Why are the leaves on my melon plants dying?

Hi folks, would be grateful with any knowledge on this one...
I am growing some sweet melons (Bastion variety) in my greenhouse. On all the plants the lowest leaves have turned yellow and then curled up and died. The upper leaves still look healthy(ish) but even these are becoming more yellowed. The plants have set some fruit and I want it! From my own research it seems that possible causes of yellowing/dying are -
(a) Powdery mildew (b) Red spider mite damage (c) Wrong soil pH, or maybe insufficient nitrogen in it
(a) and (b) are possible, and I have done what I can to treat/prevent.
For (c) I tested my compost and was surprised to discover that it was alkaline - this was surprising because I added a sizeable quantity of farmyard manure which I thought would have made it acidic. If I want to try to lower the soil pH is it safe to use a sequestered iron solution, or would this be too much of a shock for melon plants. Is it also possible that the manure itself is causing this problem e.g. if it was not fully broken down and has thereby "scorched" the plants?
Any advice/opinions from the melon growers out there would be appreciated!
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