Nutrient Deficiency Azalea

This is not an infectious disease, but a nutrient deficiency which could be caused by a couple of things, mainly water stress (too much or too little) and/or the wrong soil pH range for the plant. Soil conditions can, and often do, change over a period of time; its apparently changed enough around the roots of this plant to cause the deficiency. Azalea needs an acidic soil pH, but at the very least you will want to fertilize with an acidic fertilizer; they are available at your local garden center. You will also want to make sure the soil around this plant is not particulary wet (too wet) for some reason (ie: water leak nearby, water running “down” slope,even if gradual), or too dry. You’d think the healthier looking plant would be suffering the same as this one since they are so close to each other, but this is often seen in landscapes due to variations that can and do occur in small areas. Hope this helps.