Shading

Low light reduces photosynthesis which in turn reduces carbohydrate production and rooting.
In low light conditions, plants naturally grow upwards in order to out compete other plants and capture more light.
When the low light is caused by permanent shading (not caused by competing plants) then this can also cause etiolation. Etiolation causes long, stretched out growth producing weak plants.
The causes of shading (trees/buildings) are also likely to reduce air movement therefore meaning the turf surface stays wet for longer meaning potential increase in disease occurrence.
Turf growing in shade also has a thinner leaf cuticle which can make the plant more susceptible to disease attack.

Approach

Where possible manage the environment to reduce the amount of shade. Encourage the turf to grow without excessive vertical growth and encourage better rooting.
Reduce nitrogen inputs versus full sun areas
Reduce irrigation compared to full sun areas
Dry surface with use of penetrant wetting agent
Aerate well to encourage good rooting and movement of surface water
Increase carbohydrate levels in plant. Select shade tolerant grass cultivars.