I use neem oil every 3 weeks or so. Neem oil is a good insecticide and fungicide. Fungicide property is more preventive than curative. I mix 5ml of neem oil with 2-3 ml of mild liquid soap first and then mix it in 1 Litre of warm water. Liquid soap helps in mixing neem oil with water.
Once the mix is ready, it would be whitish in color and you have to spray when the water is warm enough.
I would spray this mixture early in the morning (~6.30 to 7.00 a.m). Spraying it later would scorch the leaves
Big plus with neem oil is it is completely safe to butterfly and bees and smokes only the bad guys