If you grow roses, you’re probably familiar with a pathogen called black spot (Diplocarpon rosae). This fungal disease causes dark spots to appear on the leaves, and once it takes hold, the leaves eventually turn yellow and fall off. During particularly bad years, the plant may become completely defoliated. Black spot is a devastating disease for rose growers across most of the country, but there’s hope! Here are a few steps you can take to keep this pathogen from destroying the roses in your garden.
1. Promote Air Circulation Around Roses
Like almost all other fungal diseases, black spot thrives in wet weather. When there’s high humidity or increased rainfall, that’s when gardeners should be on the look-out for early signs of black spot. The sooner the pathogen is noticed, the sooner action can be taken and the better the control. Although you can’t control the weather, your roses should have plenty of air circulating around them. Make sure they are properly spaced and pruned.
2. Water Roses Low and Early
Because black spot thrives on wet foliage, irrigate your roses only at ground level. Use drip irrigation, or apply water by-hand only to the root zone of the plant, instead of using over-head sprinklers. Irrigate in the morning whenever possible, so any foliage that happens to get wet has time to dry before nightfall.
3. Clean Up The Rose Garden
Black spot easily overwinters on any fallen foliage that’s allowed to remain in the garden. The spores readily splash up from the ground the following spring, infecting the newly emerging foliage and causing a new outbreak. Every autumn, pick up all of the fallen rose leaves in your garden and dispose of them in the garbage or the burn pile to prevent re-infection.
4. Apply Fungicides To Your Roses
In early spring and throughout the growing season, ideally before the disease takes hold, use a commercially prepared biological fungicide, such as one based on either potassium bicarbonate or Bacillus subtilis as a preventative measure. When applied according to label instructions, these products do an excellent job keeping black spot at bay.
You can also make your own anti-fungicidal mixture by mixing 1 gallon of water with 2 tablespoons of horticultural oil and 4 teaspoons of baking soda. Called the “Cornell mixture,” this recipe was developed and researched at Cornell University, and is commonly recommended for use against many common fungal organisms, including black spot and powdery mildew. After mixing it well, spray it on both upper and lower leaf surfaces ever two to three weeks throughout the growing season.
5. Grow Resistant Rose Varieties
Another option is to replace all of your black-spot-prone rose varieties with resistant choices. You can avoid the need to spray by planting selections with a known resistance to the pathogen. There are many on the market, and your local nursery should be able to help you find a good cultivar for your growing zone.