Yellowing Bedding Plants

A garden center in Lincoln, NE is wondering what is happening to a few varieties of annual bedding plants they have in stock.  They carry well over 100 different varieties of annual plants, and the problem only persists with certain plants such as begonia, lantana, annual phlox, and especially petunia.  Every year, a few varieties of plants start to develop yellowing growing points.  The rest of the plant seems fine and healthy, but once the yellowing starts, the new growth is slow, small, and flowering usually stops after a few weeks.  The plants are in a variety of places throughout the nursery, including outdoors in full sun all day, to shade, and greenhouse conditions.  Plants are put in these locations according to their light requirements.  All plants are watered with fertigation water every day.