away from your roses. In an area with poor air circulation, the spores have nowhere to go but back onto your plant and the surrounding soil.
- Water correctly. Try to avoid overhead irrigation, which wets the foliage. It's more efficient to water at ground level anyway; you lose less water to evaporation. Also, avoid watering late in the day. Water evaporates much slower in cooler evening and nighttime temperatures.
- Remove leaves that show signs of infection. As soon as you see black spot on your rose foliage, remove any infected leaves. Throw these leaves away. Don't put them in your compost pile. If you check your roses regularly and remove infected foliage immediately, you'll have a good chance of keeping black spot under control and keep it from infecting other parts of the plant.
- Keep a clean garden. Pick up and throw away any fallen rose foliage regularly. Especially in late winter, rake up the area around your roses, dispose of any debris, and give the entire area a good three-inch deep layer of mulch. If you do this before new foliage begins to emerge, it's fairly unlikely that you'll have any major issues with black spot.
- Choose resistant cultivars. If you've had persistent problems with black spot, be sure to make sure that any new roses you add to your garden are resistant. Your local cooperative extension service will be able to provide you with suggestions.
- Prevention is the best method. The old remedy of treating black spot with a baking soda spray has been shown to be ineffective. The only way to control black spot organically is to practice the prevention and maintenance tasks above.