Help...need advice about possibly "sick" seedlings. I know Zinnias are one of the easiest annuals to grow however I'm pretty much of a newbie at starting seeds indoors and so have several questions.

I have a few trays started indoors which are doing very well and have gotten pretty large. All the plants have 3 sets of true leaves.

BUT in the past two days I notice a few leaves on two of the plants that look "sick".

See 4 photos at photobucket:

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I wonder if anyone can tell me what's going wrong? At first I thought could be white fly. However nothing flies up from the plant and I don't see little specs that can move. There are no teensy white flecks on the underside of the leaves either. On the pics of the teo plants you'll see several leaves whose edges look discolored and sort of rough and dry. Is there anything I can do to save these two plants...and I wonder if this is going to spread to all the rest? Should I just cut off those leaves? Or should I destroy the two plants? Could this be caused by overwatering? Or too close to the lights? Or lack of enough of enough moving air?

Any ideas will be appreciated. These trays of plants have been doing so well I hate to loose them now.

ALSO..

The plants are about 4 to 6 inches tall. I'm in wisconsin and the temp is 38 degrees F today...it's overcast and windy. Warmer and humid weather is predicted starting tomorrow. Can new plant this size be placed outside to begin hardening yet. What temp should it be outside to start hardening?

I know I should put them in a shady area protected from wind (like my porch). Should they be out there all day and brought into the warmer house at night? Or could they be out all day and then taken into an UNHEATED garage for the night?

http://s27.photobucket.com/albums/c192/jellybean7777/?action=view&current=PICT0002sick.jpg

http://i27.photobucket.com/albums/c192/jellybean7777/PICT0003sick.jpg

http://s27.photobucket.com/albums/c192/jellybean7777/?action=view&current=PICT0004sick.jpg

http://i27.photobucket.com/albums/c192/jellybean7777/PICT0005sick.jpg