Hello doctordlewis and thank you for your question.

You are correct — space can be very limited in the average backyard garden. Unfortunately, you do need to keep red and black raspberries separated by 100 feet or more. But once you understand why, you may be able to develop a workaround, since you know your space better than I do.

The reason for the separation between black and red raspberries is to prevent easily transmitted diseases from infecting vulnerable plants. This applies to hybrid varieties as well. Red raspberries can carry viruses such as mosaic without showing any signs or symptoms. This invisible threat can infect your black raspberries via aphids, which travel on the wind and typically arrive in your area in early June. Read more about aphids on raspberries in this Penn State fact sheet: https://extension.psu.edu/aphids-on-raspberries-in-home-gardens

Hybrids such as boysenberries and marionberries are susceptible to the same viruses, including Raspberry Bushy Dwarf Virus (RBDV) and the crumbly fruit virus complex, which affect both red and black raspberries as well as marionberries, boysenberries, and loganberries. What makes RBDV particularly challenging is that it spreads through pollen via pollinators such as honeybees, meaning physical distance alone offers incomplete protection in a typical backyard setting. https://extension.umn.edu/plant-diseases/viruses-backyard-fruit#symptoms-of-raspberry-leaf-curl-virus-1793861

So what should you do if you want to grow both black and red raspberries in your backyard garden? Separation by distance remains the safest approach. If you can place a physical barrier between them — such as a home or garage — and position the black raspberries upwind from the red raspberries, that would be helpful as well. Be sure to start with certified disease-free plants, and keep a close eye on them beginning in mid-May, inspecting regularly for the arrival of aphids. Once aphids appear, treating for them promptly can reduce the chance of an outbreak.

Review the attached fact sheets and take the recommended precautions to help delay disease pressure. Over time, that pressure will build when plants are too close together, and infection becomes increasingly likely. What you observed at the Marion County Master Gardener Demonstration Garden was probably for display purposes only and should not be taken as a planting recommendation.

If you have any additional questions, please do not hesitate to ask.