Symbolic meaning red apples

All of these references point to similar ideas. Erased, Anohana and Death Note all have similar plot lines dealing with death:

Erased

29 year old Satoru Fujinuma is sent back in time 18 years to prevent the events leading to his mother's death, which began with a series of kidnappings in his 5th grade year.

Anohana

Five childhood companions reunite when the ghost of their dead friends appears, and demands they grant her final wish.

Death Note

An intelligent high school student goes on a secret crusade to eliminate criminals from the world after discovering a notebook capable of killing anyone whose name is written into it.

Apples represent a few things in myth and fiction, one being death and the other being rebirth--usually in the form of having your eyes opened (revealed knowledge). A common one is from Genesis 3:5:

"For God knows that when you eat from it your eyes will be opened, and you will be like God, knowing good and evil."

The use of the apple also shows up in Disney films, one being Snow White, where Snow White eats a poisoned apple. This type of common theme is called an "archetype," or the perfect representation selected across time to convey an idea. These types of ideas can be rooted in human biology and passed down through our genes, the apple specifically is one of those ideas. Human vision is precisely evolved to detect danger(blood), ripe fruit as well as human sexuality (blood in the blushing of cheeks and red lips--also why sports cars are red, they're more sexually appealing that way).

A BIT OF PRE-HISTORY LESSON

The specific correlation here is that humans evolved to detect danger through acute color vision to see blood and death (red, like apples). Simultaneously the acute vision lead to the ability to see time and look into the future where you could possibly die. So humans started to store up food and gather resources. Women couldn't always take care of themselves in history due to child birth, so men did all of the hunter gather stuff. So what happens is that men gained selection pressure through resources over women (best hunter got the best women, survival). Simultaneously women had less danger to contend with and evolved to convey sexuality (ripeness) through their skin, lips and cheeks. This would appeal to hunter gatherers and the best hunters would flock to the most sexually ripe women. Men also achieved this feat, in that sexuality (ripeness) is correlated with fitness (ability to hunt).

Now when it comes to reproduction, the main function is to pass on your genetics. So whomever holds genetic selection power can pass on their genetics more easily. So if you were less visually appealing it usually correlated to your ability to either provide (men) or reproduce (women).

This gave women sexual selection power over men in that they can decline men the ability to pass on genes, e.g. death because their looks correlated to their ability to gather resources. Which is important for childbirth and care. This made men subconscious of their looks and abilities. Hence why in Genesis Eve gives Adam the apple, she judged him and made him aware and self-conscious.

This play between men and women was in essence what caused the awakening of consciousness, ability to see death and the future as well as to see ripe fruit. Hence the correlation between the different stories with women, apples and death.

UTILIZATION OF ARCHETYPES

So when this sort of archetype is utilized in media in this manner it's usually subconsciously being used to convey an idea that your physiology may understand, but consciously you don't. That's why these shows are so captivating, they have ideas rooted in their substructure that appeal to you at your core and represent ideas spanning the entirety of human evolution/existence.

I don't have a copy on hand to check, but according to the Death Note Wiki:

"in Death Note 13: How to Read, Tsugumi Ohba states that there was no underlying reason for his choice of apples. He is quoted as saying that he liked the way the red apples contrast with Ryuk's dark appearance."

So so much for that idea. Doesn't surprise anyone.

A lot of the time authors of popular fiction put something in their book because they thought it sounded or looked good or it was something that reminded them of a friend or family member and the moment the reviewers, critics and teachers get hold of it it becomes some awesome symbol of death or evil or whatever.

But even though the apple symbolism turns out to be readers' bias rather that author intent, Death Note stil has tons of symbolism to discuss, most notably the ubiquitous doves and crows ...

Forbidden fruit - Death Note makes many references to Christianity. The apple is popularly thought to be the forbidden fruit in Genesis, which the snake told Eve would make her smarter, open her eyes, and elevate her to the status of God. ... Eating the apple was the Original sin and the first symbol of temptation.

Here is an answer to a similar question.

Many anime use this symbolic meaning of apples.

Hopefully that answers your question :)