I recently returned home after camping with my family to find four of my tomato plants had been attacked by blight while I was away. All the symptoms were there – blotches on the leaves and stems, fruit turning brown and sections of the plant dying. My first concern was how to prevent it spreading to my many other tomatoes and to keep it from affecting next year’s crop. Understanding how plant diseases and pests spread is central to this as may plant diseases and pests can survive over winter in a garden, ready to attack young seedlings the following year.
The Usual Suspects
The following pests and diseases all commonly survive in gardens and need dealing with efficiently to prevent their spread:
- Blight: One of the worst diseases of potatoes and tomatoes, blight is spread by spores blown from one area to another. Infected plants left on the soil from a previous year or the tubers of infected potatoes in the soil can harbour the disease which then spreads rapidly during warm, moist conditions. So, disposing of all blight affected plants is essential. In the case of my blight-attacked tomatoes I seem to have been able to remove the problem before it transferred to other healthy plants in the garden and greenhouse but it often spreads before the problem is spotted.
- Downy Mildew: Most mildews (which generally affect squash and vine plants) survive in plant debris through winter. Clearing away all old plants is particularly important, including any diseased leaves on the soil.
- Viruses: Diseased plants and some weeds can retain virus infections such as cucumber mosaic virus (which is not limited to cucumbers but also affects other squash plants, lettuce and spinach) and infected plants must be destroyed. Failure to do this makes it likely that the virus will then be spread by aphids when they suck the sap from plant stems.
- Cabbage root fly and carrot root fly: Common winged insect pests like root fly survive as larvae in the soil around the plant. Burning infected plants and moving the crops to new areas for at least 3 years is advised combined with the usual protection of these plants using root collars for brassicas and fine mesh for carrots.
- Aphids: Usually aphids overwinter as eggs on a perennial plant, typically trees. However, brassicas such as cabbages and broccoli are often grown through the winter and aphids can lay eggs in stalks left in the ground. It is important to harvest these plants or pull them up before the aphids produce their next generation in late spring.
- Root aphids: Other aphids attack the roots of plants such as lettuce and these have been found to overwinter in soil so if this has been a problem it is wise to avoid growing the same plant in that space next year.
- Slugs and snails: These love all ‘nooks and crannies’ and in particular stones, wood and under sheet mulch. Removing or checking these sanctuaries will reduce the problem of controlling slugs the following year.
![Aphid on lettuce]()
Eliminate Everything?
From this list it sounds as if the best idea would be to completely level the entire vegetable garden over winter, removing everything and leaving just bare soil. I know gardeners who do this but there are problems with this approach:
- Beneficial insects need homes over winter too. I once started cutting back a low hedge in my vegetable garden during winter, only to find that I was destroying the habitat for a number of ladybirds which were sheltering there. I now make sure that I leave some good places to encourage insects like them to stay. After all, they are going to be the best defence I have against aphids the next year.
- Bare soil gets eroded over winter if exposed to harsh rain, snow and wind. So this is a good opportunity to sow green manures or add a layer of mulch. Some gardeners cover all dormant areas with black plastic which really helps eliminate weeds but can also harbour slugs and produce a lot of water run-off if it’s not permeable.
![Jeremy in his greenhouse]()
The Best Advice
Garden hygiene isn’t the only method used to control pests. Equally important are crop rotation, encouraging beneficial insects and feeding the soil well. However, the following principles are a good checklist to follow wherever possible:
- Remove all diseased plant material as soon as it is spotted. This should only be composted if you are using very hot composting techniques, otherwise it should be disposed of in a bin or by deep burying.
- Clean all tools and hands immediately after handling infected plant material to ensure that you are not spreading disease around the garden yourself.
- Be wary of spreading soil from areas that have suffered from problems such as clubroot, root aphids etc. Cleaning footwear may be necessary.
- Keep plants cut back and well spaced so that air can circulate well. Damp areas with little ventilation help fungal diseases, moulds and many others to survive. Our Garden Planning Tool can help get the spacing right to prevent overcrowding.
- Clean greenhouses, cold frames and cloches (row covers) well each year.
Cleaning up at the end of the season is certainly not my favourite activity and it can easily seem like just another chore to add to the list of jobs. Instead I try to see it as part of the very early preparation for next year: ensuring that the garden is in perfect condition for starting plants in spring and trusting that the rewards next year will be well worth it.
