Black Spot is a foliar disease that attacks roses of nearly all strains. It’s characterized by black spots that appear on leaves in either round or irregular shapes. The spots are usually surrounded by yellowed leaf areas (chlorosis) and the spots, if left to progress, often coalesce and take over entire leaves.
Diseased leaves will often begin to shed from the plant as it attempts to protect itself. This repeated defoliation can severely weaken the plant and will almost always lead to a lack of budding and flowering as the plant pours its energy into survival. If left long enough, Black Spot will likely reach to stems and even the canes of the bush itself.
How to Treat Black Spot on Roses
If your plants are infected and shedding leaves, make sure to rake and clean them up carefully and often. The fallen leaves can spread the spores of Black Spot by blowing into other plants. Affected plants should be heavily pruned to improve air circulation (the spores of this blight require relatively damp conditions). Watering at ground level is also recommended (even if your roses aren’t infected).
Infected leaves that have been removed should not be composted and used on other rose plants, but should instead be either composted for use in other areas of the garden or burnt.
Roses themselves should not be planted too close together and should be in an area with good sun and air circulation. Some strains of rose, especially the bushy, heirloom varieties, are particularly fungus resistant.
Organic fungicide options are readily available at most garden stores. A do-it-yourself option is to make your own by mixing a couple of tablespoons of baking soda (potassium bicarbonate) into water and stirring until dissolved. Spray applications that cover leaves can be used every 10-14 days. Be aware that this will likely remove or damage the waxy coating inherent on rose leaves, so applying too often can severely damage the plant.
Other alternative options for fungicide include products with copper infused in them, lime sulfur (only use when the plants are dormant), neem oil (a preventative), sulfur (dust, wettable, etc.) and commercially hydrogen dioxide is also available.
Prevention of Black Spot on Roses
An ounce of prevention beats a pound of cure in all cases. Using the above information regarding air flow and sunlight, knowing how the fungus spreads and survives is important to knowing what to do to prevent it.
Black Spot, being a fungi, requires specific conditions to thrive and propagate. It takes about 2 weeks for the spores of the fungus to form and they must be exposed to at least 7 hours of moisture before germination. Lower temperatures (65F) is best for germination and 75-80F is best for development once established. Over 85 usually kills or inhibits the fungus and a lack of moisture for more than 24 hours will likely kill most of it.
The fungus can overwinter on mulched leaves and on living canes, but spores cannot survive the soil for longer than a month under good conditions.
Taking all of these things into account, cleaning up fallen leaves and pruning back bushes adequately for winter is the most obvious preventative for Black Spot.