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I noticed black spots all over our tree and not sure what the problem is. I have sprayed with neem oil and soap but it doesn't seem to be helping any.

Recommendations would be much appreciated.

It looks like insects, and neem oil will do the job but you need to give it a chance to work it is not instant. I would not spray either, I water it into the soil as it is systemic and will last. Also it will not kill other insects. If you have only applied in the last few days then wait, if longer then do it again.

How's much neem should I add when watering the tree?

Also, if I spray neem and insecticidal soap on the leaves does it have to be washed off or is it good to let it stay on there?

I am confused You do not know what the problem is, yet you decide to spray with Neem? Why?

It looks like mould, mould after winter is normally a sign of an earlier infection of aphids.

- Wash the plant; The mould should come off (NO rubbing and pulling, just a gentle wash)
- Check for aphids and scale

1ml Washing up liquid 5ml neem oil 1ltr water

What should I wash it in?

I use a garden hose with showerhead, give it a good rainstorm, manually gently wash the worst offending leaves off.

Over summer this should diapear as the plant grows and drops the old foliage.