white flowered jasmine climber is caked in black mould

Quite likely it is the result of either aphids or scale which is attacking your jasmin. The insects secrete a sticky substance called honeydew. This falls on the leaves and then a black sooty type mold grows on the honeydew. Have a careful look at the stems of the jasmin for aphids or for little brownish bumps which will scrape off with your fingernail. These will be the scale insects. I would think that scale is the more likely culprit.

If you treat the jasmin for the insects and get rid of them the problems will stop. The mold can be washed off the foliage with the hose.

Your problem is sooty leaf mould it is caused by Scale insects if you look closely on the underside of the leaves you will see Scale insects. The scales look like tiny limpets suctioned to the rib on the back of the leaf As they feed on the sap of your plant they produce a by-product which drops onto the upper surface of leaves below known as Honey Dew. The honey dew is sticky and clear, but forms a great base for the sooty mould to grow on. The mould is a separate fungus that
grows on the honey dew and does no real harm to the plant To treat sooty mould the best method is a long process of wiping each leaf clean. Then this needs
to be followed up with and insecticide such as Provadoto remove the primary pest, the scale insect .