Perfectly healthy plants can be affected by air, water or soil borne diseases at any time. Understanding the causes, identifying them early and taking effective action will help you establish and maintain a healthy vegetable garden.
Anthracnose. This is a fungal disease present in Europe since the 1990s affecting tomatoes, beans and cucumbers and many other plants. It causes lesions on plant stems, leaves and fruit which develop into spore groups during warm, wet weather. Affected plants should be destroyed.
Anthracnose disease attacks all plant parts at any growth stage. The symptoms are most visible on leaves and ripe fruits. At first, anthracnose generally appears on leaves as small and irregular yellow, brown, dark-brown, or black spots. The spots can expand and merge to cover the whole affected area. The color of the infected part darkens as it ages. Infected fruit has small, watersoaked, sunken, circular spots that may increase in size up to 1.2 cm in diameter.
Organic treatment is a copper type spray similar to that used to control potato blight below.
Blight. Potatoes and tomatoes are affected by this fungal organism. Leaves will initially shrivel and rot before the disease spreads to the fruit which will turn brown and decay. Blight thrives in warm, wet weather commonly in late summer.
Early varieties of potato will normally crop before these conditions arrive. It is good practice to rotate seed stock or to use blight resistant varieties. Affected plants should be destroyed and not composted.
Club Root. Affecting brassicas, the first signs of infection are wilting, blueish leaves and a dying plant. Roots become swollen and distorted, restricting growth and yield. Alkaline soils can help to curtail, although not eliminate, the disease.
The disease is will be brought into a new garden with infected plants, it is common in allotment gardens where traffic between plots is high but isolated gardens rarely have it.
Once you have the disease in the garden you will have to live with it as the spores remain in the soil for up to 9 years. You can avoid growing brassicas in the garden for some time or take the following precautions: Lime the soil the previous Autumn (club root thrives in acidic conditions), use raised beds (clubroot likes wet conditions), burn or otherwise dispose of brassisca roots rather than adding them to the compost.
Damping Off Disease. Damping off is caused by a variety of different soil borne fungi causing young seedlings to rapidly fail. Plants under stress due to high temperature or waterlogging are particularly prone.
Using good quality seed compost, thorough cleaning of pots and trays and good ventilation will help to prevent this condition. Slight underwatering at seedling stage is also more advisable as plants create a stronger root system searching for water and are far less likely to be troubled by damping off disease.
Downy Mildew. This appears as a white downy growth on the undersides of leaves and along stems. It can be avoided by improving air circulation to eliminate the damp, humid conditions that cause it. Selective pruning can help with this to improve air flow.
Water in the early morning, avoid splashing the leaves and give plants time to dry out during the day. Severely infected plants should be removed. If plants are grown in a greenhouse a soil irrigation system can be beneficial as it reduces the need for foliar watering and keeps the leaves dry.
Fusarium Wilt. This fungal disease spreads from the plant root to the capillary vessels in the stem, impairing the plant’s ability to draw up water. The plant will start to wilt and leaves will turn yellow, progressing from the older foliage to the new as the growing season develops.
When plant stems are cut lengthways the brown striped of diseased capillaries will be readily apparent. Suspected plants should be removed and disposed of as early as possible and in severe cases compost should be replaced.
Leaf Spot. This disease is present in a bacterial or a fungal form and can affect most plant species. It appears as a black or brown spot, often with a yellow halo, which will spread and eventually destroy the affected leaf. The disease is common in warm, moist conditions and will spread easily when watering. There is no remedy other than preventive care. Fungicides can be used when early symptoms appear, particularly with brassicas.
Mosaic Virus. The leaves of beans, tomatoes, and peppers, are affected by mosaic virus, displaying mottled green and yellow foliage. Leaf curl and wrinkle occurs and plant growth is often stunted. Preventive measures are encouraged, such as planting resistant varieties and deterring pests, especially aphids and leafhoppers, which spread the disease. Remove and destroy any infected plants.
Powdery Mildew. This fungus specific to each host plant displays a white powdery coating on the surface of leaves and fruits which causes plants to become distorted and growth to die back. Affected plants also often display dark brown or bright yellow spots.
If it isn't treated, the problem can cause plants to die or fruiting to fail. Removal and disposal of infected leaves and stems will help prevent the development of spores for the next season.
Rusts. Common rusts can affect everything from asparagus to beans, carrots, and onions. Infected plants develop reddish brown powdery spots on leaves and stems. Rust diseases are not fatal but may result in damage to the foliage preventing flowering and fruiting. Prevent infection by providing good air circulation around crops and remove any seriously affected plants.
Most diseases, particularly fungal based, can be prevented by exercising good gardening practices. Choosing certified seed and rootstock will ensure your plants have a good start and carry a good degree of resistance to many diseases. Be diligent in cleaning tools, trays, pots and anything that comes in contact with your plants and soil. Clean and disinfect your greenhouse at the start of the growing season to eliminate any problems remaining from the previous year. Sensible watering and good ventilation will help prevent the growth and spread of many fungal organisms.