Write the mushroom species, crop stage, first observation date and affected material. Describe what is visible: slowed colonisation, altered colour, damaged caps, soft tissue, poor pinning or insects. Include how many bags or which bed area appear affected and whether the pattern is expanding. These observations are more useful than writing a disease name inferred from one image.
Take a room overview, a view of the affected bed or bags, a close view and a healthy comparison at the same stage. Keep lighting and distance consistent. Mark the affected area on a room map. Do not open suspect bags or move material through clean areas simply to improve a photograph. Ask the receiving specialist how to collect and send a sample when one is required.