I have multiple boxwoods that were transplanted last fall and new ones purchased this spring (didn't get planted till fall). They are developing light green/chartreuse leaves and branches. Some appear to be changing now to yellow. I have attached two pictures. Could this be underwatering over the summer, or should I check the soil for mineral/absorption issues, or is this a disease problem. Seemed fine until mid to late August.
We viewed your photos. This does not look like a disease or insect issue. The boxwoods look like they are failing to establish themselves. When plants decline within the first year of planting it can usually be attributed to environmental and site conditions such as poor planting techniques, poor drainage, poor soils, etc. You will have to check for this. Boxwoods have a shallow root system and require moisture during dry periods and if planted in the fall watered until the ground freezes.
If the boxwoods were container plants, they can be pot bound and hard to keep watered. Container plants establish faster if you disturb the “around the pot” growth direction of the roots. Use a sharp knife or blade to cut four one-inch-deep cuts the length of the root ball. New roots will rapidly grow from the cut areas of the roots. See our website for planting techniques http://extension.umd.edu/hgic/trees-and-shrubs/planting-process and our Publication on Common Abiotic Plant Problems http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG86%20Common%20Abio...for more information.
If you suspect the new plants may be pot bound, pull up the plant and look at the root system and see if the roots have spread out. If the root system is still in the shape of the container they are probably pot bound.
Watering - Check soil moisture of newly planted trees and shrubs at least once a week. Soil that is moist or damp to the touch is fine. If the soil begins to dry out, water the plant thoroughly. Do not overwater; however, you can easily drown newly planted trees and shrubs through too much tender loving care with the hose. Make sure mulch is no thicker than several inches and keep away from the base of the stem.
See post planting care http://extension.umd.edu/hgic/trees-and-shrubs/post-planting-care
Also, see our boxwood publication http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG52_IPM_Boxwood.pdf