Cucumber Mosaic Virus
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
Remove affected material now
Take out the affected leaves, shoots or whole plants before the next wet spell, bag them as you cut, and wash your hands and tools before touching healthy plants.
Carried by insects
An insect moves it plant to plant, so controlling the carrier matters as much as treating the symptom. Check for the vector before assuming the plants infected each other directly.
It carries over to next season in soil, debris or seed — which is why rotation matters here, not just treatment.
Keep out of the compost
This survives a garden heap. Bag it for municipal green waste, which is composted industrially at temperatures a garden heap cannot reach, or burn it where that is permitted.
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
Full problem lists for these crops
About
Cucumber mosaic virus has the widest host range of any plant virus — over 1,200 species, from courgettes and tomatoes to spinach, lilies and delphiniums. Aphids move it from plant to plant in seconds, and many common weeds carry it silently through the winter.
Symptoms
Yellow mosaic mottling and puckering of young leaves, which often become narrow and strap-like. Plants are stunted, flowers are streaked or malformed, and cucumber fruits are warty, pale and bitter.
Organic Treatment
Pull out infected plants at once — the virus is systemic and no plant recovers. Wash hands and tools after handling, control aphids early, and keep a strict weed-free margin around susceptible beds.
Chemical Treatment
No chemical treats a virus. Insecticide against aphids helps only marginally: transmission is non-persistent and happens faster than any spray can act.
Prevention
Grow resistant varieties where they exist, especially of courgette and cucumber. Remove perennial weeds such as chickweed and groundsel that host the virus, and use reflective or straw mulch, which measurably reduces aphid landings.