Grey Mildew

there is a mix you can use that isnt any chemicals, its a mixture of biacrb and something else will try and find it

found a link for more details and its baking soda not bicarb

http://gardening.about.com/od/gardenproblems/a/PowderyMildew.htm
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Hi Melita and welcome to GoY you are spot on with your assumption that the mildew is caused because your garden is so enclosed. Is there a way for you to take down or lower part of the fence?

Go to http://gardening.about.com/od/gardenproblems/a/PowderyMildew.htm for advice on controlling. Do make sure you don't leave plant detritus lying around over the winter as this is a common cause of powdery mildew.

Bit drastic lowering fences which wont stop this, it happens in greenhouses and on indoor plants to. Powdery mildew is one of the most common and easily recognized plant diseases, some plants are more susceptible than others.Try and find powdery mildew resistant cultivars
Don’t plant non-resistant varieties in the shade,
Remove and destroy all infected plant debrie and disinfect all tools and pots. change compost in pots.
Improve air circulation by thinning and pruning Don’t use fertilizer until the problem is corrected. Powdery mildew favors young, succulent growth Don’t water your plants from above
Apply a fungicide check the label to be sure they are safe and effective on the type of plant that is infected. With most fungicides will need repeat applications every 7 - 14 days for continuous protection - as per instructions. Cheer up I am sure you can over come this be controlling it.
I do wonder if you need to look at the type of plants you are growing in a narrow enclosed garden? good luck.

Narrow and enclosed shouldn't be an issue on its own, mildew hits every year and my own susceptible plants on the balcony (windy, and open) still get it. Crowded planting, though, will encourage it (which is probably why I get it on the balcony, its stuffed), so I wonder whether your garden is very crowded - improving that might help. Otherwise, invest in a good systemic fungicide that will treat most of your plants, spray at the first sign of mildew next year.

I know someone once who had this problem so I helped lay a path along the fence on both sides. Not only did it clear the mildew but made it easier to maintain the fence.