What causes black spots on leaves of my oregano plant?
Black spot is a fungi, it thrives in humidity, summer rain, or excess watering.It is a big problem in the northwest and in California.It can be spread by water splashing up on a plant.It also can lay dormant over winter in leaves, and garden soil .It can be remedied fairly easy with a spray of 1tsp cooking oil, 2 tsp baking powder in a gallon of water.Spray weekly, and you will probably have to through out your season. If you do not think it is the fungi there is another possibility. Snails leave dropping that look very similar to the fungi.(snail bait will take care of them) You can tell the apart by how the scrape off the plant if they fall right off…..then it snail poo if you feel a powdery substance when you scrape…..fungi. Unless you are seeing chewing evidence I would bet it to be fungi.
