There is white powdery stuff growing up the branches on my holly. Is this a fungus?
Yes, this is most likely a fungus called powdery mildew. It is common and can affect holly shrubs, but will typically not seriously harm them. A spray of whole milk diluted in water has been shown to help kill powdery mildew fungus. These articles discuss other ways to treat and prevent powdery mildew:
https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm
https://www.gardeningknowhow.com/plant-problems/disease/get-the-cure-for-powdery-mildew.htm
http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/powdery-mildew/
Are holly shrubs deer resistant?
Holly shrubs are considered relatively deer resistant. Unfortunately, deer will eat holly and pretty much anything else if they are hungry enough, so there are no guarantees.
Here are some more ideas for deer resistant plants:
https://www.gardeningknowhow.com/plant-problems/pests/deer/deer-resistant-plants.htm
My shrub is gorgeous and full of bright red berries. I'd love to use them for decorating for the holidays, but it's only September 26! How long do they hold on into the season and, if I cut them now, can they be preserved? I'd love to preserve the greenery also, not just the berries.
This may depend on the variety, but many holly varieties will ripen berries throughout the fall and early winter.
These articles may give you more information on preserving the branches:
https://pubs.ext.vt.edu/430/430-470/430-470.html
http://www2.ca.uky.edu/agcomm/pubs/ho/ho70/ho70.htm
We planted a 3 ft. tall holly bush from a nursery in July in central Florida. After the very hot summer and watering often, it is now losing its leaves. The leaves are very green. It faces north and gets some shade. What is going on?
This sounds like a case of delayed transplant shock, a very common condition in recently planted shrubs. It occurs because the roots have not fully established in the soil yet, making the plant ore susceptible to stress. Hot weather can make the problem worse. See this article:
My holly bush has berries, but they are not red but a light orange. What causes this? Will they turn red?
Some varieties of holly produce orange berries naturally. This is probably what is going on. If your bush has previously produced red berries, see this article:
I have 6 holly bushes I've shaped into trees that over 15 years old. They have always been healthy up until last year when one of them started turning yellow (leaves). Right now the other 5 are dark green with few berries and this one oddball has yellow leaves and full, I mean FULL, of berries. Do you have any idea what is wrong and how I can bring it back to health? The picture I'm uploading was this summer and the leaves are yellow, but no berries. Now it's full of berries and less leaves, but the other 5 trees look the same.
This is vary well a deficiency in iron, called iron chlorosis.
Here is a link with more information.
https://www.gardeningknowhow.com/ornamental/shrubs/holly/holly-yellow-leaves.htm
Why did the trunk of my holly bush turn black?
This could be caused by Scale, which it them will result in sooty mold--the black on the stems.
I would treat the plant with Neem OIl.
https://www.gardeningknowhow.com/ornamental/shrubs/holly/diseases-of-holly-bushes-pests-and-diseases-damaging-holly-bushes.htm
https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm