Manganese 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
Manganese becomes unavailable in alkaline, over-limed or waterlogged soil, and in peaty soils that are naturally low in it. It is commonly confused with magnesium and iron deficiency, and the pattern of yellowing is what separates them.
Symptoms
Yellowing between the veins of the youngest and middle leaves, with the veins themselves and a band either side staying green — a broader green edging than iron deficiency shows. Beetroot leaves turn red-purple; peas develop the brown "marsh spot" inside the seed.
Organic Treatment
Lower the pH with sulphur or an acidic mulch of pine needles or composted bark, and improve drainage in waterlogged ground. A foliar seaweed feed containing trace elements gives a quick temporary correction.
Chemical Treatment
Manganese sulphate applied as a foliar spray at 2 g per litre corrects the symptoms within a week or two. Soil application is far less effective in alkaline conditions because the manganese is locked up again immediately.
Prevention
Do not over-lime — this is the commonest cause. Test soil pH before adding lime, keep beds well drained, and use foliar trace-element feeds routinely on known alkaline or peaty ground.