Get help with Plant Deficiency in your garden

Hi, I have some seedlings, among which some melons (or squash, don’t remember), that are at the third true leaf. The plants look pretty robust, but the leaves are discolored along the veins, I’d say white/gray/greenish. I’ve made a google image search, but couldn’t find that problem; yet I am convinced there must be a website that lists all kinds of plant problems and with pictures.
Any suggestions? Thanks.

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Whether squash or melons they are cucurbits so will need similar conditions.
Cucurbit Crop Management

http://ag.arizona.edu/pubs/garden/mg/pathology/corn.html

http://www.lane-ag.org/wm-world/cucurbit_manual/e-853_2.html

http://aussieorganicgardening.com/blog/?p=204

http://www.dpi.vic.gov.au/DPI/nreninf.nsf/childdocs/4E954120C589980BCA256F160020685D?Open&Layout=DPI+Site~Printer+Friendly

http://www.google.com/url?sa=t&source=web&ct=res&cd=5&url=http%3A%2F%2Fwww.uwex.edu%2FCES%2Fwihort%2Ffruitveggies%2FDiseases%2FCucurbitsD.ppt&ei=p4vbSfHUCJeytAOZx8W5Bw&usg=AFQjCNEAA5xiJf6YQXtyWF6tBE0Nb8NyXA

http://www.planetnatural.com/site/xdpy/kb/n-p-k-organic-fertilizers.html