Black spot disease ruins roses, turning them bare and skeleton-like. The fungus spreads easily, especially in warm and humid conditions. If left untreated, the disease can kill the entire plant. The fungus can also infect young canes and flowers, causing them to blister and develop red spotting. The weakened plants become less healthy, so they are more susceptible to other diseases and pests. The good news is that with proper care and treatment, roses can recover from black spot.
The first step to treating a black spot outbreak is to prune away and dispose of any infected leaves or stems. Be sure to sanitize your pruners to prevent spreading the fungal spores to other plants in your garden. Infected canes should be removed, as well.
It is important to monitor roses regularly, especially in warm and humid weather. Ideally, you should check them every 7 days to see if they are showing any signs of a black spot outbreak. If you do notice any spotting, treat them immediately with an approved fungicide such as Daconil or neem oil. These products can be found at most local nursery or garden centers.
Preventing black spot is much easier than treating it once it starts to show up on your roses. The fungus overwinters on infected fallen leaf matter and stems, and germinates in spring when conditions are right. The fungus spreads by splashing of the spores from fruiting bodies, called acervuli, that form in black spots on leaves and stems. The spores then fall on and infect new tissue, starting the cycle all over again.
Fungicides can be purchased at most local garden centers and are very effective against black spot when applied according to package instructions. Organic fungicides such as neem oil and baking soda also work. Neem oil, in particular, is very effective because it contains a natural plant hormone that discourages fungus growth.
Other methods to help reduce fungus problems include reducing weed competition and opening up the space around your roses so that air can circulate freely. Watering early in the day and keeping the foliage as dry as possible are important, too. Situating your roses where they will receive full sun all day, particularly morning sun, helps them dry quickly of the dew that can encourage black spot fungus.
In general, choosing a breed of rose that is suited to your climate and knowing how to prune them properly can help reduce black spot problems. Fungicides are also available for purchase that can be rubbed into the plant to protect it from diseases such as black spot.