See attached plant. Leaves seem to have some issue as identified by dark spots. Any idea what could be wrong?
In general the leaf is showing signs of necrosis, which is quite simply death of tissue which then turns black. When it dries the spot on the leaf may leave a hole. Death of tissue occurs when the food supply is shut off or diminishes to the point that the whole plant is deprived of nourishment. The plant does not cut off supply to one whole leaf, but rather fails to send nutrients to the extremities, so the effect is seen in the whole plant and is most obvious at leaf tips. Growth slows as the immediate soil is exhausted.
Taking the case of the plant as indicated in the photo we appear to be dealing with Periwinkle (Vinca major or perhaps V. minor). This plant, due to its ability to survive in low light conditions and form a dense, weed-discouraging mat, is frequently used at the base of trees to provide cover where grass does not grow well. Unfortunately it is often planted and then it fades from the TLC list. The problem is that then you have two plants fighting for nutrients, the tree and the ground cover. The base of tree scenario is also frequently in some kind of tree well providing limited resources.
When freshly planted the periwinkle soon forms a solid mat. It does its weeding very well and may be wonderful for 10 years so you forget it. But then it becomes a problem that requires hard work to renew. Not only is the matting dense and so does not yield to tools easily but you risk damage to the tree roots. It survives by receiving organic matter from the tree and the occasional bird that perches and poops. After a while, necrosis is inevitable unless you can establish a kind of stasis; enough nutrient to maintain existing growth but minimize new growth.