Cucumber Mosaic Virus
I think my courgette plants have mosaic cucumber disease. Large healthy courgette plants gradually become shrivelled and yellowish.
From your description of the problem with your courgettes, it does sound as though Cucumber Mosaic Virus (CMV) is the culprit. The source of infection may not be as easy to track down as CMV, despite its name, is able to infect many different species of plant, not just vegetables and not limited to the Cucurbit family. Many ornamental and herbaceous plants can have the disease without showing the severe symptoms that you find on your courgette plants. CMV has been detected in more than 700 plant species in 86 families.
Some examples below:
Vegetables: Cucumbers and courgettes, tomatoes, peppers, spinach, celery and carrots
Flowers: Gladioli, lilies, alstroemeria, begonia, carnation, crocus, cyclamen, dahlia, freesia, hyacinth, hydrangea, impatiens, narcissus, nerine, orchids, pelargonium, phlox, tulip and zantadeschia
Weeds: Stellaria media (chickweed), Senecio vulgaris (groundsel)
CMV is spread by sap feeding insects, aphids being the most common vector. Unfortunately, resistant varieties are not always foolproof and it may be necessary to take other precautions. Controlling aphids in the garden is one way that you can reduce the likelihood of infection getting to your plants. Grow flowers amongst your vegetables that will attract aphid predators such as ladybirds and hover flies.
Another way to prevent virus infection may actually be to isolate the courgette plants in a netted area to prevent aphids from feeding. Use a fine meshed netting, such as the enviromesh available from The Organic Gardening Catalogue. Use this when you plant out the young plants in early summer.
If you would like further advice on your organic growing, why not become a member of Garden Organic? It costs as little as £2.75 a month, and not only gives you full access to our advisory service, but also free or discounted entrance to many gardens across the UK. Call 02476 308210 or email firstname.lastname@example.org