Q.Hibiscus disease
What is wrong with hibiscus plants that have black spots on the leaves? I saw a group of plants at a nursery and all looked ok but had black spots on the leaves. Did not know if I should buy one or if it would infect my other garden plants. Please advise.

I can't say for sure what the plants may have or the reasons, but I would not advise purchasing anything but a healthy plant specimen.
Here are some articles that will help you.
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/answers-to-what-causes-hibiscus-leaves-turning-yellow.htm
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/how-to-care-for-hibiscus-plants.htm