Plants can be compromised by insect attack and disease from fungal infection or virus.
The range and target of attack is varied. For example, black spot is a fungus that affects the photosynthesizing tissue of roses; caterpillars chew leaves reducing the photosynthesizing tissue of plants; phytophthora is a fungus that targets plant roots reducing the uptake of water; nematodes target plant roots reducing the uptake of water.
The qualified gardeners have been trained to notice and diagnose insect attack and disease as a precursor to treating the attack appropriately. For example, plant foliage may become covered in a black sooty substance. This is caused by aphids which secrete honeydew, and this in turn attracts Sooty mould, a fungus that covers leaves and stems in a black sticky substance that blocks light and reduces photosynthesis.
Treating the aphids with a systemic insecticide will eliminate the insect attack and the fungus at the same time. If you are dealing with a fruit tree that is consumable then you cannot use a systemic insecticide but rather a contact spray like pyrethrum.