Morning Glories will often shed the lower older leaves as the days start to get shorter and the nutrients are absorbed from the older leaves and relocated to the newer growth and in preparation for flowering...

The yellowed leaves near the base of a MG plant may be the simple result of the relatively harmless maturation of the vine or the yellowed leaves may represent a more significant pathological process...and determining the difference is not always extremely easy...especially by 'remote'...

Checking the underside of any sickly looking leaves for any signs of aphids or spidermites should automatically be the 1st troubleshooting step...and if there are no pathological organisms present,then proceed to consider the metabolic status of the plant in question...

Water stress can make plants more susceptible to sun burn and to other pathological processes...

I recommend keeping the plants well hydrated...

Bloom Booster (which is phosphorus based) may also be of some help...

TTY,...

Ron