Tomato diseases leave readable signatures: late blight starts as watery grey-green leaf spots that brown quickly, early blight draws concentric "target" rings on older leaves, powdery mildew dusts the leaf surface white, bacterial spots scatter small dark dots with yellow halos, and mosaic viruses mottle and deform young growth. Blossom-end rot — a dark sunken patch on the fruit tip — is not a disease at all but calcium deficiency from uneven watering, so no spray will fix it.
Because several conditions mimic each other, diagnosis should start with questions: where did symptoms begin, is there a coating, what was the weather? If in doubt, photograph the symptom and upload it to Zemedelko's assistant — its photo-diagnosis mode ranks the likely causes, asks clarifying questions like an agronomist, and always states its confidence. If treatment follows, remember the spray journal required by Ordinance 12 — see our post on electronic farm journals.