Tony Dickerson answers your questions
QHave we lost the ba le with allotment pests and diseases? Garry O’Sullivan, Hillingdon, Middlesex
AIt’s dispiriting when various diseases and ailments wipe out crops seemingly overnight. However, most of the major culprits can be tackled and almost all by using organic and cultural measures without resorting to chemicals.
Top of the problem list will be slugs and snails. Keeping plots clean and tidy will eliminate most of their hiding places. Hand-picking after rain works well with slugs and snails, as do beer traps, and both these methods are environmentally friendly. Those described as organic ferric (iron) phosphatebased slug pellets in shops are suitable for organic gardeners, and are just as effective as non-organic (those based on metaldehyde). If you are worried about children, pets and wildlife, however, use nematodes instead. Whitefly on brassicas and blackfly on beans can be kept in check with regular applications of SB Plant Invigorator. Check plants regularly and spray on as soon as you see infestations. This product is also useful against powdery mildew on courgettes and peas and various fungal infections of brassicas. It creates a protective layer on the leaves and is used at Wisley in the Model Veg Garden.
Barriers of a different sort in the form of nets and meshes are also effective. Netting protects brassicas against caterpillars in summer and pigeons all year. Fine Enviromesh secured over carrots and leeks prevent carrot root fly, leek moth and allium leaf miner. I keep these in place from sowing to November. Root collars will keep cabbage root fly from damaging brassicas.
Tomato blight is best avoided by overhead protection from the rain, and a polytunnel is the best investment for keen growers. The same disease on potatoes can be sidestepped by growing second earlies, which usually mature before blight becomes rampant on maincrop potatoes.
Two diseases, though, will defeat most efforts – onion white rot and club root on brassicas. The former stays active in the soil for 20 years. Be careful not to walk the spores from infected soil on boots around plots. Reduce club root by liming the soil in autumn and growing transplants on in pots to a good size.