Since it is October in Canada I assume this is an indoor plant. It looks like a physiological stress which is often diagnosed by color changes from the periphery and leaf drop.
Leaf drop in citrus is often related to weather, either too cold or prolonged heat and dry soil.
What conditions is this being kept under? Are you watering a lot?
This doesn’t seem to be an infection and with best practices (stable temperatures, regular watering) new leaves should appear and remain healthy
Posted by: Kelsee Baranowski
Posted: October 9, 2017