Question
What are they and how do they work?

Thank you.

Answer
Marianne:
I usually hear the term "opportunistic microorganisms" used in my profession.  This essentiality means that a microorganism does not a disease normally, but when the plant becomes stressed or in a weakened state, then the mocroorganism does have the ability to cause a disease.  Plants can become more susceptible to many plant diseases when they are stressed for a variety of reasons.  This is why, as "plant custodians", we should pay a lot of attention to good nutrition and other environmental factors that promote a vigorous plant.

Regards
Steve