Question
what is the name of this fungal disease with white fruiting bodies on orange leaves and whats its cure?

Answer
Without seeing a picture, it's hard to say for certain.  Possibly powdery mildew.  If you are able to supply an image or a longer description, I might be able to tell you if this is the problem.

If the problem is powdery mildew, you should remove and discard the symptomatic leaves.  Once you see the white stuff, they cannot be cured and there are lots of spores in the white growth that can cause new infections.  I'm not sure if your plant is indoors or outside.  If it is inside, increasing the amount of light often helps solve powdery mildew problems.  Increase air circulation by thinning the plant or moving it away from other objects.

If you are going to use a fungicide, you will need to make sure the product is safe for indoor use if your plant is inside.  Some type of fungicidal/insecticidal soap is often useful, but will need to be applied every week until the problem is cleared up.  If the plant is outside, a fungicide containing sulfur will be quite effective.  I have also heard of using a homemade baking soda mix to treat the problem, but don't know how effective it is.  You would mix up a solution of 1 tablespoon baking soda in a gallon of water and spray it on the plant.