Dipladenia Sanderii

Question

What causes the leaves to turn yellow & then fall off. The plants are in pots in a sunny position in Bundaberg.

Answer 1

From the appearance and random pattern of the yellow spots, it appears to be a virus infecting the older leaves. If so, there is no chemical control. Since the new growth appears clear of the disease, the plant may be “out-growing” it. Keep the plant as healthy as possible with good fertility (including micronutrients) and water management.
Viruses are often spread by insect pest vectors, like leaf hoppers and psyllids. Preventive pest management may help prevent spread of the disorder.
I don’t know your local issues and resources there on the Sunshine Coast, but check with a local nursery for organic fertilizer and pesticide products.