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Diseases in the Vegetable Garden

Lee rust disease

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 diseaseAnthracnose. 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 on potato leavesBlight. 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 on brassica plantClub 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 beansproutDamping 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.

Recognising downy mildew on squash leavesDowny 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 wiltFusarium 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 pn tomato plantsLeaf 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.
Recognising powdery mildewPowdery 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.

Leek rust on leak leafRusts. 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.

  1. Pouncer

    My whole vegetable garden is a narrow strip behind the house and has been very successful for years. It is now covered in rust:
    – mini plastic greenhouse which is never closed: cucumbers rusted; aubergines and peppers are ok;
    – two large raised beds: artichokes, garlic rusted; beans rusting now; tomatoes are ok;
    – two large oak tubs: courgettes just started rusting;
    – pots with potatoes and pots with herbs are fine.
    I don’t know where to start – should I just clear at the end of the season and leave next fallow or green manure. I try to ‘use all the space’ as one is exhorted to do and yes I do improve with compost and horse manure from time to time. Presumably i should be spacing my plants more widely. I guess i’ve answered my own question!

    1. Andrew

      Hi Pouncer
      I am not being lazy but yes, you have just answered your own question. Good air circulation will help a lot so next season I would leave the garden more open even if it does mean a reduction in overall yield.

  2. Sheree

    Hi! I haven’t planted in my raised bed because I believed it had something I don’t remember now! Seems the info was to use Copper in the soil… or don’t use the bed for a couple of years. I did the latter. Now I want to plant in the raised bed again, but I have concerns with the overall health of the soil. It needs to be sanitized of deseases before planting. Do you have suggestions for a general overhaul of the soil to fight deseases? If not, can you suggest someone that might help… or reference online information?

    I appreciate any simple plan you may share or refer to online.

    Thanks much!

    1. Andrew

      Hi, Its hard to comment without knowing which particular disease you had in the soil. But, generally speaking, leaving the bed dormant for a few seasons should allow the soil to recover and most diseases will have died off. Be sure to add soil improvers to complement the structure and rebuild its nutrient content, these will help to create healthy soil.

  3. Mary Cullen

    I had a honey suckle which was growing but covered in little bugs and black flies. I tried to treat it with soap and water solution and left it on for about 4 hours then hosed it off. The flowers have never bloomed on it so I have cut it right down . What are the chances of it for next year.

    1. Andrew

      Hi Mary, apologies for the delay in replying. It should grow back again next year. Keep an eye on it early in spring next year and treat for aphids as soon as you see any. Aphids are a big problem in spring but balance is generally restored in the garden in summer when predatory insects like ladybirds arrive. You could try planting some lavender or similar around the honeysucle to attract ladybirds. Hope this helps

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