Pests and diseases
The stealthy munch of the slug, the devastating carrot fly and the disheartening potato blight - all growers can suffer such ghastly set backs. Here is an overview of some common Pests and Diseases and suggestions on how to treat them organically.
See also Slugs and snails.
But before we start, remember that organic growing works on the principle of prevention, rather than cure. To help prevent pest and disease problems:
- Maintain a healthy, active soil with plenty of nutrients, using compost and manures. See Managing your soil
- Use crop rotation, see How to grow vegetables & herbs the organic way, to prevent harmful diseases building up in the soil
- Work with nature by creating a diversity of plant life. Mix flowers with vegetables to attract beneficial wildlife, such as caterpillar-eating birds and aphid-eating insects.
- Never control nature with toxic chemicals and sprays, as they kill not only the pest but create a health hazard for the birds and other wildlife that eat the pests. See Neonicotinoids and Glyphosate
- Choose plants and varieties that are suited to the site and soil or growing medium
- Be vigilant - keep out pests, such as slugs, caterpillars and pigeons, by constantly checking your barriers, traps and covers. Especially after rain or in damp conditions.
Organic pest control for fruit and vegetables
These are listed alphabetically. See also Nematodes.
More in-depth information is available on individual fact sheets for Garden Organic members. Why not become a Garden Organic member? 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 email@example.com
Allium Leaf Miner
What is it? A tiny white maggot which feeds on leeks and onions, causing similar damage as the leek moth caterpillar (see below).
Symptoms: White spots, leaf splits and distorted plants which eventually rot.
Prevention and/or treatment: Clear away any debris at harvest, dig over soil to expose pupae to predators. Do not put infected plants or debris straight into the compost heap. Instead destroy them by soaking them for a couple of weeks then bury the mush. To prevent occurrence, cover susceptible plants with an ultra-fine mesh cover (the fly is tiny). The eggs are laid throughout the year, and can overwinter in the soil, so it is difficult to create total barrier control.
What is it?: Red and black ants are most common in the garden. Red ants tend to sting, while black ants are less aggressive. New nests are created in late summer.
Symptoms: Nests can undermine garden, greenhouse and potted plants, causing them to wilt or die. Some species build mounds on lawns, making mowing difficult and spoils the lawn's appearance.
Prevention and/or treatment: It is impossible to eliminate ants from a garden, so to some extent you have to learn to live with them. There are some actions you can take – encourage their natural predators such as slow worms and frogs, douse the nest with water (boiling, if you prefer), use grease bands on fruit trees; similarly grease greenhouse staging legs, or stand them in a moat of water as ants can’t cross water.
What is it?: Aphids are sap-sucking insects that can be found on a very wide range of plants - and in roots, stems and leaves. Often known as greenfly or blackfly, they are one of the most common pests. They can also carry viruses.
Symptoms: In large numbers clustered around tender young growth they cause young shoots to become weak and distorted, sometimes killing the plant.
Prevention and/or treatment: Avoid using too much nitrogen-rich fertiliser which encourages soft leafy growth which is attractive to aphids. Encourage creatures that feed on aphids, such as birds, insects and their larvae, earwigs and bats. Grow flowers that attract hoverflies, lacewings and ladybirds (see Flowers and herbs to attract beneficial insects). During winter, hang up pieces of fat in fruit trees and above rose bushes to attract blue tits which eat aphid eggs. Inspect plants regularly and squash any aphids that are seen. Pick off heavily infested shoots and leaves and drop into a bucket of soapy water. A strong jet of water can also dislodge them.
Apple codling moth larvae
What is it?: Known as apple maggots, the larvae are pinkish-white with mottled brown heads, 18-20 mm long. The adult moth is mottled grey-brown in colour, about 8 mm long.
Symptoms: They burrow into the core leaving a prominent, red-ringed, entry hole blocked by dry ‘frass’ (maggot droppings). A large proportion of the fruit flesh can be eaten away and the cavity becomes filled with brown frass.
Prevention and/or treatment: Pheromone traps can be effective, as well as attaching greasebands around the tree. Encourage earwigs (which eat the moth eggs) and Blue tits (which eat the cocoons). Remove and destroy windfalls, birds (including hens) will clear away pupae, and check that tree ties do not contain cocoons.
Pigeons love brassicas! They can strip a plant in winter when there is little other source of nutrition. Other garden birds love berries. To deter them it is worth thinking of two behavioural traits – neophobia (the fear of the new) and habituation (the ability to become tolerant of stimulus). Therefore using deterrents such as static scarecrow will only have limited effectiveness. Either use a complete barrier, such as netting, making sure there is no entrance or possibility of the bird getting trapped, or a simple system of string and wire threaded across the vegetable patch (CDs are optional!). This appears to make it difficult for birds to judge flight access.
What is it?: Small white winged insects on the underside of leaves of brassica plants, which fly up in clouds when disturbed. The young whitefly, known as ‘scales’, remain on the leaves.
Symptoms: The whitefly themselves can cause severe distortion and stunted growth, and the scales make leaves unappetising and covered in sticky honeydew that is exuded by the feeding insects. Sooty or black moulds often grow on the honeydew.
Prevention and/or treatment: Most plants can tolerate quite a high infestation. Limitation measures include creating a healthy soil, which produces strong, resistant plant growth. Fennel, coriander and cow parsley will attract parasitic wasps which lay eggs inside the whitefly scales. Their larvae consume them from the inside out. Remove infected leaves before the immobile young whitefly ‘scales’ turn into adults. Alternatively insecticidal soap or sprays based on vegetable oils can be effective. Biocontrols will only work in a greenhouse/polytunnel.
What is it?: Adult flies are about 8 mm long, shiny black with reddish head, orange legs and transparent wings. The larvae are 8-10 mm long, creamy-white in colour.
Symptoms: Young seedlings can die, and mature plants are infested with rusty brown tunnels. There may be no foliage symptoms.
Prevention and/or treatment: Flies are attracted by the smell of bruised foliage, so pull carrots and weed around them on a dry evening with no wind – or on a very windy day. Similarly sow sparsely to avoid the need for thinning. There is some evidence that growing carrots with onions (four rows of onions to one of carrots) can help minimise damage, again because the onion masks the smell. Fleece gives excellent protection as does a vertical fence-like barrier around three or four rows of carrots. This needs only be 70 cm–1 m high, as carrot flies are weak fliers. Growing carrots in a container on a table top also helps, as it lifts them above ground level.
What is it?: The common earwig can be a nuisance in the garden, damaging the petals of flowers such as dahlias, clematis, delphiniums, pansies and chrysanthemums. However, earwigs do also have a beneficial role in the garden, feeding on aphids and other small insect pests, including the apple codling moth.
Symptoms: Large ragged holes in flower petals. Although often found in cavities inside fruits (peaches, apples, pears) earwigs are not usually responsible for the initial damage; they tend to take over and extend a wound caused by other fruit pests.
Prevention and/or treatment: Create strategically placed nests (using upturned flowerpots with straw and newspaper) which can be emptied of earwig inhabitants by shaking them over a bowl of soapy water.
What is it?: Sawfly larvae are green with black spots and a shiny black head. When fully grown they reach approximately 30 mm. The eggs are about 1 mm long and pale greenish white in colour. They feed on the leaf edges and are difficult to spot.
Symptoms: The larvae strip the plant of its leaves, leaving a weakened and defoliated plant that often produces a poor crop the following year.
Prevention and/or treatment: The adult sawfly lays its eggs from April onwards, so inspect bushes from then. New generations will be hatching low down in the bush. Larvae should be picked off by hand or sprayed – either with direct jets of water or with a nematode control (available online). Remove mulches from around the plant in late autumn, to allow birds (hens particularly!) to clear up the cocoons in the soil.
What is it?: Adult whiteflies are small moth-like insects, 1-2 mm long, white/creamy yellow in colour, with white wings.
Symptoms: Covers the underside of leaves which turn yellow from the sticky honeydew excreted, and can develop sooty black moulds.
Prevention and/or treatment: Prevention, as always, is healthy soil that creates strong growth and resistant plants. Yellow sticky tapes can help, especially if you tap the plant directly underneath. Spray badly infected plants with insecticidal soap. During winter, scrub the greenhouse down using warm soapy water, to remove the eggs of any overwintering pests. Ensure any overwintering plant stock, such as fuchsias, are clear of whitefly infestations. Throw out badly infected plants. Bio controls are available.
What is it?: The adult is a tiny, inconspicuous, brown moth. The caterpillars are up to 13 mm long, yellow-green in colour with grey-brown patches and a yellowish brown head.
Symptoms: The caterpillars tunnel into the plant, creating brown and white patches on the leaves and eating the stem and bulb.
Prevention and/or treatment: Destroy infected plants, clearing all debris and digging over ground after harvest. Pick off caterpillars by hand. Use horticultural fleece to protect plants from egg laying moths. Encourage predators such as birds, frogs, bats etc who will eat the larvae.
The mole can live anywhere where there is sufficient depth of well-drained top soil, but is most common in grassland and deciduous woodland – and organic gardens where there are high populations of earthworms! Mole hills are made of a very fine-textured, friable soil and were traditionally used to make potting composts. Mole hills can be unsightly in a lawn, and make mowing difficult. There are ways of trapping moles humanely (probably best left to professionals) but you can also deter them from creating their tunnels by flooding, noise, vibration, the smell of human urine, barriers and even digging them out. Spurge (Euphorbia) can also repel moles.
New Zealand Flatworm
What is it?. This invader came to Britain in the 60s. It is a threat to our native earthworm as it destroys them. They are easily recognised, with their flat bodies (5-15cms long), dark markings on the top above a pale underside, pointed at both ends and covered in sticky mucus.
Symptoms. The flatworm encircles the earthworm and covers it in mucus, which causes the worm to disintegrate and become digestible. Flatworms can travel - but they are usually found in cool, damp conditions. Hence their presence in the northwest of the UK, N Ireland and Scotland. Warmer temperatures in the south can kill them. Their eggs resemble hard, flattened black currants.
Prevention. It is difficult to eradicate the flatworm. Best practice is to create refuges ie stones, logs, weighted down polythene sheets in cool shady areas. Then, once the worms have gathered, destroy them – either by putting in hot water (>30c), sprinkling with salt, or squashing them. This has to be done regularly, as part of a sustained campaign. It is recommend that you do not touch them, the mucus can cause skin irritation. Wear gloves. Any sightings should be logged onto the OPAL survey web page.
Pea and bean weevil
What is it?: Adult weevils are small, brown/grey in colour and short-snouted, between 5- 6 mm in length. The larvae are legless, white with a brown head and are found in the soil around leguminous crops.
Symptoms: Semi-circular notches eaten out of the edges of the leaves of peas, broad beans, clovers and vetches. Severe infestations can cause seedling losses, especially in cold wet conditions. Older plants are little affected, although they can get into the pods and beans themselves.
Prevention and/or treatment: It is generally recognised that there is little you can do to protect your plants from this pest, except, as always, encourage strong, fast growth by providing plants with the best possible growing conditions.
What is it?: The yellowish-brown larvae are 6-8 mm long, with brown markings and brown heads. They mature into brown adult beetles 3-4 mm long, covered with fine hair, which lay tiny cream eggs in fruit blossoms
Symptoms: Only occasionally causes severe damage. Burrows into the fruit, causing it to become hard or distorted.
Prevention and/or treatment: Fork the soil around the canes at the end of the season to bring the beetles and pupae to the surface, to be eaten by birds. Repeat this several times through the autumn/winter. This technique effectively disrupts the life cycle of the beetle
Slugs and snails
What is it?: Slugs and snails are soft bodied, gastropod molluscs that move along on a single muscular foot and secrete slime. They scrape their food up with a spiky, rasping tongue. Snails tend to hibernate in the winter and are unable to move through the soil, whereas slugs can be active all year round both above and below ground. Snails are able to climb higher as they retreat into their shells to prevent drying out. Both slugs and snails mostly feed by night.
Symptoms: The tell-tale trail of slime, the seedlings completely eaten, the large holes in leaves, and even the hollowing out of potato tubers are all depressing indicators of slug presence.
Prevention and/or treatment: Protection of vulnerable plants is the key - and it's important not to rely on only one method. Always renew barriers after rain, and accept that some damage is inevitable. The following may help: dig to disrupt both slug and its eggs; encourage natural controls such a beetles, frogs, birds and hedgehogs; frequently inspect your plants and hand pick off (particularly in damp weather and at night); create barriers of dry material which slugs find hard to traverse, such as grit, sheep wool – and renew when wet after rain. You can also put a thick layer of dry oats or bran around small vulnerable plants for slugs to gorge on and dehydrate- making easy pickings for the birds. Again, renew when wet after rain. If you use traps (a can or saucer with dregs of beer) empty them frequently. To avoid killing ground beetles which eat the slugs, it would be better to put your beer into a saucer with raised edges. Use of nemotodes (microscopic organisms, available to buy online) can have some success, but they only work once in a season, and the conditions are very specific for the nemotodes to function. If you have to use slug pellets (and yes, we all lose our patience at some stage!) make sure they are approved for organic growing, use SPARINGLY, and store safely. Most contain ferric phosphate, which will break down in the soil. However they also contain other chemicals which can affect earthworms. Try the Organic Gardening Catalogue. Using non organic slug pellets is to be avoided at all costs. Not only do they kill the slugs, but they also can affect the hedgehogs, thrushes, frogs and other wildlife that eat the slugs.
What is it?: Vine weevil larvae are up to 1 cm in length with a plump, creamy white body and a brown head. When found, they are usually curled into a ‘C’. The adults, also 1 cm long, are dull matt black with ridges running down their back and a pronounced ‘snout’.
Symptoms: Serious pests of a wide variety of plants, particularly Fuchsia, Primula, Cyclamen and Begonia. The larvae are the most damaging, usually found in the compost of container grown plants, but they can also attack the roots of plants in open ground. Adult weevils make holes in leaves, which usually don’t harm the plant. Plants wilt as though short of water, and when taken out of the pot the root system will have virtually vanished and the larvae will be clearly visible in the compost.
Prevention and/or treatment: Be vigilant, if caught in time, plants can be saved either by re-potting in a new pot with fresh soil, or by re-planting in a different place. Larvae should be destroyed. Put a strip of wide PVC tape, such as brown plastic parcel tape, around individual pots and tubs, and smear this liberally with insect barrier glue that the weevils cannot cross. You can often trap the adults on a warm August night. Use of nemotodes (microscopic organisms, available to buy online) can have some success, but the conditions are very specific for the nemotodes to function.
What is it?: Larvae are tough skinned, cylindrical, golden yellow to orange brown in colour and reach up to 25 mm in length when mature. Three pairs of thin small legs are located behind the head. Wireworms are the larvae of various species of click beetle
Symptoms: They attack the underground parts of plants, damaging roots, tubers, corms and stems. Favourites are potato, beetroot and carrot as well anemone, dahlia, and gladioli. Small holes 2-3 mm across are seen on the outside of the tuber or root. A network of tunnels is often invaded and enlarged by other pests such as slugs or woodlice. Further bacterial and fungal rots may develop making them unsuitable for storage.
Prevention and/or treatment: It is not easy to prevent or control the larvae – thorough digging before planting and harvesting will expose them to predators such as birds. Harvesting a potato and other root crops early will limit the damage
Organic Disease Control
These are listed alphabetically under Alliums (leeks and onions), Apples and Pears, Beans, Beets, Brassicas, Courgettes (and marrows), Peas, Potatoes and Tomatoes.
More in-depth information is available on individual fact sheets for Garden Organic members. Why not become a Garden Organic member? 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.
Alliums - Leeks and onions
What is it?: White rot
Symptoms: A disease that attacks the roots of all alliums, including ornamental and eating onions, and the necks of garlic. Roots rot and a white fluffy mould may be present. Plants, apparently growing well, suddenly start to die. Older leaves turn yellow and wilt, and examination will reveal that roots have become stunted or rotten. There is a characteristic sour smell.
Prevention and/or cure: This disease can last for 20 years in the soil, however it doesn’t spread, so try to use as long a crop rotation as possible. Quarantine is the best method of avoiding white rot. Infectious soil can be transferred on footwear, tools and seedlings. Dispose of diseased material in your municipal waste where it can be composted to high heat.
What is it?: Downy mildew
Symptoms: Affects onions, shallots and chives. Leaves develop a grey/ purplish mould and die back. It is more widespread in cool wet summers.
Prevention and/or cure: Clear away and destroy all onion debris at the end of the season. Do not compost infected material. Avoid damp, poorly drained, sheltered sites. Use wide spacing and keep well weeded to allow good airflow through the crop. Only use firm healthy sets; destroy any that sprout prematurely.
What is it?: Leek rust
Symptoms: Rusty red pustules develop on leaves in late summer. In severe attacks leaves may turn yellow and die, and plant size may be reduced.
Prevention and/or cure: These may disappear if weather turns cold or wet. Water crops if dry in summer. It seems that plants grown on soils high in nitrogen and low in potassium are more susceptible to attack by leek rust. Use only well-rotted manure (fresh is high in nitrogen).
Apples and Pears
What is it?: Scab
Symptoms: This is a widespread fungal infection causing blemished fruit, as well as on young leaves. Leaves will curl and drop prematurely; fruits develop brown corky patches which can crack.
Prevention and/or cure: Clear up fallen leaves and infected fruits. Watering fallen leaves with diluted urine, or any other high nitrogen liquid manure (such as nettle ‘tea’) will help kill spores. Prune trees regularly to maintain an open centre to increase air circulation. Apples that are particularly susceptible to scab are: Cox’s Orange Pippin, Gala, James Grieve, and Laxton’s Superb. Pear: Williams, Bon Crétien.
What is it?: Apple powdery mildew.
Symptoms: This is a serious and common fungal disease of apples. It also infects pear, quince, peach, medlar. A white powdery coating appears on leaves and shoots, as well as flower buds in spring. Blossom may be affected, causing it to wither and drop. Leaves become distorted, narrow and folded, then turn brittle and fall. A harsh winter will reduce the risk of infection; it spreads most rapidly in summer when warm, sunny days are accompanied by humid nights.
Prevention and/or cure: Pruning is the best way to prevent infestation. In winter, cut out any shoots and buds that have been infected with mildew, they will appear silvery/grey, and buds distorted. In spring, carefully remove infected leaves and shoots. Prune directly into a bag to prevent spores from spreading. Check trees weekly through the season and carry on cutting out infection. On small trees this can be a very effective method of controlling mildew, if done thoroughly. Prunings should be buried in an active compost heap or sent to your local council’s green waste recycling centre.
What is it?: Fruit tree leaf curl
Symptoms: This is not a disease, but results from infestations of small yellow-green aphids feeding young shoots in Spring and Summer, causing leaf curl. It can damage young trees.
Prevention and/or cure: Encourage beneficial insects (see How to grow flowers the organic way) and birds to eat aphids, and take heart that fruiting will not be affected. Pick off infected leaves and destroy.
What is it?: Broad Bean chocolate spot
Symptoms: Dark brown spots on leaves, stems and pods. Plants die prematurely.
Prevention and/or cure: Good air circulation reduces incidence of the fungus. Grow in well-drained soil and improve drainage if waterlogged. Use a wider spacing between plants and between rows. More commonly a problem on autumn sowings, on wet soils. Remove badly infected plants to the compost heap and don’t save the seeds.
What is it?: Grey mould - Botrytis
Symptoms: Grey water-soaked lesions caused by botrytis on bean pods are common when flowering coincides with wet periods.
Prevention and/or cure: Poorly drained and very sheltered sites should be avoided. Be sure to remove any petals adhering to the pods, and don’t save the seeds from infected plants.
What is it?: Flowers failing to set pods.
Prevention and/or cure: This can be due to a variety of causes:
Dry soil - water runner beans in dry weather (the soil not the foliage) applying about six litres per square metre twice a week, depending on soil type.
Poor pollination - Runner bean flowers must be pollinated, by bees in the main, in order to set pods. Bees may ignore runner bean flowers in favour of others with a more sugar rich nectar. Things tend to improve in August when the more attractive blooms have finished flowering. Bees also tend not to be so active in cold, windy or wet weather.
Flowers destroyed - remains of runner bean flowers on the ground around the plants, indicates that birds, particularly sparrows, have been destroying the flowers.
Tiny, shiny black beetles, known as pollen beetles are often found runner bean flowers in the summer, and are blamed for poor pod set. Although these beetles do feed on pollen they are not usually to blame.
Beets – Beetroot, Chard and Spinach
What is it?: Powdery Mildew
Symptoms: Creates a grey powdery coating on the leaves, usually in hot dry weather.
Prevention and/or cure: Improve soil water holding capacity, grow resistant varieties of spinach, water in very dry weather, and keep picking to generate new leaves. This mildew is different to that affecting peas or courgettes, so they won’t catch it from each other (although they appear under similar dry conditions).
Brassicas – cabbage, broccoli, cauliflower and kale
What is it?: Clubroot
Symptoms: First sign of the disease can be wilting of plants, particularly during dry weather. Subsequently, plants may appear stunted or sickly and the foliage develops a purple-red tinge. Infected roots swell and distort, often producing either a single large gall (‘club’), or a cluster of smaller galls.
Prevention and/or cure: This disease can last for 20 years or more in the soil without you growing a cabbage. It affects nearly all members of the cabbage family, often arrives on infected plants, and there is no known cure. Choose resistant varieties. Adding lime to the soil will help. To give plants a healthy start, raise plants in 7 cm pots, then transplant. They will still be partially affected but may reach maturity. Ensure good hygiene, don’t spread the soil with your boots or tools to other areas.
Courgettes, Marrows and Pumpkins
What is it?: Powdery Mildew
Symptoms: Forms a powdery white coating on leaves. It usually appears at the end of the season when growth is declining anyway, so is nothing to worry about.
Prevention and/or cure: If it appears earlier in the season, water the plants well if the soil is dry.
What is it?: Cucumber mosaic virus
Symptoms: The young leaves develop a mottled or mosaic yellowing; growth is reduced and the leaves are distorted. Fruit tends to be pock marked and cropping will be poor.
Prevention and/or cure: Infected plants should be pulled out and put in the compost heap. As this virus can be found in hundreds of different plants the only answer is to grow varieties with some resistance.
What is it?: Powdery mildew
Symptoms: White powdery coating on leaves and pods. Common in dry, hot weather.
Prevention and/or cure: Keep plants watered and mulched in dry weather. Avoid sowings that will crop in the height of summer. Spray with bicarbonate of soda solution (2g per litre of water).
What is it?: Potato blight
Symptoms: One of the commonest potato diseases. Dark brown blotches that are often surrounded by a yellow halo that quickly spreads to rot the whole leaf. Leaves quickly become wet and rotten.
Prevention and/or cure: Often spread by infected tubers being left to sprout on compost heaps. Warm ,wet and still weather causes a rapid spread. Dry weather can halt the disease, so it’s worth removing the first few infected plants. Cut off the potato tops (haulms) and burn them. Don’t harvest any tubers for 3 weeks, this allows the skin to set. If you notice potatoes almost at the soil surface, mulch the rows with leaves or straw, or even cover with more soil, to prevent the tubers going green – which renders them inedible. Where blight is a regular problem you can reduce its effects by growing more resistant varieties, using deep ridges to allow the blight to be washed off by rain, or by growing early varieties which should give a decent crop before blight strikes.
See here for more in formation on potato blight.
What is it?: Scab
Symptoms: Unsightly brown marks on the tubers, but potatoes can be peeled and eaten.
Prevention and/or cure: More common on dry soil and those that are more alkaline. Grow resistant varieties, use certified seed, improve the water holding capacity of the soil, and never add lime before a potato crop.
What is it?: Tomato blight
Symptoms: Caused by the fungus-like oomycete, that also causes late blight of potatoes. Dark brown/ blackish round patches appear in the foliage, often surrounded by a pale yellow halo that quickly spreads to rot the whole leaf. The underside develops a downy white coating of spores in moist conditions, particularly at night. Dark streaks and spots may develop on infected stems. Fruits develop dark markings, quickly developing a dryish brown rot. A whitish-grey mould may accompany this.
Prevention and/or cure: The fungus is not poisonous to humans, however fruit are not pleasant to eat and will not ripen or store. To prevent or control, keep the plant leaves dry. Water the soil, not the leaves. Infection occurs in warm, moist airless conditions. Increase air flow between plants, particularly in greenhouse and polytunnel. Growing earlier maturing and smaller fruiting varieties might allow you to harvest fruit before blight strikes. There are also some resistant breeds, such as Crimson Crush. Leaves and stems of plants affected by blight can be added to your compost heap; the fungus will not survive in dead plant material. Do not compost blighted fruit, as the fungal spores can survive in seeds to grow and reproduce next spring, carrying blight onto your new crops.
If you're new to organic growing, and you still have the remains of chemical pesticides in your potting shed, see this website which explains how to dispose of them safely.