Powdery Mildews on Bonsai!

Bonsai plants, those tiny trees you hear about in films like "the Karate Kid" and perhaps even in "Gremlins" - you know... the kind being continuously pruned to perfection by old Japanese or Chinese caretakers who also possess incredible martial arts skills! - are not immune to plant pathogens.

Just recently, I received e-mail from a bonsai fanatic on the West Coast (Vancouver) who was a "frantic fanatic" because his self-cultivated bonsais of garry oak and maple were each afflicted with powdery mildew. He thought that since the mildew affected his oak bonsai first, that it passed it on to his maple - consequently he's worried about his entire bonsai collection.

"Not to worry!" I told him. Why? Because powdery mildew is highly species-specific. What attacks oaks will not affect maple. The fact that two separate species of his bonsai collection were affected by two very different powdery mildew pathogens suggested to me that he could improve his cultural practices. While it's true that currently the West Coast (mainly California/Washington States) are experiencing an onslaught of powdery mildew affecting oaks, maple powdery mildew is not usually a problem except in years with high humidity. Again, good cultural practices would help to control and eliminate the pathogens from the bonsais.

So... cultural practices... bonsais... technically, the whole idea behind the practice of bonsai is that daily and continued maintenance of the trees provides the maintainer with a routine for self-cultivation! In this case, self-cultivation is the buddhist idea of maintaining one's self through the maintenance of other living organisms... after all, we are all of the same origin (according to Buddha)! In this case, it's essentially true - we keep ourselves clean to prevent becoming sick - why should it be any different for the trees? The cultural practices designed to prevent or control powdery mildew is a routine of cleanliness and maintenance. Incorporation of these practices may also prolong the longetivity of bonsais and prevent other diseases.

Quite simply to control/prevent powdery mildews:

1. Improve lighting conditions
2. Increase air circulation/reduce overall humidity (by means of a fain, or increase passive air circulation through careful pruning)
3. Water ONLY in the morning
4. Discard/destroy affected leaves after they fall off
5. Replace the soil completely
6. Use safer soap on all leaf surfaces, including the bark

Good luck!