Potassium 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
Potassium deficiency reduces fruit quality and disease resistance, common on sandy and light soils and in heavy-fruiting crops like tomatoes, potatoes and fruit trees.
Symptoms
Yellowing and browning (scorch) of leaf margins on older leaves while the centre stays green, weak stems, and poor, uneven fruit colour and ripening.
Organic Treatment
Apply wood ash, kelp meal, or a high-potash organic fertiliser, and mulch with compost. Comfrey liquid feed is a good organic potassium source.
Chemical Treatment
Sulfate of potash or a high-potassium soluble feed corrects deficiency during fruiting.
Prevention
Test and balance soil nutrients, feed fruiting crops with potassium, build organic matter, and avoid leaching on sandy soils.