Magnesium Deficiency
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.
The signs that lead here
About
Magnesium deficiency is widespread on sandy, acidic and heavily potassium-fertilised soils, common in tomatoes, potatoes and roses, since magnesium is the central atom of chlorophyll.
Symptoms
Yellowing between the veins of older lower leaves while the veins stay green, often with reddish or purple tints and curling margins, progressing upward over time.
Organic Treatment
Apply Epsom salts (magnesium sulfate) as a foliar spray or soil drench for fast relief, and add dolomitic lime to acidic soils to correct it long-term.
Chemical Treatment
Magnesium sulfate soil application; dolomitic limestone where pH also needs raising.
Prevention
Maintain balanced fertility, avoid excessive potassium, lime acidic soils with dolomite, and build organic matter.