Hi vim,
It is hard to tell from the picture, but from what I can see it appears that the leaves have a "purple" tinge in the middle before the hole appears which could indicate a phosphorus deficiency.
The necrosis could also be caused by a magnesium deficiency, which I have here in Seattle due to our soft water. I dose with MgSO4 (Epsom Salts) per Rex Griggs' recommendation for my soft water and the plants respond fairly quickly.
Here is a link to a chart I use to diagnose problems (it may take a little time to load).
http://www.finostrom.com.gr/images/a...lizers/map.htm