No, black spot contagious other plants is a common fear but the truth is this fungus only attacks roses. Diplocarpon rosae has evolved to infect the Rosa genus alone. Your tomatoes, hydrangeas, and every other plant in the bed are safe from it. The fungus cannot grow on any species outside the rose family.
I used to panic every time dark marks showed up on plants near my rose bed. One year my phlox and coneflowers had leaf spots at the same time my roses broke out with black spot. I was sure the disease had jumped across the whole border. A local extension agent set me straight. She told me those other spots came from different fungi that each target their own host plants.
The Diplocarpon rosae host range is limited to roses for a good reason. This fungus co-evolved with the Rosa genus over thousands of years. Its spores, called conidia, only latch onto rose leaf cells. The leaf surface gives the spore a chemical signal to germinate. Other plant species lack that signal. The spore lands, fails to penetrate, and dies without causing any harm at all.
Black spot spread garden wide among your roses happens fast though. The main paths are rain splash, dew, overhead watering, your hands, and sick transplants. Maryland Extension lists all these routes. One sick bush showers spores onto nearby roses during a single storm when plants sit too close.
Space your roses 3 to 4 feet (0.9 to 1.2 m) apart to limit splash between bushes. Good spacing lets air flow through the canopy and dry leaves faster after rain. Wet leaves give spores the moisture they need to sprout. Anything that speeds drying cuts your infection risk. Skip the overhead sprinklers and water at the base in the early morning hours.
Check new roses for spots before you plant them in your beds. Quarantine new bushes away from healthy plants for two to three weeks and watch for signs of disease. One sick transplant from a nursery can bring spores into a garden that was clean all season long. This step takes patience but saves you from a big headache later.
Your non-rose plants need no special shield from black spot at all. Focus your work on the roses. Keep them spaced out, prune for air flow, spray on time, and clean up fallen leaves. Those steps block the fungus from jumping between your rose bushes. That is the only real spread risk you face in your garden.
Read the full article: Black Spot Roses: Prevention and Treatment Plan
