Full stem is covered like white powder like disease. How to cure this?
The powdery residue is from a fungal issue called powdery mildew. You can treat the plant with neem oil. Here is more information: https://www.gardeningknowhow.com/plant-problems/disease/get-the-cure-for-powdery-mildew.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
Have fertilized and water is automatic but leaves continue to yellow and fall off - even with fertilizer plants don't have deep green healthy looking leaves.
There are several things that can cause leaf yellowing in hibiscus. Thisx article will help you pinpoint the cause in yours: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/answers-to-what-causes-hibiscus-leaves-turning-yellow.htm
This article should help: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/how-to-care-for-hibiscus-plants.htm
I think I may have overfertilized my hibiscus. I just bought it and read the tag which said to fertilize once a month. I used Miracle Grow-the strength for houseplants. I noticed later that it may already have a time-release fertilizer in it. (little green beads) The leaves are very limp and no longer dark green. Please help!
If you've only fertilized once, it probably hasn't been overfertilized. Hibiscus are heavy feeders, but a constant overfertilization will burn the roots. Has the plant been kept wet? If so, you've tried too hard. Let the plant dry slightly between waterings, then water thoroughly. If you've been keeping the plant on the dry side, let it soak overnight so the soilball can become rehydrated.
What can I do to correct the yellow leaves with black spots on my hibiscus plant?
Your hibiscus probably has a fungus called black spot. It is fairly common and affects other plants, especially roses. This link is to an article with information about black spot: https://www.gardeningknowhow.com/ornamental/flowers/roses/black-spot-roses.htm
yellowing leaves on hibiscus can occur when the plant gets too dry. Leaf age can sometimes cause yellowing.
The leaves are turning yellow and dropping off. What can I do to save this plant?
This should help you identify the problem: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/answers-to-what-causes-hibiscus-leaves-turning-yellow.htm
I live in Wisconsin and I've had Hibiscus plants planted outside for at least 5 years now. In fall I always cut the stalks down to about 6 inches. Every year I get new shoots that grow off the old stalks. This year I didn't get as many stalks. I'd like to know the proper way to take care of my plants for many more years of enjoyment.
Hopefully this article will answer your question: http://homeguides.sfgate.com/care-perennial-hibiscus-tree-year-round-42113.html