They Don't Live Alone

Malus domestica has many interactions with other organisms, some beneficial and some not so beneficial.  This includes bacteria, fungi, insects, worms and mammals.
Bacteria are used by Malus domestica by means of nitrogen fixation.  Many species of bacteria are capable of making useable nitrogen for organisms, like Malus domestica.  When the nitrogen is collected in the form of ammonia, the apple tree in return will give off carbohydrates for the bacteria.  This is called a mutualistic interaction because both organisms are benefitted.
Fungi also interact with apples.  In a mutualistic way, fungi from the phylum Glomeromycota form endomycorrhizae.  These fungi reside on the outside and outer layer of the roots.  From here, the fungi help in the absorption of water.  They also help absorb nutrients, like phosphorous and potassium.  As a token of gratitude, the fungi are given a food source in the form of carbohydrates/sugars.
 The reproductive conidia of Venturia inaequalis erupting through the cuticle of a crabapple leaf
Fungi can also not be so friendly.  Venturia inaequalis is the fungi responsible for the disease known as apple scab.  This disease creates brownish-black spots on the leaves and fruit of apples.  This fungi causes the leaves to curl and shrivel and causes the fruit to become misshapen and cracked.  The disease often times will cause apples to fall before they are fully developed.  This is called a parasitic interaction because the fungi are harming the plant.
Insects have both mutualistic and parasitic interactions with the apple.  The main pollinators of Malus domestica are insects, like bees .  This is a mutualistic relationship that helps the plant reproduce and gives the insect food in the form of nectar.  There are many insects that are parasitic to the apple.  Parasitic insects include aphids, moths, maggots, and mites.  Fruitworms and other worms can also be parasitic to apples.
Many mammals like the white-tailed deer, moose, and horse eat apples as a source of energy, including humans.  Humans grow apples in orchards all around the world to use as food (see Habitat and Geography).  Although mammals are eating the organism, it is actually a mutualistic interaction.  When mammals eat apple seeds, they are eventually defecated and relocated.  This helps apple trees transport their seeds to other possible growing areas. 

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