Necrosis means dying-dead tissue regardless of whether it is plant of animal, and typically is seen as a darkening of tissue, esp if it is "meat-flesh-muscle-skin" (Necro means BLACK-dead.) However cells of plants or animals tissues-organs die naturally throughout life when they reach the end of their natural life cycle. If an animal is single celled and dies,that is its END.
When a patch of visible tissue looks dead-dying macroscopically, we call it "necrotic." Necrosis is common in gangrene, a bacterial infection microscopically, for example. Necrosis may also imply a "spreading" of the diseased-dying tissues amongst the healthy cells which are being OVERTAKEN-PROCESSED by the putrification microscopic organisms of nature, as is common in gangrene of diabetes (or non-diabetes) after neuropathy increases loss of feeling and thus an injured limb has no feeling and the wounds natural healing does not take place and the microbes begin to take advantage and get the upper hand. The high blood sugar makes it ideal for the putrification microbes to grow and prosper and speed the process of "parasitism" of tissues we don't want to die so that it occurs faster than cell multiplication necessary for healing. Basically if cell death is FASTER than cell mitosis-healing,then we would call that necrosis. It may or not be "rotting" per se. Rotting is a "slang" non-technical term usually ascribed to meat-foodstuffs putrifying by bacteria-fungus. It describes the NATURAL microscopic process of putrification by natures laws, but we may not like it or think of it as "good," THE MAJORITY of organisms-cells of this planet are involved with microscopic putrification processes (bacteria and fungia specifically) and it helps to clean up and recycle the micro and macromolecules of all organic material of life so that new-fresh tissues can be made from the old dead stuff. Without necrosis and putrification the planet would be overrun with piles of garbage! A compost pile is the perfect example of putrification-rotting process that we like and do not associate with disease, but macroscopically the composte pile is not any different from necrotic meat. Putrid refers to the smell of putrification and is accompanied by specific gaseous by-products these putrification microbes make while they are doing their clean up work. Warm-humid weather makes the process of putrification-necrosis happen faster. The tropics are an ideal place for wounds to become necrotic faster, for the body or compost piles to rot faster. Hot humid days in NY york during a garbage strike are horrible examples too. As global warming increases it will increase-move the amount of tropical zones on this planet and thus putrification and necrosis "diseases" of plants or animal tissues. The microbes are having a great time and are thriving while we think it is "bad," but we need these processes! BTW, the more preservatives we use in our food, the slower the putrification process becomes. Some say that the natural rottingof human cadaveris slower if the person who owned that vehicle before death ate a lot of preservtives. Also, some corpses insealed coffins putrify slower than innon-sealed coffins. Some laws require coffins to be sealed so that the "disease: organisms don't get in the soil and spread. How fast will your body rot after death? Have you eated a lot of preservatives?Will you be buried-cremated? Will the coffin be sealed-unsealed? Will your body macromolecules be easily recycled back into quickly or more slowly. If you die in a hot jungle your body may be gone quickly, esp if flesh eating scavengers like beetles get to it before they find your body.
Personally, I had part of my plantar fascia on the bottom of my rt foot become necrotic after I stepped on a toothpick and they could not find it and get it out after 6 hours in the ER and several xrays, which can't see wood. Within three months I was very ill. When they did exploratory surgery, they found the toothpick embedded between bones of a joint and part of the bottom of my foot was "dead" necrotic. They removed toothpick finally, cleaned the necrosis out and the infection was controlled with antibiotics. Only some of the tissue was able to heal and regrow. I do not have the normal amount of plantar fascia now 22 years later.