There is normally no need for chemical control as the plants will grow through the disease once the growing conditions change to become less humid.
If the leaves look particularly unsightly, you can prune them off with a pair of secateurs or a hedgetrimmer. Disinfect the tools before using them on other plants. Clear up any diseased leaves to prevent re-infection and burn or dispose of them. Do not compost them.
Food-grade Potassium bicarbonate at a rate of 5 grams per litre of water sprayed until run-off has been shown to control powdery mildew on many plants and is now used commercially. A number of applications may be necessary.
Products such as Bayer Fungus Fighter should result in control of powdery mildew.
Shot-hole disease can affect all varieties of Prunus laurocerasus and Prunus lusitanica and is caused by a bacterial pathogen called Pseudomonas syringae pv syringae or the fungi Stigmina and Eupropolella.
Shot-hole disease tends to be more of a problem
on nurseries that use overhead irrigation (sprinklers) to water their plants as the disease is spread from plant to plant by water splash on the leaves. Shot-hole is not a disease that will threaten the life of a laurel and most plants will grow out of the problem once the growing conditions change (i.e. the laurel are planted into the ground where they do not need to be watered with sprinklers or the weather becomes drier) and new leaves are produced that cover up the diseased leaves.
Brown leaf spots between 2 and 10mm in diameter that eventually drop out to leave the leaf looking like it has been shot with a shot-gun.
Chemical control is not necessary. Laurel plants will grow through the disease when the weather or growing conditions change. When the plants put on new growth, it will cover up the disease.
Do not water the leaves, water the soil around the base of the plant when watering a plant.