Blossom End Rot
This problem spreads through handling, insects or soil rather than the weather, so there is no live forecast risk — focus on prevention below.
When it strikes
Fix the cause, not the leaf
No spray will help here. The trigger is soil, water supply, nutrition or temperature, so change the growing condition and judge the result on new growth, not on damaged leaves.
Not contagious
Nothing spreads from plant to plant here. Neighbouring plants showing the same signs are reacting to the same soil, water or weather, so fix the cause rather than isolating plants.
Safe to compost
No pathogen carries over on this material, so it can go on an ordinary heap with the rest of the garden waste.
When and where to look
This problem has no weather threshold. Check it on every routine walk round the bed, especially after handling plants and after bringing new stock in.
Rule these out first
Same category, and at least one host crop in common — so they are genuinely confusable in the same part of the garden.
The signs that lead here
Full problem lists for these crops
About
Blossom end rot is a physiological disorder of tomatoes, peppers, squash and melons caused by a calcium shortage in the fruit, usually triggered by uneven watering rather than low soil calcium.
Symptoms
A sunken, leathery, dark brown-black patch at the blossom (bottom) end of the fruit, often on the first fruit of the season or during dry spells; the rest of the fruit may ripen normally.
Organic Treatment
Keep soil evenly moist with mulch and consistent watering, which is the real fix; foliar calcium offers only minor help. Remove affected fruit.
Chemical Treatment
Calcium chloride foliar sprays give limited relief; the disorder is corrected by water management, not chemicals.
Prevention
Water deeply and consistently, mulch to buffer moisture, avoid heavy nitrogen and root damage, and ensure adequate soil calcium and pH.