This looks like iron chlorosis, but to be sure, I would send a sample to the Plant and Soil Analysis Lab in Madison for diagnosis. One thing I suspect is that the water pH is too high, leading to raising of the pH in the soil. I see this all the time in nursery production where a well is dug near limestone. There are some excellent publications on how to acidify greenhouse water, manage micronutrients, the pore-through method, etc. from N.C. State University (http://www.ces.ncsu.edu/depts/hort/floriculture/cfr/index.html) and scroll down to #553, 558, and 560. They are written for the greenhouse grower. A water injection system is probably what is needed for this grower. I would get a test done of their water as well as the tissue and container media. Does the grower have access to a pH meter? They can easily test their water and container media pH as well as soluble salts. A 2:1 saturated paste extract can be done with the container media and deionized water. Let it sit for about 1/2 hour, then measure the pH and EC. To help get the petunias back on track, you can use a foliar application of fertilizer labeled for foliar applications (not all of them are). Make sure the fertilizer contains Fe and Mn as both can be competing for root uptake. The grower may also want to apply a slow release, granular fertilizer (Osmocote, Nutricote, etc.) that contains micronutrients as well as NPK, Ca, Mg, and S. Hope this helps, Laura
