Posted: 03/12/2016 at 09:57
Are you in the southern hemisphere? Not the time of year for marrows here in the UK.
Your plant looks as if it's suffering from powdery mildew - it's a plant specific problem which will often attack plants weakened by stress, usually drought. It won't be the same variety of mildew that affected your tomatoes, but the same conditions in the same spot are likely to have caused it.
I would remove the worst affected leaves, give the plant plenty of water and mulch around the area with a thick layer of manure/compost to help the soil retain as much moisture as possible, and ensure that the plant gets plenty of water - all marrows, courgettes, tomatoes etc are thirsty plants.
In the future dig as much compost/manure and other organic matter as possible into the area when preparing the area for your planting.
Good luck - I hope that helps.
Last edited: 03 December 2016 09:58:10