Why are my redrobin leaves going yellow?
It is best to take care of signs of issues right away with Photinia.
Yellowing leaves can be an indication of excess or poorly draining water.
Nitrogen deficiency is also a common cause of yellowing.
It is best to have a soil test done to ensure the correct application of a nitrogen rich fertilizer.
Inspect the plant for insects, an infestation can also cause the stress and yellowing of leaves.
Treat with Neem Oil for most insects.
Here are some articles with more information.