Question
Hi Mr. McCann,

 My Periwinkle plant is experiencing severe yellowing in
leaves. I watered it generously every morning. I've checked
the leaves but did not find any mealybugs or other pests.

 How should I remedy the issue?

Answer
Periwinkle Vinca runs its course and when it is past its prime needs to be pruned back to encourage new growth; We have it here in Florida growing wild, and as is the case with yours, it will eventually become leggy and yellowed. I just cut it back to almost ground level, and eventually new growth, that is just like it was when you first planted it,begins.