Some of the common diseases affecting fruit and vegetable growing are listed alphabetically here, under Alliums (leeks and onions), Apples and Pears, Beans, Beets, Brassicas, Courgettes (and marrows), Peas, Potatoes and Tomatoes.

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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.

Beans

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, indicate that birds, particularly sparrows, have been destroying the flowers.
Tiny, shiny black beetles, known as pollen beetles are often found in runner bean flowers in the summer, and are blamed for poor pod set. Although these beetles do feed on the 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 pockmarked 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.

Peas

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).

Potatoes

What is it? Potato blight

Symptoms: One of the commonest potato diseases. Dark brown blotches 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 information 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.

Tomatoes

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.