Symptom
Mottled or mosaic leaves
A mosaic of light and dark patches, rather than distinct spots, usually means a virus. Nutrient shortages and weedkiller can look similar, but make a more regular pattern.
Look for regularity. Viruses make irregular mosaics on scattered plants; nutrient shortages make repeating patterns on the same leaves of every plant.
Likely causes
Ordered from the usual answer to the less likely one. Each links to the full problem.