A healthy plant is the result of an on-going landscape management program as outlined in the calendar. Part of the program is regular inspection of the plants to catch any problems before they become serious. An Ounce of Prevention Preventive measures may be the only control for some plant problems. Disease controls are designed to prevent diseases and can not help much after a plant has succumbed to a fungus or bacterium.
Every ornamental plant has characteristic traits, some good and some bad.
The management required by that landscape plant is based on the expression of those traits. If most traits are good, the plant may be relatively trouble free. If the traits are mostly bad, the plant may be more trouble than the good traits are worth. For instance, many Siberian elms, which are usually sold as Chinese elms, are planted because they grow fast. That's the good trait.