Understanding whether a tree is healthy or ill can be attributed to it's surroundings as well as it's own general health. Primary affects can be detrimental to the health of a tree, whereas secondary affects that are not usually harmful can take a hold of an already weakened state.
A common problem is poor nursery management, thus being that a plant that is root-bound in the pot will have a compromised root system and grow poorly in the ground. Understanding that the root system is where vital minerals and water is collected and transported through the entire tree, it makes sense that if this is compromised, the ongoing health will be also compromised, leaving it vulnerable for more secondary factors (i.e. pests, fungi or bacteria) to destroy it.
Poor soil and nutrition can also lead to a weakened state for a tree.
We'll take a look at some of the common causes for poor health of a tree or plant:
Fungi - predominately seen on decaying wood or fruit known as Rust, There is approximately 7000 species of fungi rust that begin as spores and land on the plant surface. It germinates and invades the host. Limited to leaves, stem and fruit. Plants with fungi rust can appear stunted, yellowed and cause deformities in the plant's normal growth. This can become hazardous when the enzymes of the fungi begin to decay the wood, resulting in limbs and branches breaking.
Bacteria - can be within the water and absorbed by the plant or tree. There is many strains of pathogen bacteria that can affect the plant or tree. Some produce ice nucleation active proteins (INA) that can freeze the water molecules at fairly high temperatures, thus making plants very vulnerable to frost damage.
Viruses - Like all other viruses, plant viruses are intracellular parasites that cannot replicate without the host. Plant to plant transmission is usually carried via an insect, animals feeding on the plants or seeds, gardening tools that scrap into the sap of a tree and then interact with another tree. Seen on leaf discolouration and flecking on the plant or tree.
Phytoplasmas - Infection of bacteria-like phytoplasm of the plant leading to yellowing of leaves, leaf size reduction, shortened internodes and branch die-back.
Insects - widespread and extremely common cause of damage is insects that feed and breed on trees.
Mites - Are microscopic pests that feed on leaves and lay their eggs that produce abnormal growths on leaves called leaf galls. infestations can be seen with twisted and curled leaves. This affects the plants ability to circulate water and carry out normal photosynthesis and over time can kill or severely weaken a tree.
Parasitic Plants - A plant that derives some or all of its nutritional requirements from another host plant. Some plants will not kill a tree, but simply share resources to survive. Whereas a strangler fig will encapsulate and outgrow its host leaving the host to die.
Weeds - Some weeds can out compete young saplings for sunlight and nutrients. Others can strangle trunks and branches.
Larger Animals - Animals that feed on the foliage and the bark can leave the tree with detrimental injuries and poor health. Most trees recover over time, but others simply wither and die.