I have seen the brown spots on dracaenae leaves surrounded by a yellow halo on many plants. The conditions that would indicate that this is a fungus/virus/bacteria are:
- plant potted up in a soil less mix (look for perlite, white crunchy particles)
- alkaline water supply (ph 7+) usually found with limestone bedrock in the water table
What is happening is that the soil less mix is delivered at ph neutral (6) or slightly acidic with a large proportion of peat moss. Over time the plant uses up the peat moss and turns it into plant matter and what is left cannot buffer the alkaline water. As the soil becomes alkaline the plant becomes stressed and susceptible to infection.
The best solution is three fold:
- move to a higher light area
- repot the plant with new soil less mix or a clay based soil
- allow the plant to dry out more between waterings