Diseases and Pests of Ornamental Plants
- Edition. Pascal P. Pirone
- Malcolm C. Shurtleff. the Iowa State University…
Healthy plants look good, grow well, and are productive. Plants remain healthy as long as conditions favor normal plant growth and development. Sometimes plants are unhealthy, and this occurs when something irritates the plant. The irritation may be somewhat continuous, acting over an extended period, or it may occur nearly instantaneously. Continuous irritation causes disease; instantaneous irritation causes injury.