Pests and Diseases
Home Vegetable Gardening
Course 1 · Chapter 7 · Pests and Diseases: Slugs, Aphids, Blight, Prevention and Organic Controls
Chapter 6 covered the routine care of watering and weeding. Even with good care, something will eat or infect some of your crops. The aim is not a spotless plot but keeping losses small, mostly by prevention and by catching problems early.
Prevention First
Healthy plants in good soil resist trouble better than stressed ones. Most of the best controls cost nothing and are done before the problem appears.
- Look often. A weekly walk round (Chapter 6) spots a few aphids or one blighted leaf while they are still easy to deal with.
- Rotate crops. Moving each crop family to a new bed each year stops soil-borne problems building up; Chapter 9 covers this.
- Keep the plot tidy. Remove dead leaves, rotting fruit and hiding places such as old boards and long grass, where slugs shelter.
- Grow resistant varieties where they exist; seed packets often say if a variety resists blight or another disease.
- Do not overcrowd. Good spacing (Chapter 5) lets air move and leaves dry quickly.
- Invite helpers. Ladybirds, hoverflies, ground beetles, frogs and birds all eat pests. Some flowers and a patch of rough ground encourage them.
Common Pests
| Pest | Signs | Organic response |
|---|---|---|
| Slugs and snails | Ragged holes in leaves, silvery trails, seedlings eaten overnight | Collect by torchlight after dark or after rain; keep the plot tidy; protect seedlings with cloches or bottle covers; encourage frogs and beetles; beer traps help a little (check) |
| Aphids (greenfly, blackfly) | Clusters of tiny insects on shoot tips; sticky residue; curled leaves | Squash by hand or rub off; wash off with a jet of water; leave ladybirds and hoverflies to eat them |
| Cabbage white caterpillars | Holes in brassica leaves; green caterpillars; yellow eggs under leaves | Pick off eggs and caterpillars; cover with fine mesh |
| Carrot fly | Rusty tunnels in carrots; maggots | Cover the crop with fine mesh; sow thinly so less thinning is needed, as the smell of bruised leaves attracts the fly (check) |
| Pigeons | Torn leaves on brassicas and peas | Net over a frame; shiny strips or discs help only briefly (check) |
Barriers
Fine insect mesh, netting and cloches physically stop pests reaching crops. They work against caterpillars, carrot fly and pigeons, and need no spray.
Sprays
Even organic sprays can harm helpful insects as well as pests. Use them as a last resort, on the affected plants only, and read the label (check).
Common Diseases
Plant diseases are mostly caused by fungi, which thrive in damp, still conditions, and by viruses, which are often spread by sap-sucking insects such as aphids.
| Disease | Signs | What to do |
|---|---|---|
| Potato and tomato blight | Brown or black patches on leaves, spreading fast in warm, damp weather; white fuzz under leaves; rotting tubers or fruit | Remove affected foliage quickly; harvest potatoes soon after cutting off blighted tops; grow resistant varieties; keep tomato leaves dry; bin infected material rather than composting it (check) |
| Powdery mildew | White powder on leaves, common on courgettes and squash in late summer | Water the soil well, mulch, space plants, remove badly affected leaves |
| Damping off | Seedlings collapse at the base, often in crowded, wet, cold trays | Sow thinly, use clean pots and fresh compost, water sparingly, give light and air (see Chapter 4) |
| Clubroot | Brassicas stunted and wilting; swollen, distorted roots | Rotate; raise the soil pH with lime if acid (see Chapter 2); buy clean plants; it cannot be cured once in the soil (check) |
Not Yet Verified, and Sources
- Source. Written from general knowledge of gardening; no specific source was read. Nothing is quoted from a text.
- Marked (check). Beer traps, the carrot fly and thinning advice, the effect of deterrents on pigeons, the use of sprays, handling of blighted material, clubroot control, and the weather conditions favouring blight. Verify against local gardening advice and product labels.
Exercises
Your newly planted lettuce seedlings have ragged holes and silvery trails across the leaves each morning. Identify the likely pest and describe three things you would do, in order, to protect the crop without chemicals.
After a warm, wet week, brown patches appear on your tomato leaves and a few potato leaves. Explain what you suspect, what you would do today, and how you would dispose of the affected material.
Design a prevention plan for a small plot growing carrots, cabbages and peas, listing one measure for each crop against its most likely pest, plus two general habits that cover the whole plot.
Quick Reference
- First: prevent, with tidiness, spacing, rotation and barriers
- Slugs: collect at night, protect seedlings, attract predators
- Aphids: rub off or wash off; let ladybirds work
- Blight: act fast in warm, wet weather; bin affected material
- Mesh: stops carrot fly and caterpillars
- Sprays: a last resort