Brown leaves in autumn can reflect several causes. Check the plant, symptom pattern and growing conditions before treating every mark as an infectious leaf blight.
Check the pattern before naming a disease
Brown leaves in autumn can reflect seasonal change, dry roots, damaged roots or infection. Note which leaves are affected and how the marks develop. A calendar season alone does not establish leaf blight.
Inspect conditions and hygiene
Review watering, drainage and crowding. Remove clearly deteriorated material as appropriate and avoid spreading debris between plants. Use clean tools and keep enough access to inspect the remaining leaves.
Choose treatment after diagnosis
For a persistent or spreading problem, obtain a reliable diagnosis. Use only a product suitable and locally labeled for the actual plant and problem, following its instructions. Do not apply a fungicide automatically to every brown patch or assume a general disinfectant will cure damage caused by cold or poor roots.
Related reading: Plant Problems: Infection or Growing Conditions?.