Pests and diseases of the vegetable garden: recognise them and act without chemicals
19 common vegetable garden problems, with the symptom, the cause, the season and the move. Almost all can be sorted without a product.
Holes in your cabbages, a white powder on the courgettes, carrots gnawed with brown tunnels. Before you buy anything, find the culprit. The right move depends on it. Most of the time, it comes down to a net, a watering or a rotation, not a product. The last part says which products the law allows private individuals in Belgium and France.
Find the culprit in three questions
Ask yourself three questions, in this order.
- Where is the damage? Leaf, root, root collar, fruit or seedling.
- What does it look like? A hole means something is eating the plant. A felt, a powder or a water-soaked spot suggests a fungus. A plant that wilts while the soil is moist points to the roots.
- When? Pests have their season. A cabbage white in March or blight in April is unlikely.
Also turn the leaf over. Eggs, aphids, whiteflies and mites hide underneath.
The months in the tables come mostly from the RHS, the British horticultural society. They hold roughly for Belgium and northern France. If your spring comes earlier, in the south-west or the south of France, bring them forward.
The ones that eat leaves
| Problem | Symptom | Season | The move |
|---|---|---|---|
| Slugs and snails | Irregular holes in tender leaves and seedlings, slime trails | Mild, damp weather | Protect only fragile plants, collect in the evening, beer traps |
| Aphids | Colonies under leaves and at the tips of stems, curled leaves | Spring and summer | Tolerate, let ladybirds and hoverflies come, squash colonies between two fingers |
| Cabbage caterpillars (cabbage whites, cabbage moth) | Holes in outer leaves, droppings, hollowed heart | May to October | Net put up before egg-laying, not touching the leaves; remove eggs and caterpillars |
| Flea beetles | Small round holes on cabbages, radishes, rocket, especially young plants | Spring and summer | Fine-mesh net on seedlings; tolerate on established plants |
| Cabbage whitefly | Small white insects that fly up when you touch the leaf, black sooty mould | All year | Tolerate, except on kale where you eat the young leaves |
| Colorado beetle | Striped yellow and black adult, orange-red larvae, foliage devoured | Spring and summer, two generations a year in Hauts-de-France | Check under leaves, remove eggs, larvae and adults |
| Red spider mite, under glass | Fine pale mottling, fine webs, yellow-green mites under the leaf | March to October | Ventilate, water the shelter; predatory mite Phytoseiulus persimilis |
For the Colorado beetle, do not mix up its eggs with those of the seven-spot ladybird. The ladybird's are pale yellow, the Colorado beetle's yellow then orange before hatching (Fredon Hauts-de-France). The guide Colorado beetles details the picking.
The ones that attack roots and stems
| Problem | Symptom | Season | The move |
|---|---|---|---|
| Carrot root fly | Brown tunnels and rust around the root, cream maggots | May to October; egg-laying late May to June, then August to September | Sow after mid-May, harvest before the end of August, full net |
| Cabbage root fly | Young cabbages that slow down, wilt and die | April to September; the 1st generation does the most damage | Cardboard or felt collar of 8 to 15 cm at the base, or net |
| Clubroot | Stunted plant, purplish leaves, swollen and deformed roots | Growing season | Lime, drain, raise plants in pots of at least 9 cm, resistant varieties |
| Leek moth | White patches on leaves, tunnels in the shaft | April to October; larvae in May to June and August to October | Insect net all season, rotation |
| Leek miner | Rows of white dots on the leaf, then maggots and pupae in the shaft | Egg-laying March to April and September to November | Net during both flight periods, rotation |
| Onion white rot | Yellow leaves that collapse, rotting bulb and roots, white felt | Growing season, especially in dry weather | Never bring in soil from an affected plot. It stays contaminated for 15 years or more |
| Damping off | Seedlings that do not come up or collapse, often with a whitish felt | Spring, indoors or under cover | Clean pots, fresh potting mix, tap water, thin sowing, ventilate |
For carrot, the RHS cites less susceptible varieties. None is 100 % resistant. To go further, read Leek moth and leek fly and Clubroot.
Diseases and disorders
| Problem | Symptom | Season | The move |
|---|---|---|---|
| Blight, tomato and potato | Water-soaked spots that quickly turn brown, white fuzz under the leaf, spotted fruit | Summer, in hot, humid weather | Remove and throw away affected parts, never in the compost; four-year rotation |
| Powdery mildew (courgette, squash) | White powdery felt on the leaves | Spring to autumn | Water regularly in the morning, mulch, space plants, avoid too much nitrogen |
| Grey mould | Grey-brown felt on dead parts, strawberries that rot | Damp weather, poorly ventilated shelter | Remove dead parts, ventilate, space plants |
| Leek rust | Bright orange pustules that release an orange powder | Midsummer to late autumn | Space plants, avoid too much nitrogen, clear debris |
| Blossom end rot (tomato, pepper) | Dark, sunken spot at the end of the fruit, opposite the stalk | Summer | Water regularly. It is not a disease |
Blossom end rot is neither a fungus nor a pest. Calcium does not reach the end of the fruit, most often because of irregular watering. The RHS judges calcium sprays of little use. Read Blossom end rot in tomatoes.
The guide Tomato blight explains how to tell it from early blight and read the risk window. For powdery mildew, read Powdery mildew, the white felt on courgettes.
The net sorts the most problems
Eight pests in the table stop at a net. But you need the right mesh. The RHS distinguishes three sizes.
| Mesh | What it stops | What gets through |
|---|---|---|
| 4 to 7 mm | Cabbage whites, if the leaves do not touch the net | Everything else |
| 1.3 to 1.4 mm | Cabbage root fly, carrot root fly, onion fly, butterflies | Aphids, flea beetles, leek miner and leek moth |
| 0.8 mm | Flea beetles, whiteflies, miners, aphids, and the flies above | Thrips, which need 0.3 to 0.6 mm |
For cabbages, the guide Cabbage white and cabbage caterpillars gives the full procedure.
Prevent rather than treat
Four habits avoid most of the problems in the table.
- Rotation. The RHS advises four years between two tomatoes in the same place outdoors, against blight. It judges it crucial against carrot root fly and useful against leek moth and leek miner.
- Regular watering, in the morning. It avoids blossom end rot and slows powdery mildew. Water mid-morning rather than in the evening.
- Space and air. Crowded plants hold moisture. Blight, grey mould and rust take advantage.
- Beneficial animals. Birds, hedgehogs and ground beetles eat root maggots, according to the RHS. The guide A living vegetable garden says how to attract them.
Products allowed for private individuals
| Belgium | France | |
|---|---|---|
| The rule | Only products authorised for non-professional use | Since 1 January 2019, no plant protection products for private individuals (Labbé law), with exceptions |
| Still allowed | Products whose authorisation number ends in G/B or G/P | Biocontrol products, those usable in organic farming and low-risk substances |
| Forbidden | Products ending in P/B, reserved for professionals | All other plant protection products |
| Where to check | Fytoweb, the website of the Belgian federal public health service | E-Phy, the Anses catalogue |
In Belgium, G means garden and B Belgium. Fytoweb also reminds you to rinse the sprayer three times. The rinse water is emptied onto the lawn or a mulch, never into the drain. Empty packaging goes to the recycling centre. In France, even products that are still allowed call for precautions in use, the Fredon Ecophyto newsletter reminds us.
These rules change. Before you buy, check on Fytoweb or on E-Phy that the product is authorised for amateurs and on your crop. The guide Products allowed in the vegetable garden goes into detail, nettle feeds included.
For blight, the app watches the forecast for your address and warns you when the risk window opens. Add your tomatoes and potatoes.
FAQ
My plant wilts while the soil is moist, what is it?
Look at the roots. On a cabbage, swollen roots mean clubroot. If young plants die after planting, think of cabbage root fly. On an onion or a leek, a rotten base with a white felt suggests white rot.
Should you treat aphids?
Most often, no. The RHS advises tolerating them. In spring, they multiply before their predators, then ladybirds and hoverflies take over. If a plant suffers, squash the colonies between two fingers.
Are slug pellets a good idea?
The RHS advises avoiding them, even organic ones, because they harm garden wildlife. Protect fragile plants instead, collect in the evening or set beer traps.
Can you compost diseased leaves?
Not those with blight. The RHS advises throwing them out with household waste or burning them. Also disinfect stakes and tools.
Why are my carrots gnawed when I saw nothing?
The carrot root fly lays at the base. Its maggots tunnel into the root underground. Sow after mid-May and harvest before the end of August. Also cover with a 1.3 mm net.
Photo: Andy Potter, Geograph, Wikimedia Commons, licence CC BY-SA 2.0
Sources
- RHS, Slugs and snails (tolérer, cibler les plantes fragiles, étude de la RHS : ruban de cuivre et écorce sans effet sur les dégâts)
- RHS, Aphids (ennemis naturels, tolérer, écraser les colonies)
- RHS, Cabbage caterpillars (piérides et noctuelle du chou, de mai à octobre)
- RHS, Flea beetles on brassicas and allied plants (altises : petits trous ronds, radis, roquette)
- RHS, Cabbage root fly (mouche du chou, collerettes de 8 à 15 cm)
- RHS, Club root (hernie du chou, spores jusqu'à 20 ans dans le sol)
- RHS, Cabbage whitefly (aleurode du chou)
- RHS, Carrot fly (mouche de la carotte, semis après la mi-mai)
- RHS, Leek moth (teigne du poireau)
- RHS, Allium leaf miner (mineuse du poireau, pontes de mars à avril et de septembre à novembre)
- RHS, Leek rust (rouille du poireau)
- RHS, Onion white rot (pourriture blanche, 15 ans ou plus dans le sol)
- RHS, Tomato blight (mildiou de la tomate et de la pomme de terre, rotation de quatre ans)
- RHS, Blossom end rot (cul noir : un arrosage irrégulier, pas une maladie)
- RHS, Powdery mildews (oïdium)
- RHS, Grey mould (pourriture grise, Botrytis cinerea)
- RHS, Damping off (fonte des semis)
- RHS, Glasshouse red spider mite (araignée rouge sous serre, de mars à octobre)
- RHS, Insect-proof mesh (mailles de 0,8 mm, 1,3 à 1,4 mm, 4 à 7 mm et les ravageurs arrêtés)
- FREDON Hauts-de-France, Le doryphore : un ravageur de la pomme de terre en recrudescence (fiche 2021/37, octobre 2021)
- SPF Santé publique (Fytoweb), Dans son jardin (produits autorisés aux amateurs, numéros en G/B ou G/P)
- FREDON Hauts-de-France, Lettre d'information Ecophyto n° 13, Comment jardiner autrement (loi Labbé, interdiction pour les particuliers au 1er janvier 2019, août 2016)
- ANSES, E-Phy, catalogue des produits phytopharmaceutiques et de leurs usages autorisés en France
He gardens in Belgium and tries out in his own vegetable patch what the app recommends. His articles always start from a source you can check and a step you can take this weekend.
