Spotty leafed marrows
If the white spots are powdery, this could be powdery mildew, which can attack marrows at this time of year. A standard fungicide for powdery mildew should cure it.
If it is not powdery mildew, it could be a soil borne disease. Try applying the new plant strengthener Grow Boost to the soil around the marrows. The natural fungi in Grow Boost can out-compete soil borne diseases preventing them starting. Normally it is better to treat the soil just before or just after planting. Grow Boost can be used for a variety of plants and crops. It is available from Harrod Horticultural.
I hope this is of some help.