Diagnosing Soil-Borne Plant Diseases: The Disease Triangle Concept

Why Plants Get Sick

Soil is full of fungi—beneficial, harmless, and pathogenic. Disease requires three factors (the "disease triangle"): a susceptible plant, the pathogen, and favorable environmental conditions. Remove any leg, and disease fails.

Example: Rust on Zoysia

Rust spores are present year-round, but disease appears only in fall when warm days and cool nights create heavy dew.

Prevention Strategies

• Plant resistant varieties: 'Miss Kim' lilac, 'David' phlox, 'Diana' Rose-of-Sharon.
• Improve soil: dig up the problem area, mix in 2 inches of soil conditioner, replant.