I had the same stains on my tree - lemongrass. Its leaves were identical to Those in your photos. They fell down and the entire Bush became almost bald. I not has found answer and in mind get to say goodbye with this plant and I simply continued to do regular watering. But after a month something happened and there were new shoots and young leaves. Of course I'm interested in hearing the comment of this professional lady. I will definitely wait for her answer for you.