Black Spots

[D.J. Feuerborn]

Hello. My neighbor has problems with his orchids (cattleyas) getting black spots. (I live 3 houses away). Neighborhood is University Hts./Hillcrest. Outside plants. On patio, in shade. Some air circulation but not great. He uses stored rain water for watering. Feeds his plants every two weeks. They all do very well Except for black spots which sometimes hits new sheathes preventing a flower from forming. Solid big black spots. Not freckled. I have a photo but don’t know how to send it of a current plant named Pot. Chief Sweet Orange. It’s always had this problem, last two years. Any knowledge you have on black Spots will be appreciated. Why and how to prevent it. Thanks!

[Deborah Halliday]

This is a bit tricky to diagnose without a picture.  Also, you are a very caring neighbor to write to us.  Here are a few ideas.  It sounds like a fungus or bacterial problem.  This is probably related to not enough air circulation, allowing the leaves and around the sheath to stay wet for too long.  Watering should be done early in the day, and a fan might help as well.  It also may be that the plants are getting too much fertilizer and not enough light, causing them to be rather soft and tender. Cattleya leaves should be a bright yellow green, not deep green.  They won't tolerate full sun all day, but depending on the location, it may be too shady for sturdy growth.  I also wonder about the amount of fertilizer.  We recommend weekly, weakly.  That is about 1/4 of the recommended strength of a balanced fertilizer (e.g. 10-10-10 or 20-20-20) with every watering.  I'm using about 100ppm nitrogen this time of year.  Spraying with a fungicide like Cleary's 3336 might help slow things down.  I hope this give you and your friend some food for thought! 

[Deborah Halliday]

It looks like these plants were able to get rained on in the winter.  Cattleyas can tolerate cool temps, but only if they are kept drier.  Cold & wet is a bad combo.  If these spots are still expanding, I'd recommend cutting off the leaf to below the spot with a clean razor blade.  The cut could be treated with some cinnamon powder.  I'd look at the bark mix to be sure it has not broken down.  If the plants have not been repotted in a couple of years, the bark could be holding too much moisture. That could lead to rot problems.  The main issue to me is letting the leaves dry off before evening, especially around the buds.  Being careful to keep water away from the buds and off the leaves as much as possible should also make rotting less of a problem.  It takes about 6 hours on wet leaves for fungus to germinate.  If there is not much air movement, a fan would also help.  The leaves need to be dry before bedtime!

On Sunday, June 28, 2020 at 12:10:26 PM UTC-7, D.J. Feuerborn wrote: