What is happening to my pothos when a leaf edge turns black and dry?

This looks like a pothos with a dried, damaged leaf edge rather than an obvious pest attack or particular leaf infection. The damage is dark brown to black and dry, while the rest of the plant looks mostly firm. Uneven moisture is the best fit, especially as you describe infrequent watering and no drainage.

That combination can be deceptive: the compost may stay too dry for a long time, but when watered, the excess has nowhere to go. Roots then alternate between drought and lack of oxygen. Pothos often shows this first as brown tips or edges on individual leaves.

What to do now

• Trim away the dry part of the leaf, or remove the whole leaf if it bothers you. The damage will not heal.
• Water when the top two to three centimetres of compost have dried, rather than leaving the whole pot dry for a long time.
• If the inner pot has no holes, repot into one with drainage holes when practical. This is particularly sensible as autumn light fades and compost dries more slowly indoors.
• Keep it in a bright position, but avoid leaves pressed against a cold window, which can also cause dark edge damage.

Watch new leaves over the next few weeks. If they emerge healthy while only older leaves have brown edges, you have probably stopped the problem. If more leaves quickly blacken despite steadier watering, lift the rootball and check for brown, soft roots.